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Measurements of mean lives of excited atoms MacKenzie, Kenneth Ross 1937

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MEASUREMENTS OP MEAN LIVES OP EXCITED ATOMS Kenneth Ross MaeKenzie A Thesis submitted f o r the Degree of Master of Arts i n the Department of , Physios THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1937. CONTENTS Introduction page 1 Ma.th.od and Theory-Outline of method used page 2 Apparatus Kerr c e l l page 3 The vacuum s t i l l page 4 O p t i c a l equipment page 4 E l e c t r i c a l equipment page 5 Method of measuring the phase di f f e r e n c e page 6 Introduction of the phase angle page 7 Measurement of Mean Lives "by the P h o t o e l e c t r i c Method Theory of the method page 9 Experimental proceedure page 11 Conclusions page 14 Bibliography page 16 P h o t o e l e c t r i c Relays Thyratron c o n t r o l l e d r e l a y page 17 An e l e c t r o n i c r e l a y page SO Methods of C o n t r o l l i n g the Pressure i n a Gas X-ray Tube page 23 F i r s t method page 25 Second method page 26 Bibliography page 28 / MEASUREMENTS OF MEAN LIYES OF EXCITE!) ATOMS INTRODUCTION The present work was undertaken with the object of applying to other substances the method described by J. H. E. G r i f f i t h s ' for the measurement of mean l i v e s of excited atoms. Most of the necessary apparatus, (some of which had previously been assembled by G-. M. Volkoff*") has been b u i l t and tested. The task of purifying the nitrobenzene was undertaken by Mr. English and i t was while waiting for the f i n a l product that the writer decided to try some measurements with a photoelectric c e l l * These investigations showed that some perturbing factors, often of a large order of magnitude^could be present i n the method outlined by Griffiths^which might e a s i l y be mistaken for the desired ef f e c t . 2 METHOD AM) THEORY Outline of method used The general outline only, w i l l be repeated here as the detailed mathematical treatment can be obtained by referr i n g to the papers G r i f f i t h s ' a n d Volkoff* It may be described as follows. Light from a discharge tube, operated by a high f r e -quency alternating potential,is allowed to pass through an op-t i c a l system consisting of a l i c o l prism, a nitrobenzene Kerr c e l l , a c a l c i t e double image prism, and a second Nicol prism, and i s then analyzed by a spectroscope. The Kerr c e l l i s oper-ated from the same source of high frequency potential as the discharge tube, and the whole system acts as an electro-optical shutter which transmits more or less l i g h t depending on the moment during the cycle at which the maximum of l i g h t from the discharge tube reaches the Kerr c e l l . The angle 6 through which the second M c o l prism must be turned from i t s crossed position i n order to make the two images of any given spectral l i n e produced by the ealcite c r y s t a l of equal instensity i s a measure of the amount of l i g h t emerging from the system. The moment during the cycle at which the maximum amount of l i g h t from the discharge tube reaches the Kerr c e l l may be varied by either delaying the l i g h t by r e f l e c -tion over a long light-path, or by introducing an e l e c t r i c a l phase-difference between the potentials on the discharge tube and the Kerrc-eell. The observations consist i n finding the values of & for any gived spectral l i n e corresponding to d i f f -3 erent values of the effective phase difference between the maximum l i g h t reaching the Kerr c e l l and the maximum potential applied to i t . The results are plotted i n the form of a graph of 8 against the phase difference. The position of maximum or min-imum of this curve, as shown i n the theoretical development given below 9 gives s u f f i c i e n t data to determine the mean l i f e of the state, a tran s i t i o n from which to some other state gives r i s e to the spectral l i n e under observation, provided the o r i -ginal phase relations between the current in the disharge tube and the potential on the Kerr c e l l are known. The result of the mathematical treatment shows that the mean l i f e of the state i s given by the relation* Jtan 2 Y = 2. PT where T i s the mean l i f e , p i s 2-IT times the o s c i l l a t o r frequency, and t i s the value of the phase difference intro-duced which makes & a maximum or minimum. The Kerr c e l l The Kerr c e l l s were made by 0. M. Yolkoff and f u l f i l l the theoretical consideration .given by him i n his paper* The theory of the c e l l s and their use i n electo-optical shutter systems w i l l not be discussed here as many detailed treatments 3 can be found i n the l i t e r a t u r e . The electrodes are of brass, 5 x 2 mm. in area and separated .5 mm. They are brazed onto tungsten leads which are sealed into pyrex tubing. Microscopic cover glasses, free.from strains are sealed on to the short To face page 4. piece of tubing with ceresin^wax, as t h i s substance i s not soluble i n nitrobenzine. The general design i s shown i n f i g . / . The v&ouum s t i l l The vacuum s t i l l used by Vollcoff f o r the p u r i f i c a t i o n .;. of the nitrobenzine was r e b u i l t with a few minor changes. (1.) An ungreased mercury s e a l stopcock was used as a r e f l u x c o n t r o l . (2.) The l i q u i d could be c o l l e c t e d i n e i t h e r bulb by s l i g h t l y bending the long piece of glass tubing. Both these changes are apparent i n f i g . 2. In t h i s way the chance of contamination i s reduced to a minimum as the nitrobenzine and i t s vapor touch nothing but glass during the whole process. O p t i c a l Equipment (i ) Spectroscope. The spectroscope used i s a constant d e v i a t i o n instrument manufactured by Adam E i l g e r . I t can be f i t t e d e i t h e r with a telescope f o r v i s u a l observation, or with a camera f o r photographic recording. The c o l l i m a t o r supplied with the instrument i s replaced by the e l e c t r o - o p t i c a l shutter described below. ( i i ) E l e c t r o - o p t i c a l shutter. The main part of the e l e c t r o - o p t i c a l shutter i s the ICerr c e l l described above. In a d d i t i o n two TJicol prisms, a c a l c i t e c r y s t a l and a s l i t are used. F i g . 3 i s a diagrammatic representation. XL Is a part of the c o l l i m a t o r supplied with the i n s t r u -ment. L i s the c o l l i m a t o r l e n s . The tube between the s l i t S and the thread at X by means of which i t i s attached to XL eon-t a i n s a c a l c i t e c r y s t a l C, and a H i e o l prism 1^ which may be 3" rotated about i t s axis which eoineides with the axis of the tube. The p o s i t i o n of the M'col prism i s marked by a pointer P attached to i t , and moving with with respect to the f i x e d d i s c D graduated i n degrees. K i s the Kerr c e l l and N, i s the p o l a r * i s i n g M e o l prism. The s l i t i s placed v e r t i c a l l y , the p l a t e s of the Kerr c e l l are set p a r a l l e l to the s l i t , the/polarising M c o l i s set with i t s plane of p o l a r i s a t i o n i n c l i n e d at an angle of 45° to the d i r e c t i o n of the applied f i e l d i n the Kerr c e l l , and the c a l c i t e c r y s t a l i s f i x e d i n such a p o s i t i o n that the plane of p o l a r i s a t i o n of one of the two images of the s l i t produced by i t i s coincident with that of the f i r s t M c o l . ( i i i ) Method of comparing i n t e n s i t i e s . The angle through which the a n a l y z i n g - M e o l prism i s turned to make the two images produced by the c a l c i t e c r y s t a l of equal i n t e n s i t y , may be found e i t h e r by matching the two i n t e n s i t i e s v i s u a l l y by observing the l i n e s i n the telescope of the spectrometer, or i t may be obtained more accurately by a photographic method. The images are f i r s t matched v i s u a l l y , then the telescope i s replaced by a camera, and several p i c t u r e s i n that region are taken varying the p o s i t i o n of the M c o l prism by a small amount, say 2°, each time. The p i c t u r e s are then examined by a miero-photometer, and the angle at which the two images are of equal i n t e n s i t y i s found by i n t e r p o l a t i o n . E l e c t r i c a l Equipment. The o s c i l l a t o r shown i n fig.¥ Is a standard Hartley c i r c u i t using a 212 D tube and designed to cover a wavelength range from 30 to 600 meters. The wavelength could be measured To face page 6. i with an accuracy .of 1 to 2fo with a wavemeter and could,-when necessary, be more accurately determined by comparison with commercial t r a n s m i t t e r s . The power supply ( f i g . 5) uses type 866 r e c t i f i e r tubes and a 1 lew. pole transformer. Method of measuring the phase differemce. Eo attempt was made to. use the v a r i a b l e l i g h t path method f o r introducing the phase d i f f e r e n c e , as a s u i t a b l e cathode ray tube was not a v a i l a b l e A c c o r d i n g l y the phase d i f f e r e n c e was introduced e l e c t r i c a l l y and measured by means of a phasemeter patterned a f t e r the one described by R. R. Law. The c i r c u i t diagram i s shown i n f i g . 6. I t has been designed as a complete u n i t which w i l l operate from the 110 v o l t a.c. l i n e . Since i t must be used i n the immediate v i c i n i t y of the o s c i l l a t o r i t has been very thoroughly shi e l d e d . Two r a d i o frequency voltages of the same frequency, but d i f f e r i n g i n phase, are applied to the grids of the tubes V, and Yz through the coupling condensers C,, C 2. The capacity required i s extremely s m a l l . "Usually the wire c a r r y i n g the r a d i o frequency voltage under t e s t was placed about one h a l f Inch or so from the terminal A or B and i n t h i s way formed the condenser. A heterodyning-voltage from the o s c i l l a t o r Y3 i s applied to the g r i d s of V, and V 2through the v a r i a b l e condensers Q3 j':C . I t can be shown mathematically, by expressing the mutual c h a r a c t e r i s t i c curves of the tubes i n the form of a Taylor's s e r i e s and applying the two voltages to i t , that the audio frequencies produced w i l l have the same phase r e l a t i o n 7 as the impressed r a d i o frequencies.. This phase di f f e r e n c e i s then measured by means of the bridge c o n s i s t i n g of E,,R' ,1,,L 2, a n d B y heterodyning aboye and below the frequency to be measured, any phase s h i f t i n the apparatus may be' cancelled out. '* This Is shown In f i g . 7, where the voltages are expressed ye pr-i o r t a l l y . When the bridge Is balanced, the phase angles are given b y the r e l a t i o n fr-tan~'£lML o s c i l l a t o r frequency < impressed frequency. &'=. -Jtan''rhi> o s c i l l a t o r freauency > imtiressed frequency. The required pkhse angle is' then G+a' . u> i s obtained by comparing z the beat with a sonometer. Introduction of the phase angle. Some d i f f i c u l t y was encountered i n the Introduction of the phase angle. The s p l i t phase method described by G r i f f i t h s ' was not used because of the large number of radio-frequency ammeters required, instead the apparatus shown i n f i g . ? was assembled. A 7T s e c t i o n f i l t e r i s used to couple the two reson-ant c i r c u i t s and w i l l Introduce an angle of l a g depending oh the constants used. The l a g i s r e f e r r e d to as the wavelength con-stant i n transmission l i n e theory. In e f f e c t the f i l t e r acts l i k e a s e c t i o n of a transmission l i n e . The t r a i n of waves w i l l t r a v e l along the l i n e with nearly the v e l o c i t y of l i g h t and any desired phase l a g can be obtained by choosing the proper length of l i n e . I t i s r e a l l y the e l e c t r i c a l analogue of the v a r i a b l e l i g h t path method. T h e o r e t i c a l l y any angle from 0 to 180° may be obtained, but i n p r a c t i c e the u s e f u l impedance range of the ff s e c t i o n 9 l i m i t s the angle to 30 or 40°. A further phase s h i f t may he r e a l i z e d "by detuning the second c i r c u i t on e i t h e r side of the resonant p o i n t . The coupling must then be strengthened to maintain the o r i g i n a l voltage across the discharge tube. This voltage was measured with a e l e c t r o s t a t i c voltmeter. Combina-tio n s which would produce the desired s h i f t , as measured with the phasemeter were found by t r i a l . The phasemeter measures the voltage across the condenser G ( i n s e r i e s with the discharge tube),which w i l l be i n phase quadrature with the .current through the tube. This voltage i s then compared with the voltage across the Kerr c e l l . The cor-r e c t operating voltage f o r the Kerr c e l l i s found by employing steady l i g h t and varying the tap P u n t i l 6 becomes equal to 45° f o r equal i n t e n s i t i e s of the two images produced by the c a l c i t e double image prism. This must be done f o r each frequency of the o s c i l l a t o r used. The voltage i s then read with an e l e c t r o -s t a t i c voltmeter and l a t e r readjusted to t h i s value when l i g h t of varying i n t e n s i t y i s used. Wo f u r t h e r progress has been made with t h i s method as the p u r i f i e d nltrobenzine i s not as yet a v a i l a b l e . The photo-e l e c t r i c method which was mentioned i n the i n t r o d u c t i o n was next attempted and w i l l be described i n d e t a i l . MEASUREMENT OF MEAN LIVES BY THE PHOTOELECTRIC METHOD Theory of the method. This method has been r e s t r i c t e d f o r the present to those s p e c t r a l lines.which can be i s o l a t e d by means of f i l t e r s ^ as s u f f i s i e n t i n t e n s i t y has not as yet been obtained by the use of a spectroscope. A discharge tube, operated by a high f r e -quency p o t e n t i a l i s used as the source of l i g h t which i s exam-ined f o r both modulation and phase*by a p h o t o e l e c t r i c c e l l . A t h e o r e t i c a l expression connecting these q u a n t i t i e s w i l l now be derived. I f at any moment there are n atoms i n a p a r t i c u l a r s t a t e , the r a t e . a t which n decreases owing to r a d i a t i o n i s proportion-a l to n and i f there were n0 atoms i n t h i s state at time t=o the number at time t i s -kt h = nb-e. ; the average l i f e i s then T=M. K I f t h i s state i s also being produced at a rate f(pt)} then da. _ Kn + fptyt p u t t i n g pt = & and ~k = ^ = f(d)xs a fu n c t i o n of the current and i s periodic, with a frequency twice that of the o s c i l l a t o r . An assumption i s now made with regard to fie) , the v a l i d -i t y of which w i l l be examined l a t e r ; namely, that the rate of e x c i t a t i o n can be represented by a f u n c t i o n . w h i c h i s sym-m e t r i c a l about i t s maximum. With t h i s a s s u m p t i o n ^ can be represented quite general-l y as a Fourier s e r i e s of the type ' • * £&) = % -f- <L a c-oo m e (z} z I t has "been shown "by G r i f f i t h s ' that the e f f e c t of the t h i r d and succeeding terms can he neglected so that the expression . takes the form -f(6) - <*-~bc<n>6 The d i f f e r e n t i a l equation then becomes de p p The complementary function w i l l vanish f o r t l a r g e , l e a v i n g as the p a r t i c u l a r i n t e g r a l ft) lip P •H'-hl To f i n d the value of 9 f o r n a maximum or minimum l e t . dn - 0 i n (v) . This leads to the r e l a t i o n do / ta-n 6 = 4- - p~r • 9 i s therefore the phase s h i f t and i s equal to Zy (the s h i f t given by G r i f f i t h s ) , sinee /(&) has a frequency twice that of the o s c i l l a t o r . Equation (f) gives the number of atoms i n the state at any moment. The l i g h t emitted w i l l be p r o p o r t i o n a l to Kn^-kpn-, the rate at which atoms leave the s t a t e . The l i g h t curve i s therefore given by the expression i p n - a. - + -k<^>e) To f i n d the values of -hpn f o r -kpn. a maximum or minimum, l e t • £k. - o i n (3) de This gives the r e l a t i o n -Apn •= a- btun& For dn- = 0, JCCCK e - t h e r e / a r e cos 0- Jk— f o r the l i g h t curve to be a minimum and aaoo = - -A f o r the l i g h t curve to be a maximum. The peaks and v a l l e y s of the l i g h t curve are therefore given by the expression .The forms of the l i g h t and e x c i t a t i o n curves are shown i n f i g . 4 and i t i s evident that the amplitude of the a.c. f l u c t u a t i o n s are i n the r a t i o / to -1—^ i f the d.c. components are drawn to the same s c a l e . I f the form of the e x c i t a t i o n curve i s known, the mean l i f e of the state can therefore be determined by measuring the d.c. and a.e. components of the l i g h t curve. For discharge tubes 1 mm. i n diameter G r i f f i t h s makes the assumption that the e x c i t a t i o n curve i s i n phase with the current curve. This should also mean that a= b as shown i n f i g . io f o r the f o l l o w i n g reasons. The rate of e x c i t a t i o n should depend on the rate at which electrons i n a c e r t a i n energy range disappear. These e l e c t r o n are being produced at a rate where f,(&) represents the current f u n c t i o n . The electrons therefore have a rrmean l i f e " and the d i f f e r e n t i a l equation would be ^ + l<>h = • '^he e x c i t a t i o n curve should therefore l a g behind the current curve by an angle ~teLn~'j- - tan~'pT' and the:,asc. components should be «' " i n the r a t i o / to _^=—_when the d.c, components are drawn to the same sc a l e . I f the angle of l a g i s assumed to be zero,-:-.the e l e c t r o n mean l i f e should be p r a c t i c a l l y zero,and the e x c i t a t i o n curve should have the form shown i n f i g . io where <x=&. Experimental Proceedure. An attempt was made to measure the meanlife of the 73s, state i n mercury, since a s u i t a b l e f i l t e r f o r the green l i n e To face page 12. • Curve'. fot~ Cetron CBI-y. VilTajjv Applied- to The Cell Fly. t3. A~7 pA d%\ i—-tl+l'I'H'I'H-AAA* . to r. — H f— l«l'H 'sTieteT. _ J Fitf. IH. 11 was a v a i l a b l e . The discharge tube was constructed as i n f i g . / / . A few drops of mercury were put i n and the a i r thoroughly pump-ed out. I t was connected across the o s c i l l a t o r tank c i r c u i t as shown (fig.//) and the discharge s t a r t e d by gently warming the tube. An attempt was f i r s t made to measure the angle of l a g with the arrangement shown i n f i g . / 2 . The phase angle between the discharge tube and the ph o t o - c e l l was varied by detuning the c o i l I and varying the coupling to maintain the voltage f a i r l y constant. This adjustment was not very c r i t i c a l as i s evident from the curve shown f o r the Cetron vacuum ph o t o - c e l l shown i n f i g . 7 3 . The current through the c e l l should be great-est when the voltage on the c e l l i s i n phase with the l i g h t var--i a t i o n s . The r e s u l t s were so e r r a t i c that no attempt was made to. draw conclusions from them. The reason seemed to be the un-stable operation of the discharge tube which seemed to vary somewhat i n i n t e n s i t y during the course of a reading. The method of measuring the r e l a t i v e amplitudes of the a.c. and d.c. components of the l i g h t curve was therefore t r i e d as the readings could be taken i n a much shorter time. The ap-paratus i s shown i n f i g . / ? . I t i s a d i r e c t coupled vacuum tube voltmeter which i s designed to measure both average and peak values. The f i r s t tube impresses an amplified s i g n a l on the g r i d of the second tube which i s biased to c u t - o f f . The photo-c e l l i s connected so that l i g h t on the c e l l w i l l decrease the pla t e current i n the f i r s t tube thus causing the plate current of the second tube to increase. The amount by which the.cathode To face page 13. 1 i # V * .I/I Tube Vj *f pkasetx.ete.f-_ [ Pea,K Voltage";. 3-H 3-2 2-7 2-7 H-H y - 3 " 2-6, 2-2 ; 2-7 7-2 /• P- /•J 13 voltage of the second tube must be increased to again reduce the plate current to zero i s a measure of the peak value of the l i g h t 1 s i g n a l * With the l i g h t cut o f f , the cathode voltage of the second tube i s decreased u n t i l the plate current equals the value i t attained with the l i g h t on. The amount by which the cathode voltage i s decreased i s a measure of the average value of the l i g h t s i g n a l . A number of readings were taken, some of which are shown i n table 1. While somewhat e r r a t i c , the readings seem to i n d i -cate that the l i g h t had l i t t l e modulation. This was rather hard to believe as the frequency used was 600 kc. and former measure-7 ments on the mercury green l i n e i n d i c a t e d a mean l i f e of about -7 to seconds. A method of roughly checking t h i s r e s u l t was t r i e d and i s shown i n fIg./5". The same vacuum tube voltmeter was used with the .second tube biased as a normal amplifier,and the plate m i l -liammeter replaced by a r e s i s t o r and coupling condenser which was connected to one tube In the phasemeter. The amplified a.c. voltage caused by the l i g h t , would be heterodyned by the o s c i b . l a t o r i n the phasemeter, and the beat note heard i n the phones. The strength of t h i s beat note should be an i n d i c a t i o n of the de-gree of modulation of the l i g h t . The beat note was very f a i n t , and often e n t i r e l y absent, confirming the r e s u l t already obtained. Other unsuspected e f f e c t s were also noticed. Continuous rushing and sputtering noises were heard i n d i c a t i n g that there other f l u c t u a t i o n s which were of the same order of magnitude as the desired e f f e c t . For c e r t a i n c r i t i c a l operating conditions which could not be maintained fo r more than a few moments, these noises almost disappeared lea v i n g a f a i n t beat note due to the high frequency f l u c t u a -t i o n . A commercial neon sign was examined i n the same way and was found to produce s i m i l a r rushing sounds and no i n d i c a t i o n of a high frequency modulation. This r e s u l t agreed with G r i f f i t h s ' statement that the l i g h t from a large diameter tube has very l i t t l e modulation. A helium and neon discharge i n c a p i l l a r y tubing , with external electrodes, gave a much strong-er i n d i c a t i o n of modulation with an absence of noise. Accordingly several mercury discharge tubes, made of c a p i l l a r y tubing with external electrodes were t r i e d . The noise was considerably reducedj (due,.no doubt, to the use of external electrodes) and the modulation s l i g h t l y increased. But i t was s t i l l . too, • weak to be detected with the vacuum tube voltmeter. However the helium and neon c a p i l l a r y tube showed about ZOfo mod-u l a t i o n , i . e . the a.c. component was about 20f> of the d;c. com-ponent. The beat note from t h i s tube was sometimes rather harsh i n d i c a t i n g a 60 cycle modulation of the t r a n s m i t t e r . Changing the power input u s u a l l y corrected t h i s however. Conclus-jdn^'. These i n v e s t i g a t i o n s seem to i n d i c a t e that i n the type o of discharge tubes used the e l e c t r o n or ion "mean l i f e " i n mercury i s f a i r l y long. The f l u c t u a t i o n s would then be small and would l a g the current f l u c t u a t i o n s by a considerable angle, provided that the theory which has been outlined i s c o r r e c t . G r i f f i t h s seems to i n f e r that the angle Is quite small, even f o r l i g h t w i t h very l i t t l e modulation. ••Unfortunately the d i f -f i c u l t i e s r e f e r r e d to prevented any measurement which would decide the issu e . The audible f l u c t u a t i o n s detected by the l a s t method would not be detected by any of the -other methods described i n t h i s and consequently, would introduce an error which would tend to decrease the measured value of the mean l i f e of the s t a t e . Another f l u c t u a t i o n Which would have the same e f f e c t i s the possible, modulation of the o s c i l l a t o r by the 60 cycles a,c. The harsh note, so produced also makes phase measurements very d i f f i c u l t . Obviously the power supply should be w e l l f i l t e r e d . In conclusion I would l i k e to thank Dr. G-.M.Shrum f o r suggesting the problem and f o r h i s help and ad«iree during i t s progress.. Thanks are also due Mr. E n g l i s h f o r the construction of the vacuum s t i l l and Mr. R.F.Christy f o r assistance i n the t h e o r e t i c a l development. / 6 BIBIilOGRAPHY 1. ^ J.H.E. G r i f f i t h s , Proe. Roy. Soc., A 143, 588 (1934) Proc. Roy. S o c , A 147, 547 (1934) 2. G-.M. Yollcoff, Thesis, TJ.B.C. 3. E.F. Kingsbury, Rev. S c i . I n s t r . , 1, 22 (1930) J.W. Beams, Rev. S c i . I n s t r . , 1, 780 (1930) Rev. Mod. Phys., 4, 133 t (1932) F.G. Dunnington, Phys. Rev., 38, 1506 (1931) 4. H.J. White, Rev. S c i . I n s t r . , 6, 22 t (1935) 5. R.R. law, Rev. S c i . I n s t r . , 4, 537, (1933) 6. A.R. Frey, Phys. Rev., 49, 305 (1936) 7. R.H. Randall, Phys. Rev., 35, 1161 (1930) / 7 J?HOTOE;EECTRIC RELAYS A Thyratron controlled relay. The conventional form of photoelectric relay , i n which a photo-cell is used i n the gri d c i r c u i t of a thyratron, i s shown i n f i g . 1. While quite satisfactory for small changes i n intensity when a strong source i s available, i t i s rather unreliable when the source i s weak. In the method outlined below the s e n s i t i v i t y i s increased about 50 times and allows r e l i a b l e operation on a very weak source. Another desirable feature i s the simplicity of design and the a v a i l a b i l i t y of the component parts. With the exception of the E G 17 thyratron, the appara-tus (as shown i n f i g . 2.) was b u i l t from standard radio parts. It w i l l be seen that the c i r c u i t , of f i g . 1. has been modified by the addition of a type 75 tube, chosen because i t s ampli-f i c a t i o n factor of 100 i s the highest of any triode on the market. The transformer shown i s an ordinary 100 watt power transformer with an 800 volt center-tapped secondary and the correct filament windings for the 75 and F & 17 thyratron. R, is a 5,000 ohm;.;Voltage divider with two adjustable taps and Rz i s a 5,000 ohm potentiometer or volume control. R3 i s a one watt res i s t o r with any value from 1 to 5 megohms or more, de-pending on the intensity of the l i g h t used. R^  should have enough resistance to l i m i t the current i n the 75 to i t s normal value of .4 milliamperes which i t should handle without heat-ing. In the present case the applied potential i s about 400 Vhoto - . r , cell V FG-17 Relai-J I—(»j—WW] ULQJUUUIJUIJL^ J 8 v o l t s , which means that R^ should, he 1 megohm or greater, and. pr e f e r a b l y of 2 watt s i z e or l a r g e r . * When the f u l l p late current of .4 milliamperes i s flow-ing i n the 75, the grid, of the thyratron i s at i t s maximum negative p o t e n t i a l , and must be more than 6 v o l t s negative with respect to i t s filament i n order to stop the a r c . Since the plate r e s i s t a n c e of the 75 i s 90,000 ohms, the p o t e n t i a l developed across i t w i l l be of the order of 40 v o l t s . There-fore the cathode of the 75 must never be l e s s than about 50 v o l t s negative with respect to the thyratron filament. Re-ference t o the c i r c u i t diagram, ( f i g . 2.), w i l l show how t h i s has been accomplished. The potentiometer has been connected across a p o r t i o n of the voltage d i v i d e r so that there i s 50 v o l t s between the points B and C. The photo-cells used were the gas f i l l e d type designed to operate at a maximum of 90 v o l t s . To f u l f i l l t h i s c o n d i tion the tap A was adjusted t i l l there was 63 v o l t s r.m.s., or 90 v o l t s peak, across the potentiometer. The r e l a y i n the plate c i r c u i t of the thyratron con-s i s t s of a mercury contactor, which i s t i l t e d by an e l e c t r o -magnet. To minimize the a.c. chatter i n the r e l a y the arma-ture was designed to swing past the pole pieces without touchy ing them. The operation can be dismissed i n a few words. The p o t e n t i a l of the g r i d of the 75 w i l l depend ofi the r e l a t i v e values of the r e s i s t a n c e R 3 and the r e s i s t a n c e of the photo-c e l l . As the r e s i s t a n c e of the p h o t o - c e l l Is v a r i e d from 11 some low value to i n f i n i t y , the g r i d w i l l assume a l l i n t e r -mediate p o t e n t i a l s between A and B. The p o t e n t i a l of the cath-ode can he v a r i e d at w i l l so that plate current w i l l s t a r t t o sn t flow f o r any desired l i g h t i n t e n s i t y . In e x a c t l y the same way the p o t e n t i a l of the g r i d of the thyratron w i l l depend on the r e l a t i v e values of Rv and the pla t e r e s i s t a n c e of the 75 tube. The arc w i l l s t a r t when the plate r e s i s t a n c e of the 75 increases to such a value that the g r i d of the thyratron becomes l e s s than 6 v o l t s negative with respect to i t s fil a m e n t , and w i l l go out when the g r i d poten-t i a l swings back past t h i s value. Adjustments The arc i n the thyratron can be turned o f f or on with i n c r e a s i n g l i g h t by interchanging the p o s i t i o n of the photo-c e l l and the resistance H 3, the anode of the ph o t o - c e l l of course being always connected to the p o s i t i v e side of the c i r -c u i t . I t wras sometimes found necessary to ehange to value of . R v and the s e t t i n g s of the taps A and B f o r d i f f e r e n t l i g h t i n t e n s i t i e s , as a s l i g h t phase s h i f t between g r i d and plate voltage u s u a l l y appeared when the l i g h t was v a r i e d . A com-b i n a t i o n which would p r a c t i c a l l y eliminate t h i s s h i f t at the desired l i g h t i n t e n s i t y could u s u a l l y be found by t r i a l . Wo attempt was made to c a l c u l a t e the values necessary to remove t h i s phase s h i f t as i t appeared to be p a r t l y due to f i e l d s set up between component parts of the apparatus, as w e l l as to unequal loading of the two halves of the transformer. A-nother phase s h i f t was observed a f t e r the arc had s t a r t e d , Zo due again, no doubt to the unequal loading of the two halves of the transformer. It caused the arc to start and stop at s l i g h t l y different points. The i n s t a b i l i t y due to this effect was however of the same order of magnitude as the i n s t a b i l i t y due to l i n e fluctuations and tube heating. A few precautions regarding the operation of the photo-c e l l are worth noting. Yery intense l i g h t , or an unduly high voltage to be avoided as the excessive ionization may damage the c e l l . Twisted leads to the photo-cell, or any other fac-tor that introduces a similar stray capacity w i l l reduce the s e n s i t i v i t y and may introduce a phase s h i f t . Under certain conditions however, this type of phase s h i f t may be used to cancel an existing phase s h i f t . Several of the above sets have been b u i l t , d i f f e r i n g s l i g h t l y i n c i r c u i t constants, and have been used for various purposes, such as sorting colored bal l s and counting people both' with infra-red and u l t r a - v i o l e t l i g h t . An electronic relay The relay to be described uses ordinary triodes, yet pos-sesses the trigger action of the thyratron relay. It w i l l be described by referring to the battery operated set-up shown in f i g . 3. The tubes used are the type 75 and 59, the l a t t e r being a power pentode, operated with the screen and suppressor grids t i e d to the plate. The operation i s as follows: Suppose that the grid of the 75 is made positive, caus-r ing the f u l l plate current to flow. Then, since the plate re-sistance of the 75 i s about 90,000 ohms, the voltage w i l l d i -To face page 21. Zl vide i n the f o l l o w i n g r a t i o : approximately 55 v o l t s across the 1 megohm r e s i s t o r A, and 11 v o l t s from plate to filament. Ac-t u a l l y the drop across A w i l l be more than 55 v o l t s due to the dra i n through Bt.and C. The p o t e n t i a l across B and C i s now &?•§-+ 11,or approximately 78 v o l t s which w i l l cause the g r i d of the 59 tube to be at a p o t e n t i a l of -15 v o l t s with respect to i t s cathode. This i s below the c u t - o f f bias f o r a plate p o t e n t i a l of 67-|- v o l t s , and so no plate current w i l l flow. Now suppose that the g r i d of the 75 i s gradually made more negative. The plate r e s i s t a n c e w i l l increase and conse-quently the plate to filament voltage w i l l r i s e * causing the g r i d of the 59 to become l e s s negative. Eventually the c u t -o f f bias of the 59 i s reached and plate current s t a r t s to flow. A voltage drop immediately appears across the 2,000cOhm r e s i s -tance D, causing the cathode of both tubes to become more posi -t i v e . This w i l l tend to decrease s t i l l f u r t h e r the current through the 75 and cause the g r i d of the 59 to. become more posi-t i v e . The a c t i o n , once s t a r t e d w i l l therfore b u i l d up u n t i l the f u l l current flows i n the 59 and the current i n the 75 i s e n t i r e l y cut o f f . The r e l a y can now be reeset by opening the plate c i r c u i t of the 59 i n the same, way as a thyratron must be r e - s e t . The above p r i n c i p l e was next applied to an a.c. operated p h o t o e l e c t r i c r e l a y i n which the r e - s e t t i n g a c t i o n takes place 60 times a second. The diagram i s shown i n f i g . 4. i n which a power transformer replaces the b a t t e r i e s , a r e l a y replaces the .c r e s i s t a n c e D, and a p h o t o - c e l l and resis t a n c e c o n t r o l the g r i d To face page 22. 2 2 offthe 75. The c i r c u i t performed e x a c t l y the same as the thy-ra t r o n c i r c u i t p r e v i o u s l y described. Some s l i g h t i n s t a b i l i t y was observed just at the t r i g -ger point but the same thing was also noticed with some thyra-t r o n designs. Since the t r i g g e r a c t i o n depends on the r e l a t i v values of the c i r c u i t components i t i s believed that a math-ematical treatment would i n d i c a t e the values f o r most r e l i a b l e operation and probably remove the s l i g h t observed i n s t a b i l i t y . In conclusion the w r i t e r would l i k e to thank Dr. Shrum f o r h i s suggestion of the work and h i s c r i t i c i s m s during i t s progress. The w r i t e r i s also indebted to Mr. W. E n g l i s h , Mr. W. Barss and Mr.. W. Eraser f o r t h e i r assistance i n the con-s t r u c t i o n of the r e l a y s . 2 3 METHODS OP CONTROLLING THE PRESSURE IN A GAS X-RAY TUBE Yarious leak valves have been described' f o r c o n t r o l l i n g , the pressure i n a Shearer type X-ray tube. Most of these r e -quire readjustment, p e r i o d i c a l l y during the time of the expo-" sure, making i t necessary f o r the operator to be constantly on hand to watch the apparatus. However, i f a tube i s i n constant use and has been operating f o r a few hours, conditions may be-come so steady that one s e t t i n g of the valve w i l l hold the pres-sure constant during the whole exposure, l a s t i n g perhaps 5 or 6 hours. Even so, i t i s u s u a l l y necessary to check the pressure from time to time. . To r e l i e v e the operator of t h i s task, automatic, controls were devised, a l l of them, i n some manner, using the f a c t that the current through the tube was diminished by decreasing pres-sure. Some methods used a solenoid i n s e r i e s with the primary of the high voltage transformer i n which an i r o n plunger, s e a l -ed i n t o the evacuated system, causes a column of mercury to open or close a small tube leading to the pump. However, such a device w i l l allow the a i r to be pumped out i n t e r m i t t e n t l y , r e s u l t i n g i n rather e r r a t i c operation of the tube. Aregulator, described by Kersten^ 3 overcomes t h i s objection. A v a r i a b l e leak valve i s geared to a small, r e v e r s i b l e d.c. motor. A small mirror i s f i x e d to the needle of the m i l l i a -meter i n s e r i e s with the X-ray tube, and r e f l e c t s a beam of l i g h t to e i t h e r of two photronic c e l l s . These c e l l s i n turn operate r e l a y s which s t a r t the motor i n the proper d i r e c t i o n To face page 24. to c orrect the pressure. The methods to "be described are s i m i l a r i n some ways to the one used by Kersten. By means of a reduction gear (made from an old alarm clock) a small r e v e r s i b l e d.e. motor i s used to turn a pinchcoek, which squeezes a piece of small bore vacuum tubing. This rubber tubing acts as the v a r i a b l e leak, and i s connected i n the system as shown i n f i g . 1. This type of leak proved to be f a r more r e l i a b l e than the needle valve previously used. Two methods of c o n t r o l l i n g the motor have been developed which s a t i s f y the demands made by the problem. These are as f o l l o w s : The tube can be made to run at any desired pressure by adjusting the pumping rate or leak valve to a c e r t a i n value. With, t h i s adjustment there i s a c e r t a i n range of pressure i n which the operation of the tube i s f a i r l y s t a b l e . Small bursts of gas,- l i b e r a t e d from parts of the tube, w i l l cause f l u c t u a -t i o n s i n t h i s range which are of no consequence. However, should the pressure gradually increase u n t i l i t exceeds the l i m i t of t h i s range, an unstable c o n d i t i o n i s reached. The increased pressure causes an increase i n current which i n turn heats the tube and causes a fu r t h e r l i b e r a t i o n of gas. Again should the pressure become too low, the reverse e f f e c t w i l l come in t o play and the discharge w i l l eventually stop. The c o n t r o l should therefore s t a r t the motor, i n the proper d i r e c t i o n , somewhere near eithe r l i m i t of the stable pressure range. I t Is also very necessary that the motor should stop at the same point at which i t s t a r t s , otherwise the v a r i a t i o n i n pressure w i l l be over compensated and o s c i l l a t i o n s from one l i m i t to the other w i l l be set up. The High Voltage Supply. As both methods operate i n the primary of the transformer, a d e s c r i p t i o n of the high voltage supply w i l l next be given. I t c o n s i s t s of a commercial X-ray machine made by the V i c t o r E l e c t r i c Go. of Chicago, the transformer g i v i n g 100,000 v o l t s maximum, when operated on a 220 v o l t l i n e . A v a r i a b l e , r e s i s t o r i n s e r i e s w i t h the primary regulates the voltage applied to the X-ray tube. The mechanical r e c t i f i e r was not used as i t was found that i t improved very l i t t l e the r e c t i f i c a t i o n a l -ready provided by the Shearer tube. The connections are shown i n f i g . 2. The F i r s t Method., This c o n s i s t s of the e l e c t r o n i c device shown i n f i g . 3. The voltmeter i s connected across the primary of the transformer. Two adjustable metal stops, insulated from the meter case, are introduced through the sides and are connected to the grids of two ordinary type 45 tubes. The needle i s connected as shown to the negative side of a 400 v o l t d.c, power supply. The r e l a y s i n the plate c i r c u i t s s t a r t or reverse the motor. The operation i s as f o l l o w s : The stops are adjusted u n t i l the needle l i e s midway be-tween them when the X-ray tube i s running i n the middle of the stable range. The high resistances r, r a , bias the g r i d s p o s i -t i v e l y , so that plate current flows and the contacts c( c zare held open. Nov; should the pressure i n the X-ray..tube decrease, Z 6 the voltage across the primary w i l l increase, and the needle w i l l touch the upper stop and charge I t negatively. Good con-ta c t i s not necessary since a p o t e n t i a l of 400 v o l t s i s used. The plate current i s immediately cut o f f , causing contact ef to close which i n turn s t a r t s the motor. The motor slowly opens the pinchcoek allowing more gas to enter the X-ray tube. When the pressure has increased s u f f i c i e n t l y , the voltage w i l l drop and the needle w i l l f a l l "back, eausing plate current to flow again. The above c i r c u i t Is not r e s t r i c t e d to t h i s p a r t i c u l a r high voltage system. An ammeter i n seri e s with the primary would be just as s a t i s f a c t o r y . In f a c t an extremely s e n s i t i v e r e l a y f o r small currents can be made by rep l a c i n g the voltmeter by a s e n s i t i v e galvanometer. The choke c o i l s and condensers shown are necessary to by pass the s t a t i c voltages generated i n the transformer primary. I n s u f f i c i e n t by-passing, or f i l t e r i n g , w i l l r e s u l t i n u n r e l i a b l e operation, since stray s t a t i c voltages w i l l reach the grids of the 45 tubes. The condensers used v/ere of the 1 mfd. telephone type and the chokes consisted of 400 turns of wire on a one h a l f inch, wooden rod. Yfith t h i s f i l t e r the operation was f a i r l y s a t i s f a c t o r y . An occasional voltage surge caused f a u l t y oper-a t i o n of the r e l a y , but i t was only momentary, and was immed-i a t e l y s e l f corrected. The Second Method. The apparatus used i n t h i s method i s shown' i n f i g . 4. and consists of a bundle of sof t i r o n wires i n a glass tube, which To face page 27. Z 7 i s suspended i n the centre of two c o i l s (3,000 turns of no. 26 enamel w i r e ) . The two c o i l s are connected as shown across the primary and s e r i e s r e s i s t a n c e R r e s p e c t i v e l y , and therefore the currents i n them w i l l "be pro p o r t i o n a l to the voltages a-eross the primary and the s e r i e s resistance R. By varying the r e s i s t a n c e s r .r the currents i n the two c o i l s can be made equal f o r any voltage across the primary, which of course i s determined by the pressure i n the X-ray tube. The i r o n rod w i l l then l i e midway between the two c o i l s ahdthe contacts, as shown i n the diagram, w i l l both be open. Now i f the pressure i n the X-ray tube i s diminished, the voltage across the prim-ary increases-while the voltage across R decreases. Consequen-t l y the current i n one- c o i l increases and the current i n the other decreases* The carbon Tamps accentuate t h i s e f f e c t since t h e i r r e s i s t a n c e s decrease with increasing current. The i r o n bar w i l l then be drawn toward the c o i l with the heavier current, and i t i s apparent from the diagram that t h i s w i l l cause one of the contacts to dip in t o the pool of mercury and s t a r t the motor, which then opens the leak'valve a. l i t t l e wider and r a i s e s the pressure. The vane ahown i n the diagram was introduced to damp the pendulum-like v i b r a t i o n s of the rod and i t s suspension. Result The apparatus described In the second method has been t e s t -ed and found to be quite r e l i a b l e . A number of photographs of : the IQ l i n e s of copper were made, using exposures ranging from one h a l f to three hours, and on two occasions the apparatus was le l e f t running f o r s i x hours, during which time the pressure was face page 28 2? e n t i r e l y automatically c o n t r o l l e d . A disadvantage i n the above method i s the f a c t that the mercury soon oxidizes'and must be replaced. A mercury contactor i n vacuum has been made but not tested as yet. I t i s shown i n f i g . 5. Since there i s no change i n p r i n c i p l e and the v e r t i c a l operation of the i r o n core has been tested and found s a t i s f a c -tory with open a i r contacts, i t i s believed that the apparatus shown i n f i g . 5. w i l l be a very simple.,'-yet r e l i a b l e c o n t r o l . In conclusion the w r i t e r wishes to thank Dr. Hennings f o r h i s advice and Mr. Eraser f o r assistance i n construction of the apparatus. Bibliography COH. Kersten, R, S. I . 6, 175 (1935). (OR. W. G-, Wyckoff and JY B, l a g s d i n , R.. S. I . 7, 35 (1936). (OJohn E. Dorn and George G l o c k l e r , R, S. I. 7, 319 (1936) f*)3?iN. White, J , S c i , I n s t . 7, 99 (1930). (5)H.; Kersten, R.S. I. 5, 5 (1934). 

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