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Electron attachment and negative ion formation in oxygen Christy, Robert Frederick 1937

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L  G  3  (1  I  ^  •& 1  i  l  ELECTRON ATTACHMENT AND NEGATIVE ION FORMATION IN OXYGEN  Robert Frederick C h r i s t y  A Thesis submitted f o r the Degree of Master of Arts i n the Department of Physics  THE UNIVERSITY OF BRITISH COLUMBIA August, 1937.  TABLE OF CONTENTS I. II.  INTRODUCTION. THEORY,  I I I . APPARATUS. (a) Tube (i) Construction (11}.Operation ( i i i ) Measurement o f M o b i l i t y (b) O s c i l l a t o r (c) Quadrant Electrometer (d) The Vacuum System IV.  CONCLUSION.  BIBLIOGRAPHY.  I L L U S T R A T I O N S  Figure 1  Facing page.  2.  Figure 2  *  "  3.  Figure 3  "  "  3.  Figure 4  "  "  4.  Figure 5  "  rt  Figure 6  "  "  6.  Figure 7  "  "  9•  Figure 8  "  "  9.  6„  ELECTRON ATTACHMENT AND NEGATIVE ION FORMATION IN OXYGEN • .  CHAPTER I INTRODUCTION^ I t was observed "by many workers ^^) that the mobil-  i t i e s of p h o t o e l e c t r i c a l l y generated ions, measured soon after l i b e r a t i o n from a metal plate i n the measuringfield, began to become abnormally great at 100 mm.  pressure i n a i r .  This was ascribed by numerous observers to a breaking up of the negative i o n c l u s t e r by impacts with gas molecules. To Wellisch(3,4,5) ;L due the c r e d i t f o r having investigated, by S  means of a m o d i f i c a t i o n ^ ) of the Rutherford^ ^A.C. 7  method,  the m o b i l i t i e s of negative ions when formed behind a gauze, at low a i r pressures., after they had come through the meshes of the gauze. He found two classes of c a r r i e r s , one of which he showed were normal ions of a m o b i l i t y constant close to the value accepted for normal negative ions, the other of which he asserted consisted of free electrons. The e f f e c t of the gauze and the weak a u x i l i a r y f i e l d back of i t was to delay the p h o t o e l e c t r i c a l l y l i b e r a t e d electrons, or electrons which were freed by radium r a d i a t i o n s , u n t i l they attached to n e u t r a l molecules to form ions. This W e l l i s c h did.not - know. In f a c t he did not compare his r e s u l t s with the r e s u l t s of other workers who had used no gauze. He assumed that an electron, when l i b e r a t e d , must have a c e r t a i n minimum energy for attachment and that I f i t has not t h i s energy i t w i l l  never attach and w i l l remain permanently f r e e . In 1920, L o e b ^ undertook the problem and repeated Wellisch'a experiments. He completely corroborated  the observations of W e l l i s c h . But  he further observed that the r e l a t i v e number of ions and free electrons depended on the pressure and a u x i l i a r y f i e l d  strength  which allowed of only one i n t e r p r e t a t i o n . I t showed that the free electrons were not permanently f r e e . In f a c t , i t showed that i o n formation was contingent on the condition that the electrons spend a s u f f i c i e n t time i n the a u x i l i a r y f i e l d before being studied. I f the pressure and f i e l d strength were such that the time was short, only electrons were obtained; i f the time was long, only ions were obtained. By increasing the a u x i l i a r y f i e l d strength so that the electrons had a very high energy, no increase' i n the number of ions formed was observed by W a h l i n ^ ) ,  Thus the W e l l i s c h theory was found to  be wrong. The experimental work and viewpoint on t h i s problem Oo)  were very much aided by a theory of J . J . Thomson. He a t t empted to explain the abnormal increase of m o b i l i t y of p h o t o e l e c t r i c a l l y l i b e r a t e d ions at low pressures as follows: The u l t r a - v i o l e t l i g h t used l i b e r a t e s electrons. These do not attach to molecules to form negative ions at t h e i r f i r s t impact. I f i t be assumed that the electron requires, on the average, n impacts before i t can attach to form an i o n , where n may be a large number, then i t i s possible to explain the phenomenon. This amounts to assuming that the attachment of an elecrtron i s a chance phenomenon, depending on where i t  Hi. s t r i k e s the molecule or under what energy conditions the impact takes place. For each chemically d i f f e r e n t gas t h i s would be d i f f e r e n t , depending on i t s chemical nature. For s i m p l i c i t y , Thomson assumed n to be a constant, independent of y e l o c i t y , c h a r a c t e r i s t i c of each gas. That t h i s i s not e n t i r e l y correct w i l l be seen l a t e r . As a f i r s t approximation, i t i s , however, s u f f i c i e n t . That electrons do make negative ions i n some gases and not i n others i s an observed f a c t . The term electron a f f i n i t y may then be applied to describe the behaviour of a molecule towards the electron. I t i s then necessary to f i n d more than a q u a l i t a t i v e measure of t h i s property. Two methods are  a v a i l a b l e . The one would be to f i n d out what the i o n i z i n g  p o t e n t i a l of the extra electron of the negative i o n i s . This, of course, could be determined by the frequency of the shortest wavelength of the l i g h t emitted when an electron i s attached to a molecule to form an i o n . This m u l t i p l i e d by the Planck constant would give the energy necessary to remove the electron. Up to the present, investigations  of t h i s nature have  yielded no r e s u l t and one cannot measure electron a f f i n i t y i n t h i s manner. The second procedure would be to get a quantita t i v e measurement of the average number of the impacts required by an electron with a given type of molecule before i t could attach. This quantity i s p r e c i s e l y the quantity defined by J. J . Thomson as n, h i s constant of attachment. .' The f i r s t attempts made to. measure n were due to LoeV°''and W a h l i n ' . The effect of the attachment, and i t s w  iv. consequent change of m o b i l i t y of the c a r r i e r , on the shape of the current-voltage curves obtained with a square- wave-form a l t e r n a t i n g current has been r i g o r o u s l y worked out by Mooney and loeb, I t i s given, together with the attempt at experimental v e r i f i c a t i o n , i n an a r t i c l e by loeb  ) .The paper doesn't give  accurate quantitative agreement because of the v a r i a t i o n i n the attachment constant with e l e c t r o n v e l o c i t y . B a i l e y ( ) , i n 1 4  Townsend's laboratory, measured the attachment constant n, or, better, i t s r e c i p r o c a l , the p r o b a b i l i t y of attachment h, by the analysis of the rate of loss of electrons and ions by l a t e r a l d i f f u s i o n i n a beam of what were i n i t i a l l y electrons moving under a f i e l d i n a i r at low pressures. He observed the values of h or n f o r a i r and found that these depended on the v e l o c i t y , or, b e t t e r , the energy of the electrons, h decreasing and n increasing as the value of the energy increased. Hence n or h i s a function of X/p (X i s f i e l d strength i n volts/cm. and p i s pressure In mm.) which must be included i n the equations of Loeb i n order to evaluate h. The complexity of the equations and the d i f f i c u l t y i n obtaining accurate values.of e l e c t r o n m o b i l i t i e s at higher pressures made an attempt to deduce the v a r i a t i o n of h as a f(X/p) from these data impossible. B a i l e y ^ ) , using the method of measuring the l a t e r a l 1 4  d i f f u s i o n of a current beam going through holes i n a series of p l a t e s , found that n increases as the v e l o c i t y of thermal a g i t a t i o n increases. This means that n i s not a constant of the chemical nature of the gas alone but depends on the average energy of a g i t a t i o n of the electron. This would be expected as  V.  i t seems that as the electron t r a v e l s faster a capture would be more d i f f i c u l t to e f f e c t . This agrees with Wahlin's r e s u l t s , who found that as the f i e l d increased the number of free electrons i s not decreased as W e l l i s c h s theory would make i t . 5  Cravath  t  i n loeb's laboratory, applied a new  method devised by loeb to the measurement of n or h. In t h i s a stream of electrons from a hot filament or other source i s driven through a known distance x of a i r or gas at low pressure by an e l e c t r i c a l f i e l d . At x cm. from the source there i s an e q u i p o t e n t i a l surface having a g r i d of f i n e p a r a l l e l wires separated from each other by 1 or 2 mm. and insulated. A l t e r nate wires are connected to the terminals.of a high-frequency a l t e r n a t i n g supply of p o t e n t i a l of the order of 1 0 cycles per. 6  second. This high-frequency f i e l d served to remove any of the very mobile electrons l e f t i n the beam a f t e r the distance x had been traversed, without removing the slow negative ions. The l a t t e r reached the other electrode creating the f i e l d , and the r a t i o of the current to the g r i d wires to the t o t a l current gave the f r a c t i o n of the electrons which had not attached i n the distance x. Using the data of Townsend and h i s pupils on electron m o b i l i t i e s i n a i r at low pressures, Cravath was able to obtain h or n as a function of the terminal v e l o c i t y or energy of the electron. His r e s u l t s l a y close to those of B a i l e y i n a i r , but i n Qg they extended to far higher values of X/p than did the previous measurements i n a i r . Cravath found that h f e l l to a minimum at about 1 or 2 electron v o l t s energy and started to r i s e again. He found that h appeared to  vi. vary with pressure when larger ranges of pressure were used. He also observed what seemed to be a detachment of electrons from negative ions i n the intense f i e l d s about the g r i d wires when a high a l t e r n a t i n g p o t e n t i a l was used. The energy of detachment was about 0.9 v o l t as estimated from the f i e l d s . The great defect i n Gravath's method l a y i n having to use electron-mobility data f r o n measurements not made on the same gases with which he was working. I t also suffered from d i f f i c u l t i e s involved .in the use of the absolute values of the currents, to the g r i d and plate which required c o r r e c t i o n f o r loss of p o s i t i v e ions to the g r i d wires, the f a i l u r e to capture a l l the electrons, e t c . Bradburyt ) i n 1933 perfected an elaborate and 17  b e a u t i f u l method. He used two grids at distances x  1  and xg from  the source. He used an outgassed a l l glass system (giving greater p u r i t y than previous workers had achieved) and photoelectrons instead of the contaminating hot filament. F i n a l l y he used a method of determining electron m o b i l i t i e s which gave r e s u l t s to f a r higher values of X/p than before and which were taken i n the same chamber on the same gas and nearly concurrently with the attachment data. The method Involved an equation given by J . J . Thomson( ) r e l a t i n g the photoelectric current 18  between p a r a l l e l plate electrodes to the m o b i l i t y of the electrons and t h e i r i n i t i a l energy (which was also determined i n the same tube). The use of the two grids eliminated the errors due to f a i l u r e to capture electrons, arid due to the capture of ions, by g i v i n g s i m i l a r values due to these losses so that the  vii. r e l a t i v e current values for x^ and x  gave the r e a l r a t i o of  2  ions and electrons. The r e s u l t s showed no pressure v a r i a t i o n i n Og "but showed that the curve f o r h as a function of energy f e l l r a p i d l y to a minimum at about 1 v o l t electron energy , and rose again at about 1.62 v o l t s , reaching a maximum and then f a l l i n g again. The r i s e at 1.62 v o l t s was due to the f a c t that at 1.62 v o l t s i n Og the electrons lose energy i n i n e l a s t i c impacts and thus suddenly have an increase i n h as the energy i s decreased by i n e l a s t i c impacts. F i n a l l y the average energy of. the electrons increases again as they pick up energy i n the f i e l d a f t e r the loss of the 1.62 v o l t s and the curve u l t i m a t e l y f a l l s . Bradbury's values agree with the p a r t i a l r e s u l t s of Bailey(3-4) nd of Cravath (-^) where they overlap. a  Bradbury also observed the detachment of electrons at high grid-wire f i e l d s observed by Cravath and at about the same energy. I t i s possible that t h i s may lead to a method of obtaining the e l e c t r o n a f f i n i t i e s of neutral molecules on a . true energy b a s i s . Recently Bloch and Bradbury( ) explained the form15  ationof negative ions by electron capture i n gases, i n which a d i s s o c i a t i o n process does not occur, by a unimoleeular process involving the e x c i t a t i o n of molecular v i b r a t i o n l e v e l s and the subsequent loss of energy by c o l l i s i o n or resonance. In order to obtain a proper order of magnitude to agree with experimental observations they assumed a change of only one v i b r a t i o n a l quantum number. This sets an upper l i m i t on the electron a f f i n i t y . For the case of Og t h i s l i m i t i s 0.17  viii. v o l t s , consistent with other observations. The theory also y i e l d s a dependence of the attachment p r o b a b i l i t y h on the average electronic energy. This dependence i s i n agreement with "Bradbury's observations i n the energy range .2 to 1.2 v o l t s ( i . e . u n t i l the electrons s t a r t making i n e l a s t i c impacts). I t , however, shows a maximum for h below the range of Bradbury's observations at .11 v o l t . I t i s the purpose of t h i s work to look for t h i s (17)  maximum i n h, using, e s s e n t i a l l y , Bradbury's two g r i d method and a recent method of h i s , t o be described l a t e r , for measuring the mobility.  1. CHAPTER I I THEORY l e t c = random v e l o c i t y of electrons, W= d r i f t v e l o c i t y of electrons, 1= number of electrons' per c c , A= mean free path, of electrons, Immobility of electrons i n cm. /sec  .Jvolt./em.,  1= free e l e c t r o n i c current passing through the gas at some point x, X= f i e l d strength i n volts/cm.  is  C-  Then the number of c o l l i s i o n s of an electron per sec. _c_  \ and the number of c o l l i s i o n s i n a cm. of d r i f t i s W A »  Thus the chance of an electron attaching to a molecule i n j  h c  d r i f t i n g a distance dx i s  &I = - I  then  r  \J\ -^fx  cLx  W - KX  but by d e f i n i t i o n  Integrating between two points (the two grids) x^ and xg, a distance x apart, gives /  Iu  hex  .  - - (i)  where I ^ and Ig are the free e l e c t r o n i c currents at x-j_ and Xg r e s p e c t i v e l y . I t has been customary to employ the m o b i l i t y equation to evaluate c and A f o r equation (l) .  r\ ~  3(r0  ^yTc  as k i s i n the u n i t s cm./see .^volt/cm. and not erne/see . ^ s t . v l t / Then s u b s t i t u t i n g i n ( l ) we get , _  3 av /m k^X ,  I,  ^  2. CHAPTER I I I APPARATUS (a) Tube. (i) Construction. It was f i r s t intended that the tube should be as nearly as possible the same as that which B r a d b u r y ^ ) used. 7  However it, was thought that the revolving g r i d and guard r i n g combinations would, make the enclosing bulb too large for the available glass blowing f a c i l i t i e s so I t was decided to construct the apparatus with two f i x e d grids (see f i g . 1). An analy s i s of the operation of the tube showed that t h i s construction would give, s a t i s f a c t o r y operation. To further s i m p l i f y the construction i t was decided to put the guard rings P, G-, H, I , J , ( f i g . 1) on the outside of the tube containing the r e s t of the apparatus. Also, as some d i f f i c u l t y was experienced i n making a T tube to admit u l t r a v i o l e t l i g h t , the zinc plate as a source of photoelectrons was replaced by a filament B. Even with the hot filament, however , the system could s t i l l be outgassed quite s a t i s f a c t o r i l y . I t was found that sealing the connecting wires through the side of the pyrex tubing strained the glass too much, giving i t a tendency to crack. To avoid t h i s the tube was designed to have seven connecting wires leading through the base and one (to the quadrant electrometer)  through the top.  For reasons to be mentioned l a t e r i n the discussion of the o s c i l l a t o r the grids should have a small capacity. Also  F i g . 3.  for s i m p l i c i t y the number of lead In wires through the base should be kept as small as possible. For these reasons the metal supports for the g r i d wires were not connected, l i k e a guard r i n g , to the p o t e n t i a l d i v i d e r but were connected to the o s c i l l a t o r just as the wires themselves. The g r i d design appears i n f i g . 3. The grids and other i n t e r n a l parts are made 1  to f i t i n a piece of 2-inch pyrex tubing. The grids are made a f a i r l y close f i t to ensure correct placing when the tube i s f i n i s h e d . One set of wires A i s mounted on a piece of t h i n sheet brass B, the other set C i s mounted on another piece of brass D, B and 3) being fastened together to a piece of sheet mica E . In order that both grids s h a l l be most e f f i c i e n t under the same conditions and f o r other reasons to be given l a t e r they are made i d e n t i c a l . Each g r i d A, B, i s supported by i t s two lead i n wires , ( f i g . . 2;)tc-The. filament C i s an oxide coated filament taken from a radio tube and Is supported by i t s two connecting wires. The plate D i s used as one end of the uniform f i e l d as, without i t , the filament would probably d i s t o r t the f i e l d . The connecting and supporting wires can be spot welded or brazed to the tungsten sealing wire: i n the f i r s t case they should be made of i r o n as copper doesn't weld to tungsten. F i g . 2 i s made as a u n i t , wires being welded or ' brazed to the outside ends of the tungsten before sealing i n glass. The f i r s t g r i d B should be near enough the filament C so that not too many electrons are attached before reaching i t  as i t i s desirable to have most of the attachment between the g r i d s , where the currents are measured. Also B should be f a r enough away from the filament to ensure that the electrons * make enough c o l l i s i o n s to acquire t h e i r constant average energy before passing B. For t h i s reason CB was chosen about 1 cm. and BA about 2 cm.; the other distances are not c r i t i c a l at a l l and were chosen to be convenient. A l l the tungsten wires are then sealed through the end of a glass tube about an inch i n diameter. This tube i s then sealed to the drawn down end of the 2-inch pyrex tubing. The c o l l e c t i n g plate i s made to f i t c l o s e l y inside the tube, attached by a connecting wire to a tungsten s e a l , and i s sealed to the drawn down end of another piece of the 2-inch tubing. The open ends of the tubing are sealed together so that the c o l l e c t i n g plate i s about 1 cm. from the g r i d D ( f i g . 1 ) . This l a s t seal i s much simpler i f the two surfaces f i t p e r f e c t l y , having been formed o r i g i n a l l y by cracking the tubing with a hot wire. A tube i s provided as i n f i g . 1 to connect to the evacuating system, ( i i ) Operation. Referring to f i g . 4, a filament B serves as a source of electrons. The guard rings F, G, H, I , J, K, together with the plates A and E, maintain a uniform variable f i e l d i n which the electrons from B t r a v e l i n t h e i r passage through the gas to the c o l l e c t i n g electrode 'E. Some of the electrons are captured by neutral molecules en route from B to E. C and D are grids such as that f i r s t used by Cravath; ^' Between alternate wires of each of these grids a high frequency  a l t e r n a t i n g f i e l d may be produced by means of the o s c i l l a t i n g c i r c u i t L (see l a t e r ) . Such a grid serves to separate electrons from a mixed current of negative ions and electrons. Electrons "reaching the g r i d as they move through the space between A and E are swept out to the g r i d wires during one-half cycle of the a l t e r n a t i n g f i e l d , but negative ions, whose m o b i l i t y may be 1000 times l e s s , do not have time to reach a g r i d wire i n a single h a l f cycle, and the continual r e v e r s a l of the g r i d f i e l d permits the d r i v i n g f i e l d to carry them beyond the influence of the g r i d . The filament and the grids are maintained at a mean p o t e n t i a l equal to that of the guard r i n g i n which they are placed and consequently do not d i s t o r t the main f i e l d f a r on either side. In actual practice conditions are not as i d e a l as described above since some electrons may escape and some Ions may be caught by the g r i d s . I t Is to avoid t h i s d i f f i c u l t y  that two  grids C and D are employed, which enable the e l e c t r o n i c compo s i t i o n of the current stream to be studied at C and D. The actual measurements are c a r r i e d out by measuring the current I  Q  of ions and electrons when the high frequency p o t e n t i a l i s  applied at neither g r i d . The high frequency p o t e n t i a l i s then applied at C and the current i - ^ of ions at C i s measured as the remaining current. The high frequency i s next cut o f f at C and applied at D, and the remaining current i g of ions at D i s measured. A c a r e f u l symbolic analysis showed that.using two grids thus eliminated a l l the serious errors of the single g r i d and  to t  5.  Fig.  The  + /2 0  potentiAI Dtvtd.tr-  Oscillator  -  Fig.  6.  6. that the e l e c t r o n i c current r a t i o i n the theory on page / XS  M  *  *  —T  m  To determine h i t remains to measure, the electron mobility. ( i i i ) Measurement of M o b i l i t y . The method Bradbury(16) describes i s a v a i l a b l e , but a better one and more convenient i s that given by Bradbury and Nielsen(20) and extended(21) to oxygen (see below). The p r i n c i p l e i s the a p p l i c a t i o n of the g r i d electron ion f i l t e r s to the e l e c t r i c a l shutter method of measuring i o n m o b i l i t i e s f i r s t used by Van de G r a a f f ^ ) . Both grids are 2 2  connected to the o s c i l l a t i n g c i r c u i t (q.v.)  at the same time  as i n f i g . 5. The voltage on the grids i s l e s s than that necessary to sweep the electrons e n t i r e l y out of the f i e l d to the g r i d wires and instead allows them to pass when the a l t e r nating g r i d voltage Is low or zero and cuts them o f f when i t i s high. Thus a s e r i e s of pulses of electrons pass through C to D. Now D has an a l t e r n a t i n g f i e l d i n phase with that of C and acts i n the same way. Thus the average current passing 33 w i l l depend on the phase of the h. f. voltage on D when the pulse of electrons reaches i t . I f the o s c i l l a t o r frequency be now adjusted to make the c o l l e c t e d current a maximum, then the maximumof the electron pulse must reach 33 when the o s c i l l a t o r y voltage i s zero, and w i l l be a maximum i f the maximum of the electron pulse reaches D when the o s c i l l a t i n g voltage i s zero. Now as t h i s maximum of the pulse l e f t C  when  the o s c i l l a t i n g  voltage was zero, the electrons must have passed from C to D  i n one h a l f cycle or some multiple thereof. Using the lowest frequency at which i t i s possible to get a maximum i n the c o l l e c t o r current ensures that the time occupied between C and D i s one h a l f a cycle. As the distance between the grids and the o s c i l l a t o r frequency aie known, the d r i f t v e l o c i t y and m o b i l i t y of the electrons can be immediately calculated. This method i s not quite as s a t i s f a c t o r y i n gases which attach electrons (such as oxygen) as i n gases which do :,i not attach, but by working at somewhat reduced pressures oxygen, etc., can also be investigated. Under these circumstances there appears a negative ion background current upon which i s superimposed the electron current to be measured. When the -4  p r o b a b i l i t y of attachment i s of the order of 10  -6  to 10 ,  the background current can be reduced s u f f i c i e n t l y to permit accurate measurement of the electron current maxima by r e s t r i c t i n g the pressure to values between 2 and 10 mm. When the measuring tube was constructed, i t was i n tended to either use t h i s method f o r measuring m o b i l i t i e s or to use the data of Brose^ )and of Nielsen and B r a d b u r y ^ ) . 22  21  The o s c i l l a t o r has not yet been adapted to these measurements, (b) O s c i l l a t o r . The o s c i l l a t o r i s a tuned plate-tuned g r i d type of conventional design. By changing the tank c o i l s i t o s c i l l a t e s at 10 to 100 metres. A wavemeter was made and i t and the o s c i l l a t o r c a l i b r a t e d against a standard wavemeter. As the o s c i l l a t o r was of low power i t was desirable to have the l a r g e s t possible voltage induced i n the o s c i l l a t -  8. ing c i r c u i t I so that a large v a r i a t i o n  i n voltage would be  a v a i l a b l e f o r operation of the g r i d s . To achieve t h i s the g r i d capacity was used wi th the pick up c o i l as a resonating c i r c u i t , ' the number of turns of the c o i l was arranged to make the c i r c u i t o s c i l l a t e at nearly the desired frequency and then the o s c i l l a t o r frequency was adjusted for resonance which could be detected by the meter i n the plate c i r c u i t of the o s c i l l a t o r or by a neon lamp near the grid leads. A more convenient method of detecting when the o s c i l l a t o r was tuned at the correct frequency would be through the use of a high frequency milliammeter i n the o s c i l l a t i n g g r i d c i r c u i t . To get maximum voltage i n the g r i d c i r c u i t the coupling c o i l E should have as many turns as possible f o r resonance, thus the grid should have no unnecessary capacity. In order that both grids s h a l l operate most e f f e c t i v e l y with the same coupling c o i l and at the same frequency they thus should be i d e n t i c a l . I t was found that the tuned plate-tuned g r i d  oscill-  ator was not as convenient as one with a single frequency cont r o l as the Hartley. The tuning of the o s c i l l a t o r to the coupled c i r c u i t L necessitated simultaneous adjusting of grid and plate capacities to vary the frequency and to keep the o s c i l l a t o r i t s e l f tuned, and made the resonance point i n the coupled c i r c u i t d i f f i c u l t to detect as the o s c i l l a t o r frequency could not thus be quickly swept up and down. For measuring the m o b i l i t y the required o s c i l l a t o r frequency i s much l e s s : from 800 to 4000 meters wavelength. For t h i s purpose I t was intended to use a more powerful Hartley  To  face  page  9.  e I hi via Fig.  7.  to tot-  Itctt-ometef  o s c i l l a t o r which was a v a i l a b l e . I t would make frequency v a r i a t ion easier and the g r i d potentials wouldn't be so c r i t i c a l to control. (c) Quadrant Electrometer. The electrometer used i s of the Dolezalek type with a s e n s i t i v i t y of 2000 mm. per v o l t . I f necessary t h i s could be increased by the use of a f i n e r suspension. I t was set, together with an earthing key ( f i g . 7) with mercury contacts, i n a metal shielding box as described by  Makower  and Geiger (24)  t  T h e  wire leading from the c o l l e c t i n g plate i n the tube was also shielded. To adjust the electrometer, the quadrants, the needle and the ease are earthed. The needle i s then adjusted to take a symmetrical p o s i t i o n with respect to the quadrants. The needle i s then connected to a source of p o t e n t i a l (100 or 200 v o l t s ) above earth through a large (about a megohm) resistance f o r protection i n case of a short c i r c u i t . Charging u s u a l l y d i s places the needle from i t s zero p o s i t i o n to which i t i s returned by adjusting the l e v e l i n g screws and then discharged. This, i s repeated several times u n t i l there i s no appreciable change i n the p o s i t i o n of the needle on charging. As the s e n s i t i v i t y of the instrument increases with needle p o t e n t i a l to a c e r t a i n point and then decreases with further increase, the needle was given the p o t e n t i a l of maximum s e n s i t i v i t y at about 150 v o l t s and the electrometer was c a l i b rated for d e f l e c t i o n against quadrant voltage. It was thought that the current might be measured by  10. a d i r e c t or a n u l l method using some electronic device (some of the high a m p l i f i c a t i o n tubes now on the market) rather than, by the time rate method of the quadrant electrometer. However the high frequency p o t e n t i a l i n the grids would be picked up e a s i l y on the c o l l e c t o r plate and i t would be quite a problem to eliminate i t i n the amplifying system, (d) The vacuum system. The vacuum system consisted of two parts ( i n function): the system for evacuating the tube and the system f o r p u r i f y i n g and  admitting pure Oxygen to the  tube. See f i g . 8.  The vacuum pumps were a double stage mercury vapour d i f f u s i o n pump backed by a Genco hyvac rotary o i l pump. A Mcleod guage measured pressures from 20 mm. to 10~^ mm.  and  gave an estimate at 10"^ mm. A mercury manometer measured pressures from 10 mm. to atmospheric pressure. A U tube trap which could, be Immersed i n l i q u i d a i r was provided to prevent mercury vapour from the pump and guages from d i f f u s i n g to the tube and contaminating the gas. An e l e c t r i c heating oven f o r the tube which could be moved In and out of place, was wound on asbestos over a t h i n metal tube and w e l l insulated from the outside with asbestos. A second c o i l was provided inside the asbestos covering to act as a resistance thermometer. The tube was baked out and outgassed at about 125° G. as the solder softens at higher temperatures. The filament was run w e l l above operating temperature during the outgassing procedure to avoid contamination l a t e r from i t . The oxitgassing  11. was,carried on to 10~ mm. s  The Oxygen p u r i f i c a t i o n system consisted of an i n l e t to take Oxygen from a commercial tank and an'escape valve E "(an 80 cm. tube dipping into a d i s h of mercury) which could be cut o f f i f pressures above atmospheric were required. The Oxygen then passed over PgOg i n a drying tube F and could be condensed i n t o a bulb by a c o i l G- immersed In l i q u i d a i r . I f necessary f o r condensation the pressure could be raised somewhat above atmospheric. When the bulb was f u l l of l i q u i d Oxygen (or as much as was required) i t was cut o f f from the tank and was ,: f r a c t i o n a l l y d i s t i l l e d . The f i r s t t h i r d was pumped o f f v i a I , then the middle t h i r d was stored under about two atmospheres pressure, measured by a mercury pressure guage L (with a closed end and a i r i n s i d e ) , i n J , the l a s t t h i r d was again pumped o f f . H was another escape valve which could be cut i n i n case of trouble. The l i t e r bulb J also contained some PgOg to ensure dryness. When the tube was to be f i l l e d with the pure Oxygen the l i q u i d a i r was put around the c o i l s It and M and the gas was slowly admitted from J . Passing through the l i q u i d a i r traps K and M" completed the p u r i f y i n g process.  JL2  ©  CHAPTER IY CONCLUSION Owing  to considerable technical d i f f a c u l t y i n con-  s t r u c t i o n of the tube which i s the important part of the apparatus, the work was not completed. Apart from a few minor d e t a i l s the apparatus has a l l been constructed, but no operating runs hare been made, I would l i k e to thank Dr. G. M. Shrum of the Physics department f o r suggesting the problem and f o r suggestions i n the  course of the work, Also I would l i k e to acknowledge the  assistance of Mr. ¥. R. Eraser, the mechanic i n the Physics department, i n construction of the g r i d s .  BIBLIOGRAPHY (1) L.B. Loeb, "Kinetic Theory of Gases", McGraw-Hill,613 (1935) (2) A.F. Kovarik, Phys. Rev., 30, 415 (1910) (3) E.M. Wellisch, Am. Jour. Sc., 39, 583 (1915) (4) E.M. Wellisch, Am. Jour, Sc., 44, 1 (1917) (5) E.M. Wellisch, Proc. Roy. S o c , A134, 427 (1931) (6) J . Franc k and Pohl, Verh. der Deuts. Phys. Ges.,IX, 69,(1907) (7) Rutherford, Proc. Camb. P h i l . S o c , IX, 401 (1898) (8) L.B. Loeb, Phys. Rev., 17, 89 (1921) (9) H.B. Wahlin, Phys... Rev., 19, 173 (1922) (10)  J . J . Thomson, P h i l . Mag., 30, 321 (1915)  (11)  0. Oldenberg, Phys. Rev., 43, 534 (1933)  (12)  L.B. Loeb, P h i l . Mag., 43, 230 (1922)  (13)  L.B. Loeb, Jour. Frank. I n s t . , 197, 45 (1924)  (14) V.A. B a i l e y , P h i l . Mag., 50, 825 (1925) (15) A.M. Cravath, Phys. Rev., 33, 605 (1929) (16)  J.S. Towns end and 'i'izzard, P r o c Roy, S o c , A88, 336 (1913) J.S.  Townsend, Jour. Frank. Inst., 200, 563 (1925)  (17)  N.E. Bradbury, Phys. Rev., 44, 883 (1933)  (18)  J . J . Thomson, "Conduction of E l e c t r i c i t y through Gases", U n i v e r s i t y Press, Cambridge, 466 (1928).  (19)  F. Bloch and N.E. Bradbury, Phys. Rev., 48, 689 (1935)  (20)  I.E. Bradbury and R.A. Nielsen, Phys, Rev., 49, 388 (1936)  (21)  R.A, Nielsen and N.E. Bradbury, Phys. Rev., 51, 69 (1937)  (22) Tan de Graaf, P h i l . Mag., 6, 210 (1929) (23) H.L. Brose, P h i l . Mag., 50, 536 (1925) (24) Makower and Geiger, " P r a c t i c a l Measurements i n Radioa c t i v i t y " , Longmans, Green and Co., London, 9 (1912)  

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