A capacitive signal pickoff device

A CAPACITIVE SIGNAL PICK(F DEVICE by WARREN W. BENJAMIN

JUN

11 19S58

e R AeR

Submitted in Partial Fulfillment of the Requirements f or the Degree of Bachelor of Science

at the

MASSACHUSETTS INSTITUTE2? &TECHNOLOGY June 1958 Signature of Author Certified by Accepted by uepartment of Economics. A 1958 Thesis Supervisor

ACKf OWIEDGENS1T

The author wishes to express his gratitude to Dr. E. B. Dane and Mr. Frederick Hopewell for suggesting the subject of this thesis and providing laboratory space and materials. Appreciation is directed to Professor Robert K. Mueller for his guidance as thesis advisor and to Mr. _{Francis Merenda for his technical assistance.} Also appreciation is expressed to the personnel of the

Instrmenta-tion Laboratory of the Massachusetts Institute of Technology for countless favors over a long period of time.

.............

...

A oli

... ....

A CAPACITIVE SIGNAL

PiC 10FF DEVICE

by

WAEEN W. BfJAMIN

Submit ted to the Department of 3Economics on May 26th, 1958, in partial fulfillment of the requirements for the degree of

Bachelor of Science.

ABSTRACT

An investigation has been made of a gaseous discharge trans-ducer for possible use as a Capacitance Signal Pickoff device that will convert the angular displacement of a two-degree-of-freedom

gyro wheel, about its combined input-output axes, to- a de voltage that is proportional to these angular changes. The possible appli-cation and limitations of the device are examined, and experimental evidence is presented to show the feasibility of the device. The sensitivity and accuracy are found to be a function of the ampli-V tude and frequency of the R.F. supply voltage, is the frequency

increases, amplitude variations in the excitation supply voltage become less important while the sensitivity decreases, The device

is designed to measure angular displacement, and it is found to have merit for this purpose,

TABLE CF CONTENTS

Chapter I Introduction

Chapter II Pickoff Considerations -Chap-ter III Preliminary Investigation Chapter IV Final Investigation

Chapter V Results and Conclusions

Appendix A Data from which figures 5and 6

were drawn

Appendix B'Derivation of the Input Impedance for the Difference Amplifier shown in figure 8

Appendix C Data from which figure 8 was drawn Bibliography Figure Figure Figure Figure Figure Figure Figure Figure 1 2 3 4

5

6 7 8 TABLE OF TIGURES Basic Transducer

Characteristics of Basic Transducer Pickoff Configaration

Basic Experimental Circuit

Sensitivity vs Excitation at 250 E Sensitivity vs Excitation at 600 C Differential Amplifier Transducer Characteristics Pate Nmuber 1

5

9 16 20 22 25 27 28 2 2 6 10 11 12 15 17

CHAPTER I INTRODUCTION

If a tube containing two internal electrodes is filled with an inert gas at a low pressure and is then excited by a radio

fre-quency electrical field of sufficient magnitude to cause ioniza-tion of the gas within the tube, a de voltage arises between the two electrodes. This phenomenon was discovered independently by L. Rhode1 in 1932 and by K. S. Lion in 1938. Three masters theses2,3,4 have been presented at M.I.T. on this subject under the supervision of the late Professor K. S. Lion.

The first of these three theses, presented by J. W. Sheets in 1949 examines the effect of varying the position of a movable excitation electrode around a gaseous discharge tube (see fig. 1). This mechanical movement gives rise to a do voltage between the two internal electrodes that varies linearly with the position of

the movable excitation electrode (see fig. 2). Sheets attributed this phenomenon to variation of electron energies in the plasma thereby causing a difference of charge to appear between the two electrodes which in turn gives rise to different potentials on the electrodes.

The second of these theses, presented by G. H. Wayne,3 de-scribes an application of the gaseous discharge transducer to an ultra-micrometer. Wayne presents data which show that the R.F. excitation frequency affects sensitivity and that as the frequency

-4

The third of these theses, presented by W. W. Woods, investi-gates the phenomenon still further. His results contradict pre-viously advanced theories and show that the electron energies in one part of the gaseous discharge do not vary appreciably with respect to those in another part as the position of the discharge tube is varied in the exciting field.

No complete explanation of the gaseous discharge phenomenon

confirmed by quantitative analysis has been put forth, but recent 22

investigations by Chenot and K. S. Lion2 suggest that the phenome-non may be caused by the following mechanism: consider that the

two internal electrodes are probes inserted in the ionized gas. If there is no external conducting path between the electrodes, each electrode will assume the potential of an isolated probe. Now, since the drift velocity of the relectrons is much higher than that

of the ions in the ionized gas, each isolated probe will take on

a charge that is slightly negative with respect to the potential

of the surrounding ionized gas. If the ionized. gas is caused by a do field, a stationary potential difference could arise between the two probes as a function of differing electron and ion tempera-tures surrounding each probe.

If the ionized gas is caused by an ac field, a comlication arises because the internal electrodes are capacitively coupled to the external exciting electrodes -- as shown by Wood.0 The result

is that the internal electrodes are not isolated probes and that their potentials oscillate in time with the frequency of the

excit-ing ac field. With a given external electrode configur~tion, the magnitude and place of the ac potential on each electrode will be

a function of the electric Parameters of the equivalent circuit as seen by each electrode, Thus, a variation of capacitances (along

4

with the inherent nonlinear character of the probes

)

C or C2will produce asymmetrical eurrents from the ionized gas to the electrodes and result in net cha;rges on each electrode which lead to a differ-ence of potential between them. These conditions suggest the

essen-tial reasons that make possible the use of a gaseous discharge tube as capacitance pickoff device.

To detect gyro wheel angular displacement, a pickoff device is used which consists of a gaseous discharge tube and capacitors 0 and 02. The asymmetry caused by varying these capacitors pro. duces an output dc voltage that appears between the two internal electrodes.

CH4PTR II

PIOKOFF ONSIDERATIONS

The aim of this chapter is to discuss the use of a set of

capacitors, used in conjunction with a gaseous discharge tube, as a signal pickoff transducer. An important requirement of a pickoff

-- the term "pickoff" being used f or that part of a gyroscopic

de-vice whose function it is to transform the output of the dede-vice into a suitable signal -- is that it gauge the desired quantity without appreciably affecting the quantity or disturbing the gyroseople

de-5

vice. For the purpose of this thesis, the desired quantity refers to the angular displacement of a two-degree-of-freedom gyro wheel about its combined input-putput axes. The method of angular measure-ment to be used consists of mounting four fixed metal plates of equal area in close proximity to the gyro wheel as shown in fig.

3.

The gyro wheel can be at ground potential via a sliding brush assembly in contact with the outside diameter of the gyro wheel. These four plates, separated by a distance 6 from the gyro wheel, f orm four capacitors all mounted in the plane of one of the input~o

output axes, A similar set of four capacitor plates is mounted in the 1ane of the other input-dutput axis. This is neither

dis-cussed nor shown since it is a similar system and therefore all

that is pertinent to one system applie to the other. It is to be noted that each fixed capacitor plate is connected in parallel to the capacitor plate mounted diagonally opposite to it so as to form a total of two capacitors instead of four. This diagonal parallel arrangement is used so that motion of the gyro wheel perpendicular

o-5-GYRO WHEEL

8

PICK-OFF

CONFIGURATION

-6-C,

C2

FG.3

to the capacitor plates will not produce a change in total capaci-tances Cl and C2. That is, when b is decreased on one side of the

gyro wheel, 6 is increased by the same amount on the other side of 1 x.Area

the wheel; and since C= C will not change value. The same conditions hold true for capatitor 02*

However, when the gyro wheel is displaced 0 degrees, both

6 and 8{ decrease an equal amount with a resulting increase in

magnitude of capacitance Cl .As &l ad {- decrease, 6 and '

in-2 2

crease with. a resulting decrease in magnitude of capacitance C2' Both capacitances Cl and 02 change by the same magnitvAe but in

opposite senses. A similar analysis holds true for angular displace-ments of negative 6 degrees.

For the given gyro wheel configuration, as shown in fig,

3,

R = 1.65 inches; therefore, for an angular displacement of one second arc -- which is equal to 4.848 microinches -- 6 will change eight microinches.

Abt ::'6RAO =

(1.65")C.000004848);Z:

8 microinches

Now, since this movement of eight microinches appears as Ab's in all four capacitance plates, the resulting change in the magnitude in total capacitance is four times what it would be had only one plate been used. If the area of each capacitance plate

is 3.5 cm2 and 8 is equal to .0762 cm, both

Cl

and

C2

will be 8.10

micro-microfarads.

The problem then is to detect changes in capacitance of the order of .005 micro-.microfarads and convert these changes to a do voltage that will be proportional to either a positive or negative

angular displacement of a two-degree-of-freeom gyro wheel about

-7-ak A

its combined input-ontput axese 'This proportional voltage can then be used to correct a gyrO system to a null position via a servo

system.

To determine the feasibility of the capacitance pickoff de-vice for possible application in the above specified system, it is necessary to determine the following important properties:

1. Sensitivity - For given capacitance changes, what is

the corresponding change in the da voltage output of the capaci-tance pickoff device2

2. Null drift -- If the two capacitances in question (Cl

and 02) remain of equal value, does the de voltage output of the transducer remain at sero3

3. TUncertainty -- Are there any fluctuations in the de voltage output with a fixed capacitance configarationt

CAPTR III

PRELIMINARY INTESTIGAT ION

In order to illustre the basip priciples of operation of the capacitance pickoff device, a simplified circuit, shown in fig. 4, was constructed. This circuit utilizes a readily available

NE-2 neon bulb ,as the basic gaseous discharge transducer. The

radio frequency seurce used is a Hewlett-Paelard Model 4000 signal generator, the 6AQ5 tube is employed as a conventional power ampli-tier, and T1 is an auto transformer utilizing a core from a

tele-vision flyback transformer. C and the two 0B2 gas tubes make up a series resonant BI.O circuit. The purpose of the two QB2

gas tubas is to insure that the gaseous discharge tube excitation voltage will have a relatively stable amplitude. When the two 032 tubes ionize, together withfl1 they form the resistance part of the

RWL circuit, These tubes, when ionized, act similarly to a resist-ance except that the voltage drop across the two tubes remains

rela-tively constant as the current passing through them increases or 6

decreases. This is true, however, only if the voltage appearing across the tubes is high etough to maintain ionization.

By varying the value of R, the amplitude of the excitation voltage appearing across the NFr2 neon bulb can be varied. The graph shown in fig.

5

presents data taken for three different lwt-age excitation magnitudes at a frequency of 250 XC. The do voltage output of the transducer is plotted against changes in Cy while C2

is held constant. The graph shown in f ig. 6 presents the same kind of data except that the frequency is 6oo XE. Higher frequencies

-9-M

.... -77 13

EX

PER IMENTAL

T

6AQ5

270

A

220K

D.C. OUT

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SENSITIVITY vs EXCITATION VOLTS at 6-00 K.C(

L o

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za

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too

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15

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V~ r r rrw'

were npt tried because a variable high frequency source was not available. During these tests, the exeitation electrode was set at an arbitrary position; and therefore, when the two capacitors were of equal magnitude, a dc voltage output was produced. In any final system, the exeitation electrode would be set at a position correspondin& to X = 0, as shown in fig. 1, so that when

isM equal to C the de voltage output would be zero.₂

As shown in the graphs of fig.

5

and fig. 6, the device is quite linear from -'2 volts to +.5 volts; and the sensitivity is a function of both excitation amplitude and frequency.

When the excitation voltage, at 250 30, is equal to

500

volts peek-to-peak (as measured on a Tektronix Oscilloscope), the sensitivity is

5

millivolts for a change in capacitance of .005 micro-mierofarads. Inereasing the frequency reduces the sensi-tivity to 4 millivolts for a change in capacitance of .005 micro-microfarads, but the effect of amplitude variations in the excit-ing electrode voltage becomes less pronounced. This corroborates the findings of Wayne. 3

Increasing the magnitude of _{the exciting electrode voltage} beyond

500

volts peak-to-peak results in greater sensitivity;

how-ever, tube life is considerably shortened.

The output impedance of the gaseous discharge transducer

is on the order of one megohm. Therefore, the device used to measure the dc voltage output must have an input impedance of ten megohms or higher in order to avoid loading effects,

Since it is the difference between the two internal elect-rode voltages that is pertinent, a difference amplifier (as shown

-13-in fig. 7) was constructed whose input impedance is greater than ten megohms (see Appendix 3). The cathode-follower action of the difference amplifier provides an impedance match so that a Sanborn recorder can be used without affecting the trensduber output4 In

order to eliminate the possibility of any temperature variation affecting the difference amplifier, boron carbon resistors were used because of their almost zero temperature coefficient,

T .

-14--470

D.C. INPUT

from

TRANSDUCER

4.7 M

3900

DIFFERENTIAL

AMPLIFIER

Fig. 7

a/5

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D.C.

OUT

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INVESTIGATIC&

In this chapter a commercially available gaseous discharge

transducer is examined and its characteristies are shown graphisally. Then the problem of keeping the excitation supply voltage at a

constant amplitude is examined, and attempts to solve it are pre-sented.

In Chapter III an fE-2 neon bulb was used as the basic gaseous discharge tube. A specially made transducer, manufactured

by the Decker Aviation Corporation, was on hand and so a plot of its characteristics was also made and is presented in fig. 8. The linearity displayed was f ound to be qui.te good and the sensitivity obtained was 10 millivolts f or a change in capacitance of .005 micro-microfarads.

A check on the uncertainty error and the null drift revealed

results that were not within allowable limits. Using a Sanborn Recorder (Type 150) driven by the difference amplifier shown in fig. 7, the uncertainty was k3 millivolts and the null drift was 8 millivolts in eight hours. A check of excitation voltage ampli-tude showed variations and dritt which were correlated with actual uncertainty and null drif t,

Now, from the graphs shown in figs.

5

and 6 of Ohapter III, it becomes apparent that variations in the amplitude of the radio freqency excitation voltage cause wide variations in the de out-put voltage of the transducer. In order that the uncertainty of

Fig.8 TRANSDUCER

CHARACTERISTICS

....- -- -- -....

1

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as low as possible, a constant amplitude radio frequency source is necessary. Not only must the amplitude of the radio frequency source be constant over a long period of time, but also it must be unaffeeted by transient changes. Very little has been written about constant amplitude sine waves _{that is applicable to this}

particular problem,

Since

in this case the amplitude of the ex-citation supply voltage is 380 volts pp. and the frequency is

250 EC. _{(The Decker Aviation Corporation recommends these values}

for long tube life and high sensitivity.) The high voltage re-quirement demands the use of a step-up transformer and thus in-herent thermal sensitivity is introduced. The 250 EC requirement megns that the transformers and coils must be wound since no com-mercially manufactured transformers and/or coils are available at

this frequency.

A self-contained oscillator amplifier was designed utilizing

a feedback system to correct for amplitude variations. Using con-ventional techniques, the output voltage is sampled and fed back

to control the bias of the oscillation. As the amplitude of the output increases, the feedback loop causes the bias to increase and _{thus reduces the amplitude of oscillation.}

Both the oscillator coil and the output transformer were wound using Litz wire and polystyrene coil forms. Since the oscillator anmplifier must have long term stability, one percent boron carbon resistors were used in order to reduce the effects of temperature,

The oscillator amplifier was not c ompleted at the time of this writing. However, it was felt that the problem of building

-18-frEI~a.hftMIEI~ImI*m.*m~*. I I

AN Mill

"T"

Ovin

ANN.'!

IRA. a constant amplitude oscillatoi amplifier was beyond the scope

of this thesis -- this constituting an independent problem within

the realms of engineering probability.

Preliminary results using the oscillator amplifier show that the uncertainty can be reduced to Al millivolt, but that the long run null drift remained on the order of 8 millivolts for an

eight-hour test.

-19-.. . ... .

BRSULTS AD CONCLUSIONS

The capacitance signal piekoff device was designed to measure the angular displacement of a gyro wheel by converting this displace-ment to a change in capacitance. This capacitance change is then measwed

using

a gaseous discharge tube. The results obtained in

this thesis show that the gaseous discharge tube will convert ehanges in capacitance to a do voltage proportional to either a positive or negative angular displacement.

It has been shown both by previous work9 and by this thesis that, in order for the gaseous discharge transducer to have a low uncertainty error, the radio frequency supply voltage must have a constant amplitude which at 250 EC requires an accuracy of one part in -one thousand using the Decker Aviation transducer.

The sensitivity at 250 ED was found to be 10 millivolts for a change of .005 micro-nicrofarads _{which corresponds t-o one second}

of arc of angular displacement.

The problems of signal output uncertainty and null drift have not been resolved; but, as- pointed out in Chapter IT, the design of a constant amplitude amplifier is within engineering possibilities. _{The null drift poses a more serious problem in} that the drift is probably due to temperature caused component changes. More experimentation would be necessary in order to de-termine the exact source of the drift.

Current signal _{generators used at the Instrumentation} Labora-tory produce 100 millivolts for an angular displacement of 1

milli-,~* '9sr!~5~ a macu I

radian. Using the capacitanee pickoff transducer, the sensitivity is

2000

millivolts for 1 milliradian of angular displacement. While this is a significant improvement, it must be remembered that the data collected for this thesis was obtained under static conditions,

the capacitors being fixed. The effects of a rotating wheel might well introduce periodic errors not observable under static conditions.

These considerations probably require more attention; however, any frequency components derived from wheel asymmetry could be filtered out.

In any further research, an investigation should be made of the characteristics of the transducer using frequencies in the range of five megacycles. While the sensitivity will be decreased,

it might prove worth while to sacrifice sensitivity for less null drift and uncertainty.

Since the most serious drawback of the capacitance pickoff is the null drift, it is suggested that Zener diodes (when they become available) be used to clip the output of the radio frequency

excitation voltage. This clipped sine wave could then be reshaped using a conventional filter. The effects of temperature drift might

well be lessened with this approach.

In sumation, the capacitance pickoff was designed to measure angular displacement of a gyro wheel and convert this angular dis-placement to a dc voltage that would be proportional to either posi-tive or negaposi-tive displacement. The pickoff device was found to accomplish this purpose with the limitation of excessive null drift which in the copinion of the author is a solvable engineering problem,

-21-singe marnr 3W

~ a - - ~u- ~ *

-APEEIH A

Data from which figa

5

a4 6 were drawn.

Fig,

5

3xeitation YoltaO 01 2 T ut Trequency

pjf d pyfd volts 400 volts pp. 18,0 27.5 2.0 250 E 400 volts pp. 15.4 27.5 1.5 250 X 400 volts pp. 13.8 27.5 1.0 250 W 400 volts pp. 11.9 27.5 .5 250 C 400 volts pp. 10.7 27.5 0 250 E 400 volts pp. 9.8 27.5 - .5 250 10 400 volts pp. 9-0 27.5 -1.0 250 EI 400 volts pp. 8,4 27.5 -1.5 250 XE 400 volts pp. 7.5 27.5 -2.0 250 EM 400 volts pp. 6.8 27.5 -2.5 250 E 450 volte pp. 16.4 27.5 2.0 250 E 450 volts pp. -139 27.5 1.5 250 m 450 volts pp. 12.1 27.5 1.0 250 EM 450 volts pp. 10.7 27.5 .5 250 EC 450 volts pp. 9.8 27.5 0 250 E 450 volts pp. 9.2 .27.5 - .5 250 XE 450 volts pp.

8.5

27.5 -1.0 250 E 450 volts pp. 7-8 27.5 -1.5 250 XC 450 volts pp.- 7.3 27.5 -2.0 250 EC 450 volts pp. 6.6 27.5 -2.5 250 EC

Fig. 5 (cent.)

Exeitation Toitage ci C2 e2eout

pf pid ft volts 500 vots pp. 14.0 27.5 2.0 250 C 500 volts pp. 12.2 27.5 1.5 250 X 500 volts op. 11.0 27.5 1.0 250 E 500 volts pp. 9.5 27.5 .5 250 X 500 volts pp. 9-0 27.5 0 250 E0 500 volts PP. 8.6 27.5 - .5 250 E 500 volts pp. 8o 27.5 -1.0 250 X 500 volts pp. 7.4 27.5 -1,5 250 X 500 volts pp. 7.0 21.5 -2.0 250 c 500 volts pP. 6.4 27.5 -2.5 250 XC Fig.

6

400 volts pp. 19.5 27.5 2.0 6oo E-400 volts pp. 16.2 27.5 1.5 600 m 400 volts pp. 14.3 27.5 1.0 6oo E 400 volts pp. 11.9 27.5 .5 600 Em 400 volts pp.' 10.5 27.5 0 600 E 400 volts pp.

9.7

27.5 - .5 6oo Ec 400 volts pp. 8.8 27.5 -1.0 600 MX 400 volts pp. 8.2 27.5 -1.5 6oom 400 volt s pp.

7.5

27.5 -2.0

6oo

EC 400 volts pp. 6.8 27.5

-2.5

600

Ec

Fig. 6 (cont.) Tout volts Excitation Voltage 01 450 volts pp 16.9 450.volts PP. 14,4 450 volts pp 12.8 450 volts pp. 10-9 450 volts pp. 10-0 450 volts pp. 9-2 450 volts pp. 8,5 450 volts p 7-9 450 volts pp. 7.2 450 volts pp- 6.6 500 volts PP. 14.8

500

volts PP- 12.8

500

volts PP. 11.5 500 volts pp. 10.2 500 volts PP. 9-4

500

volts pp. 8-500 voltsr PP. 8.2 500 volts PP. 7-7 500 volts PP- 7.0 500 volts PP. 6.5 Frequency 02 pad 27.5 27.5 27.5

27.5

27.5 27.5

27.5

27.5

27.5 27.5 27.5 27.5

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6oo

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600 W

600 xc

600

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600

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6oo

mc

600

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600 xc

600 E

600 XC

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APPThDI B

Derivation of the Input Impedance, for the Difference Amplifier shown in figare 8.

Using an incremental model shown difference amplif ler is analyzed.

below of one-half of the

where:

r =5.3K p +p = 165 ,RSR.=-470 Re 1=4.5K -R =4.?meg 9 g

se

=i(r+R)+ R(1iP=i(rP + RE+RL)+Ri

and since eg iRg - i₉R

p(R- Rx) =p(rp +RK+ B) + pil - iR- i r₉ o-- aR)- i (r +

T

1

rp

IR -iR, - R i = 0 RL+ RJ + R) = 0 +R R (+1) = 0 ) '' 1p ~r + ₁ + p+1)

Now, substitution of equation 2 into equation 1 gives: in ra in =gl't+ +r + p e in i g+ L₁(pR - Bg) +L(pR - B1) rp + IL + RKp+ 1)

Using the values given in the above figure:

Z g 24 megohms.

AIPFKDUC

Data from which figur 8 was drawn.

Excitation Voltage C2 Vout Yrequency

ppfd ppfd volts 380 volts pp4 11.00 10 2.0 250 EC 380 volts pp. 10.72 10 1.5 250 E 380 volts pp. 10.52 10 1.0 250 EC 380 volts pp. 10.26 10 .5 250 XC 380 volts pp., 10.00 10 0 250 E0 380 volts pp. 9.74 10 .5 250 E 380 volts pp, 9.52 10 -1.0 250 EM 380 volts pp. 9424 10 -1.5 250 EM 380 volts pp. 9.00 10 -2.0 250 E

di7m

B LBLIOGRAPFY

1. Lion, Kurt ,: "Mechanital-]1leetric Transducer"; The Review of Scientific Instrumente, Volume 27, No. 4, pp. 222-225, April 1956,

2o Sheets, John ., 8.M. Physics, M.I.T. 1947: 'An Experimental Study of a Rectifier Effect in High Frequency Discharges".

3,

Wayne, George H,, S. M. Electrical Engineering, M.I.T. 1949; "Investigation of a Glow-Lamp Ultra-Micrometer".

4. Woods, W. William, 5.M. Electrical Engineering, M.I.T. 1951:

"A Gaseous Discharge Mechano-Electric Transducer".

5.

Weems,

William

R.: "An Introduction to the Study of Gyro-scopic Instruments", pp. 7-.

6. Francis, T. J: "Fundamentals of Discharge Tube Circuits";

Methuen's Monographs on Physical Subjects.