A METHOD OF STUDYING THE EFFECT OF TEMPERATUREON PHOTOELECTRIC CURRENTSDimiter Ramadanoff Citation: Review of Scientific Instruments 1, 768 (1930); doi:10.1063/1.1748666 View online: http://dx.doi.org/10.1063/1.1748666 View Table of Contents:http://scitation.aip.org/content/aip/journal/rsi/1/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A novel empirical study of the photoelectric effect in thin gold films Am. J. Phys. 71, 766 (2003); 10.1119/1.1574040 Experimental study of the surface photoelectric effect in cesiated thin silverfilms J. Appl. Phys. 64, 4580 (1988); 10.1063/1.341261 A Sensitive Photoelectric Method for Measuring the Faraday Effect Rev. Sci. Instrum. 24, 23 (1953); 10.1063/1.1770512 A Method for Modulation in Photoelectric Recording Rev. Sci. Instrum. 22, 847 (1951); 10.1063/1.1745790 A Photoelectric Method for Tracing Current Wave Forms Rev. Sci. Instrum. 8, 385 (1937); 10.1063/1.1752193

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A METHOD OF STUDYING THE EFFECT OF TEMPER­A TURE ON PHOTOELECTRIC CURRENTS

By DIMITER RAMADANOFF

[DEPARTMENT OF PHYSICS, CORNELL UNIVERSITY, ITHACA, N. Y. RECEIVED AUG. 20, 1930]

INTRODUCTION

One of the main difficulties which have confronted various investi­gators in studying the variation of photoelectric current with tempera­ture has been to separate the thermionic from the photoelectric current. The thermionic current at higher temperatures (600°C or above) is many times larger than the photoelectric current and masks the whole photoelectric phenomena. Its presence is therefore very un­desirable and many schemes have been used in the past to prevent it from flowing through the galvanometer used for the measurement of the photoelectric current. One of the most common methods is to balance the thermionic current and allow only the photoelectric current to flow through the measuring instrument.l This method is simple and permits the use of a more sensitive galvanometer. The difficulty however with such arrangement is that a slight change in the thermionic current, which may be produced by a change in the temperature of the photoelectric surface or by other causes, will be sufficient to destroy the balance and cause large currents to flow through the galvanometer. With the balancing method it is rather difficult to obtain consistent data, particularly at temperatures around 800D C. After trying various schemes used in the past, the writer has resorted to the use of a transformer coupled amplifier and a cathode ray oscillo­graph. The method has proved to be of value not only for qualitative study but also with a fair degree of accuracy for quantitative measure­ments.

ApPARATUS

The arrangement of the apparatus is shown in Fig. 1. The photo­electric cell used in these experiments had two electrodes, a positive collector and a cathode which consists of a platinum strip coated with barium by a special process. The platinum strip was heated by passing electric current through it. The saturation voltage was rather low (8 to 10 volts) because the electrodes are set 2-3 em apart. The

l Phys. Rev., 34, 1566; 1929.

768

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December, 1930J MEASURING PHOTOELECTRIC CURRENTS 769

photoelectric current was made pulsating by interrupting with a per­forated disk the light which was illuminating the cell. The disk had circular holes, 1.5 inch in diameter, which were punched at equal distances around the periphery. The wave shape of the photoelectric current produced by this arrangement is not sinusoidal. If a sine wave of current is desired, then the holes must have the the shape of the form y =A sin2x the amplitude of which should be equal to the length of the platinum strip. The photoelectric current will be com­posed of two components, a direct current and an alternating current of double frequency. Since direct current cannot flow through the

,--------------------------,

c y

y'

Shield

FIG. 1.

amplifier only the true ac component will be impressed on the oscillo­scope plates, For quantitative results a correction naturally should be made for the filtered dc component. Even then a perfect sine wave will not be attained because different patches of photoelectric material on the platinum strip will not be equally photosensitive. Such difference in sensitivity of course will tend to distort the wave shape of the photo­electric current. A better and perhaps more satisfactory procedure would be to use a diffused light, while the variation of the quantity of light admitted is sinusoidal. Under these conditions the whole photo­electric surface would be illuminated at all times and the above men­tioned patches will have no effect on the wave shape of the current. The frequency of the current depends on the speed of the disk, and the num­ber of holes, and can easily be varied from a few cycles to 1000 cycles per sec. or more by changing the speed of the motor. In taking measure-

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770 DIMITER RAMADAN OFF [R.S.I., 1

ments however it will be more desirable to keep the frequency constant 'at some satisfactory value since most amplifiers do not have a perfectly flat frequency characteristic.

The current from the photoelectric cell is fed to a two-stage trans­former-coupled amplifier. The condenser C blocks any direct current such as the thermionic current and allows only pulsating, i.e. the true photoelectric current,to be amplified. It may be of interest to mention also that the two six megohm resistances on both sides of the condenser C were found to give the largest voltage output at YY'; any further increase in these resistances has but slight effect upon the arri.-

FIG. 2.

plitude of the ac wave. The terminals YY' are connected to the plates of a Bedell-Reich "Oscilloscope,"2 a visual cathode ray oscillograph.

'The cathode spot is projected on the fluorescent screen of the tube and its movements can be clearly observed. The Oscilloscope shows on the screen the true wave shape of the alternating voltage impressed on its plates, since a linear time axis is used. The writer has found this visual oscillograph very convenient for the study of the temperature effect of the photoelectric current. One could at a glance notice on the screen any changes in magnitude or wave shape of the photoelectric current due to temperature variation of the photoelectric surface. Since the vertical deflections of the cathode. spot are proportional to the ac voltage applied on the plates, one could obtain with fair degree of accuracy the relative magnitude of the photoelectric current at various temperatures without the necessity of calibration. However if it is desired to have the actual values of the photoelectric current, then the "Oscilloscope" and amplifier must be calibrated as a unit. A potentiom­eter capable of supplying small ac emfs should be substituted for the photoelectric cell. By reducing the resistance Rl to a few ohms, rather

2 J. of the A.I.E.E., 46,563; 1927.

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December, 1930] MEASURING PHOTOELECTRIC CURRENTS 771

large currents will produce small potential drops, and therefore suitable deflections on the oscilloscope. The alternating currents flowing through the input resistance can be measured with a sensitive vacuum ther­mocouple ammeter and a calibration curve plotted of oscilloscope deflections against current.

An oscillogram showing the amplitude and wave shape of the pulsating photoelectric current as it appeared on the fluorescent screen is shown in Fig. 2. This oscillogram is a photograph taken with a Graflex camera with 1.5 minute exposure. The temperature of the platinum strip coated with Ba was just below visible red. At higher temperatures the amplitude of the current may be 5 to 10 times greater.

CONCLUSIONS

The apparatus here described has some distinct advantages over many methods used in the past to study temperature effect of photo­electric current:

1. Sluggish currents which may flow due to the action of light on the photoelectric surface are easily eliminated by the high speed disk and only true photoelectric currents allowed to flow.

2. The thermionic current, which is very objectionable for satis­factory measurements of the photoelectric current by ordinary methods, is completely filtered out.

3. Any rapid changes in the photoelectric current which may occur at some critical temperature and which cannot be detected by a gal­vanometer, can be easily observed on the cathode ray oscillograph.

4. The amplification can be increased for small photoelectric currents to meet the requirements.

5. The photoelectric cell and amplifier occupy sufficiently small space to permit enclosure in a copper screen shield. Such a shield has been found entirely satisfactory with the two stage amplifier.

A paper containing data which was obtained with the apparatus described herein will soon be submitted for publication in the Physical Review.

The writer is indebted to Prof.E. Merritt for his valuable suggestions and Prof. F. Bedell for providing the "Oscilloscope."

•

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