Photoelectric cell

The invention: The first devices to make practical use of the photoelectric

effect, photoelectric cells were of decisive importance in

the electron theory of metals.

The people behind the invention:

Julius Elster (1854-1920), a German experimental physicist

Hans Friedrich Geitel (1855-1923), a German physicist

Wilhelm Hallwachs (1859-1922), a German physicist

Early Photoelectric Cells

The photoelectric effect was known to science in the early

nineteenth century when the French physicist Alexandre-Edmond

Becquerel wrote of it in connection with his work on glass-enclosed

primary batteries. He discovered that the voltage of his batteries increased

with intensified illumination and that green light produced

the highest voltage. Since Becquerel researched batteries exclusively,

however, the liquid-type photocell was not discovered until

1929, when the Wein and Arcturus cells were introduced commercially.

These cells were miniature voltaic cells arranged so that light

falling on one side of the front plate generated a considerable

amount of electrical energy. The cells had short lives, unfortunately;

when subjected to cold, the electrolyte froze, and when subjected to

heat, the gas generated would expand and explode the cells.

What came to be known as the photoelectric cell, a device connecting

light and electricity, had its beginnings in the 1880’s. At

that time, scientists noticed that a negatively charged metal plate

lost its charge much more quickly in the light (especially ultraviolet

light) than in the dark. Several years later, researchers demonstrated

that this phenomenon was not an “ionization” effect because

of the air’s increased conductivity, since the phenomenon

took place in a vacuum but did not take place if the plate were positively

charged. Instead, the phenomenon had to be attributed to

the light that excited the electrons of the metal and caused them to

fly off: Aneutral plate even acquired a slight positive charge under the influence of strong light. Study of this effect not only contributed

evidence to an electronic theory of matter—and, as a result of

some brilliant mathematical work by the physicist Albert Einstein,

later increased knowledge of the nature of radiant energy—but

also further linked the studies of light and electricity. It even explained

certain chemical phenomena, such as the process of photography.

It is important to note that all the experimental work on

photoelectricity accomplished prior to the work of Julius Elster

and Hans Friedrich Geitel was carried out before the existence of

the electron was known.

Explaining Photoelectric Emission

After the English physicist Sir Joseph John Thomson’s discovery

of the electron in 1897, investigators soon realized that the photoelectric

effect was caused by the emission of electrons under the influence

of radiation. The fundamental theory of photoelectric emission

was put forward by Einstein in 1905 on the basis of the German

physicist Max Planck’s quantum theory (1900). Thus, it was not surprising

that light was found to have an electronic effect. Since it was

known that the longer radio waves could shake electrons into resonant

oscillations and the shorter X rays could detach electrons from

the atoms of gases, the intermediate waves of visual light would

have been expected to have some effect upon electrons—such as detaching

them from metal plates and therefore setting up a difference

of potential. The photoelectric cell, developed by Elster and Geitel

in 1904, was a practical device that made use of this effect.

In 1888,Wilhelm Hallwachs observed that an electrically charged

zinc electrode loses its charge when exposed to ultraviolet radiation

if the charge is negative, but is able to retain a positive charge under

the same conditions. The following year, Elster and Geitel discovered

a photoelectric effect caused by visible light; however, they

used the alkali metals potassium and sodium for their experiments

instead of zinc.

The Elster-Geitel photocell (a vacuum emission cell, as opposed to

a gas-filled cell) consisted of an evacuated glass bulb containing two

electrodes. The cathode consisted of a thin film of a rare, chemically

active metal (such as potassium) that lost its electrons fairly readily; the anode was simply a wire sealed in to complete the circuit. This anode

was maintained at a positive potential in order to collect the negative

charges released by light from the cathode. The Elster-Geitel

photocell resembled two other types of vacuum tubes in existence at

the time: the cathode-ray tube, in which the cathode emitted electrons

under the influence of a high potential, and the thermionic

valve (a valve that permits the passage of current in one direction only), in which it emitted electrons under the influence of heat. Like

both of these vacuum tubes, the photoelectric cell could be classified

as an “electronic” device.

The new cell, then, emitted electrons when stimulated by light, and

at a rate proportional to the intensity of the light. Hence, a current

could be obtained from the cell. Yet Elster and Geitel found that their

photoelectric currents fell off gradually; they therefore spoke of “fatigue”

(instability). It was discovered later that most of this change was

not a direct effect of a photoelectric current’s passage; it was not even

an indirect effect but was caused by oxidation of the cathode by the air.

Since all modern cathodes are enclosed in sealed vessels, that source of

change has been completely abolished. Nevertheless, the changes that

persist in modern cathodes often are indirect effects of light that can be

produced independently of any photoelectric current.

Impact

The Elster-Geitel photocell was, for some twenty years, used in

all emission cells adapted for the visible spectrum, and throughout

the twentieth century, the photoelectric cell has had a wide variety

of applications in numerous fields. For example, if products leaving

a factory on a conveyor belt were passed between a light and a cell,

they could be counted as they interrupted the beam. Persons entering

a building could be counted also, and if invisible ultraviolet rays

were used, those persons could be detected without their knowledge.

Simple relay circuits could be arranged that would automatically

switch on street lamps when it grew dark. The sensitivity of

the cell with an amplifying circuit enabled it to “see” objects too

faint for the human eye, such as minor stars or certain lines in the

spectra of elements excited by a flame or discharge. The fact that the

current depended on the intensity of the light made it possible to

construct photoelectric meters that could judge the strength of illumination

without risking human error—for example, to determine

the right exposure for a photograph.

A further use for the cell was to make talking films possible. The

early “talkies” had depended on gramophone records, but it was very

difficult to keep the records in time with the film. Now, the waves of

speech and music could be recorded in a “sound track” by turning the sound first into current through a microphone and then into light with

a neon tube or magnetic shutter; next, the variations in the intensity of

this light on the side of the film were photographed. By reversing the

process and running the film between a light and a photoelectric cell,

the visual signals could be converted back to sound.