12 October 2009

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.
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.


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