the hot and cold junctions. Consequently, they
are widely used to measure temperature and as
heat-sensing devices in automatic tempererature
control equipment. Thermocouples generally can
be subjected to much greater temperatures than
ordinary thermometers, such as the mercury or
alcohol types.
VOLTAGE PRODUCED BY LIGHT.--
When light strikes the surface of a substance, it
may dislodge electrons from their orbits around
the surface atoms of the substance. This occurs
because the light has energy, the same as any
moving force.
Some substances, mostly metallic ones, are far
more sensitive to light than others. That is, more
electrons will be dislodged and emitted from the
surface of a highly sensitive metal, with a given
amount of light, than will be emitted from a less
sensitive substance. Upon losing electrons, the
photosensitive (light sensitive) metal becomes
positively charged, and an electric force is created.
Voltage produced in this manner is referred to as
a photoelectric voltage.
Figure 4-7.--Voltage produced by light.
The photosensitive materials most commonly
used to produce a photoelectric voltage are various
copper oxide. This additional layer serves two
compounds of silver oxide or copper oxide. A
purposes:
complete device that operates on the photo-
electric principle is referred to as a photoelectric
1. It is EXTREMELY thin to permit the
cell. Many sizes and types of photoelectric cells
penetration of light to the copper oxide.
are in use, each of which serves the special
2. It also accumulates the electrons emitted
purpose for which it was designed. Nearly all,
by the copper oxide.
however, have some of the basic features of the
photoelectric cells shown in figure 4-7.
An externally connected wire completes the
electron path, the same as in the reflector type
The cell (fig. 4-7, view A) has a curved light-
of cell. The photocell's voltage is used as needed
sensitive surface focused on the central anode.
by connection of the external wires to some other
When light from the direction shown strikes the
device, which amplifies (enlarges) it to a usable
sensitive surface, it emits electrons toward the
level.
anode. The more intense the light, the greater is
A photocell's power capacity is very small.
the number of electrons emitted. When a wire is
However, it reacts to light-intensity variations in
connected between the filament and the back, or
a very short time. This characteristic makes the
dark side, the accumulated electrons will flow to
photocell very useful in detecting or accurately
the dark side. These electrons will eventually pass
controlling a great number of processes or
through the metal of the reflector and replace the
operations. For instance, the photoelectric
electrons leaving the light-sensitive surface. Thus,
cell, or some form of the photoelectric principle,
light energy is converted to a flow of electrons,
is used in television cameras, automatic
and a usable current is developed.
manufacturing process controls, door openers,
The cell (fig. 4-7, view B) is constructed in
burglar alarms, and so forth.
layers. A base plate of pure copper is coated with
light-sensitive copper oxide. An additional
VOLTAGE PRODUCED BY CHEMICAL
semitransparent layer of metal is placed over the
ACTION .-- Up to this point, it has been shown
4-7
