the force is removed, the crystal is decompressed
figure 4-5, view A, electrons tend to move through
and immediately causes an electric force in the
the crystal as shown. This tendency creates an
opposite direction, view B. Thus, the crystal is
electrical difference of potential between the two
able to convert mechanical force, either pressure
opposite faces of the crystal. (The fundamental
or tension, to electrical force.
reasons for this action are not known. However,
The power capacity of a crystal is extremely
the action is predictable, and therefore useful.)
small. However, they are useful because of their
If an external wire is connected while the pressure
extreme sensitivity to changes of mechanical force
and emf are present, electrons will flow. If the
or changes in temperature. Due to other
pressure is held constant, the electron flow will
characteristics not mentioned here, crystals are
continue until the charges are equalized. When
most widely used in communication equipment.
VOLTAGE PRODUCED BY HEAT.--
When a length of metal, such as copper, is heated
at one end, electrons tend to move away from the
hot end toward the cooler end. This is true of most
metals. However, in some metals, such as iron,
the opposite takes place and electrons tend to
move TOWARD the hot end. Figure 4-6 shows
this characteristic. The negative charges (electrons)
are moving through the copper away from the
heat and through the iron toward the heat. They
cross from the iron to the copper at the hot
junction and from the copper through the
current meter to the iron at the cold junction. This
device is generally referred to as a thermocouple.
Thermocouples have somewhat greater power
capacities than crystals, but their capacity is still
very small if compared to some other sources. The
thermoelectric voltage in a thermocouple depends
Figure 4-5.--Quartz crystal. A. Compressed. B. De-
mainly on the difference in temperature between
compressed.
Figure 4-6.--Voltage produced by heat.
4-6
