According to these conditions, when a con-
under some other circumstances can even cause
ductor or conductors move across a magnetic field
physical damage to the aircraft. Most individuals
so as to cut the lines of force, electrons within the
are familiar with static electricity and have
conductor are impelled in one direction or
probably received unpleasant shocks from
another. Thus, an electric force, or voltage, is
friction electricity upon sliding across dry seat
created.
covers or walking across dry carpets, and then
coming in contact with some other object.
In figure 4-3, note the presence of the three
conditions needed for creating an induced voltage:
VOLTAGE PRODUCED BY PRESSURE.--
1. There is a conductor (copper wire).
This action is referred to as piezoelectricity. It is
produced by compressing or decompressing
2. There is a magnetic field between the poles
crystals of certain substances. To study this form
of the C-shaped magnet.
of electricity, the meaning of the word crystal
must first be understood. In a crystal, the
3. There is relative motion. The wire is moved
molecules are arranged in an orderly and uniform
back and forth ACROSS the magnetic
manner. Figure 4-4 shows a substance in its non-
field.
crystallized state and its crystallized state.
For the sake of simplicity, assume that the
In figure 4-3, view A, the conductor is
molecules of this particular substance are spherical
moving TOWARD the front of the page. The
(ball-shaped). In the noncrystallized state, view
right-hand end becomes negative, and the left-
A, note that the molecules are arranged
hand end becomes positive. This occurs because
irregularly. In the crystallized state, view B, the
of the magnetically induced emf acting on the
molecules are arranged in a regular and uniform
electrons in the copper. The conductor is
manner. This illustrates the major physical
stopped, view B, motion is eliminated (one of the
difference between crystal and noncrystal forms
three required conditions), and there is no longer
of matter. Natural crystalline matter is rare; an
an induced emf. Consequently, there is no longer
example of matter that is crystalline in its natural
any difference in potential between the two ends
form is diamond, which is crystalline carbon.
of the wire. The conductor at view C is moving
Most crystals are manufactured.
away from the front of the page. An induced emf
Crystals of certain substances, such as
is again created. However, note carefully that
Rochelle salt or quartz, exhibit peculiar electrical
the REVERSAL OF MOTION has caused a
characteristics. These characteristics, or effects,
REVERSAL OF DIRECTION in the induced
are referred to as piezoelectric. For instance,
emf.
when a crystal of quartz is compressed, as in
If a path for electron flow is provided between
the ends of the conductor, electrons will leave the
negative end and flow to the positive end. This
condition is shown in view D. Electron flow will
continue as long as the emf exists. In studying
figure 4-3, note that the induced emf could also
have been created by holding the conductor
stationary and moving the magnetic field back and
forth.
VOLTAGE PRODUCED BY FRICTION.--
This is the least used of the six methods of
producing voltages. Its main application is in Van
de Graf generators, used by some laboratories to
produce high voltages. As a rule, friction
electricity (often referred to as static electricity)
is a nuisance. For instance, a flying aircraft
accumulates electric charges from the `friction
between its skin and the passing air. These charges
Figure 4-4.--Molecules of noncrystallized and crystallized
often interfere with radio communication, and
matter.
4-5
