The flow of current in an open circuit may be shown by con-
necting a small incandescent lamp and a condenser in series to
alternating-current supply mains. With each reversal of the
alternating supply voltage a momentary current flows through
the lamp, and the repeated pulses of current heat the lamp
filament to incandescence.
The dielectric. — ^The insulating material between the metal
plates A and B in Fig. 81 is called the dielectric. In general
the insulating material surrounding a charged body or between
two charged bodies is called a dielectric.
Electrostatic attraction. — ^The charged metal plates A and B
in Fig. 81 have an evident attraction for each other when the
dielectric is a fluid like air or oil. This electrostatic attraction of
two charged metal bodies may be shown as follows: A gold leaf
is hung along side of a vertical brass strip. When the gold
leaf is connected to one terminal of a battery and the brass
strip to the other terminal of the battery the gold leaf is
attracted by the brass. It is necessary in this arrangement to
cover the face of the brass plate with a layer of paper to prevent
short circuiting the battery by the gold leaf. An electromotive



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124 ADVANCED ELECTRICITY AND MAGNETIMS.
force of 100 volts is sufficient to produce a very perceptible
attraction with this arrangement.
