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22 RICHARDSON'S BLUEBOOK OF PROJECTION
work. Connect them to an incandescent light globe, for example, and light will be produced by the heating of its filament to incandescence. Connected to a motor they will cause its rotary element to revolve against the resistance of a load, thus producing power. Connect them to a suitable resistance coil and heat will be produced.
This electrical condition is termed Polarity. It represents the electrical affinity a wire negatively charged has for a wire positively charged from the same power source. It is the effect of what we call electrical pressure, measured in volts.
Negative and Positive
When a circuit is charged with direct current one conductor is always negative; the other positive. (73) When a circuit is charged with alternating current each wire is alternately negative and positive many times each second. In the 'case of 60 cycle current each wire is negative 60 times each second and positive 60 times each second.
Voltage
(74) Voltage, also known as electro-motive force, is commonly abbreviated into E.M.F., or just E. Electric current is generally considered as having both pressure and volume. In its action it is very similar to that of water. Water, however, may be made the subject of physical examination. We can watch its action. We can feel it or weigh it. Electricity, on the other hand, is an impalpable substance or force. It is without appreciable weight, though scientists assure us it has an infinitesimal
weight. . .
We cannot see it except in the form of light which is not electricity' but a product of its power. We cannot, feel it except in the form of a shock, which again is not electricity but its effect in passing through the tissues
of the body. '
Voltage, in its action, corresponds to pressure applied to water in a pipe, or to steam in a boiler. An ordinary