(2) Electricity in locomotion, or current electricity, in which electricity flows along wires, through solutions and other conductors when it is able to do work.
(3) Electricity in rotation, or magnetism in which electric whirls produce attraction and repulsion, and:
(4) Electricity in vibration, or radiation in which electric charges moving to and fro millions of times a second set up waves in the ether which our eyes can see and which we call light. Then there are waves much too short for the eye to see and these are called ultra-violet waves; there are waves too long for the eye to see and these are the infra-red waves which we can feel for they are heat waves, and finally there are very long waves set up in the ether by surging high frequency currents in wires and these are called electric or wireless waves.[2]
Static Electricity.—You can think of electricity as being a fluid, like water, for it has both quantity and pressure, and in many ways it acts like a fluid.
If you filled a tank, raised above the ground, with water, the latter would be at rest, but it would be under pressure too and the moment a hole was bored in any part of the tank below the level of the water it would squirt out; in other words the potential water would be changed into kinetic water or water in locomotion. If, now, you charge a Leyden jar, or a condenser, with electricity it will be at rest until you bring the alternate coatings of tin-foil closely together when a spark will result and a current will flow.
Static electricity is generated by friction and by induction, but the electricity so produced is very small in quantity and very high in pressure. A Leyden jar, or other condenser can be charged, though, with a low pressure current of electricity, as in a spark coil.
Current Electricity.—Whenever electricity flows in a wire, or other conductor, it acts like water flowing through a pipe and it is then called current electricity. The two most common ways to generate a current of electricity is by means of a chemical battery and by a dynamo electric machine.
A current of electricity may have a small current strength, as its quantity is called, and a high voltage, as its pressure is called, like the discharge of a Leyden jar, or it may have a large current strength and a low voltage, as a current generated by a battery.
A direct current, see Fig. 49, is a current which flows steadily in one direction and this can be generated by a battery or a dynamo. An interrupted current, see Fig. 50, is a current that is made and broken a number of times a minute and this is usually done by a vibrator, or interruptor as it is often called. A pulsating current, see Fig. 51, is one whose current strength is varied. One way to produce a pulsating current is to talk into a telephone transmitter which is connected with a battery.