Waves and ripples in water, air, and æther

by Sir J. A. Fleming

It is necessary, in the next place, to point out that an electric current has a directive quality. It belongs to that category of things like forces and movements, which have direction as well as magnitude. It is not completely defined by the answer to the question—How much? We must also ask—In what direction? The direction of an electric current is settled by holding a small compass needle near to the conductor or wire in which the current exists. The little magnet will set itself with its north pole in one direction or in the opposite, across the wire. That is to say, the axis of the compass needle places itself at right angles to that of the wire. The direction of the electric current is decided in accordance with the following conventional rule: Imagine yourself placed with your arms extended straight out like a cross, and that the wire conveying the current is placed before your face in a vertical position. Imagine, also, that the position in which the compass needle naturally sets when held between you and the wire is such that its North pole is on your right-hand side. Then the current would be said to move upwards in the wire. A current which is always in one and the same direction in a wire is called a continuous, direct, or one-way current.

A current which periodically changes its direction so that it is first in one direction and then in the other is called an alternating or two-way current.

I can now show you two experiments, the employment of which will enable us always to decide whether a current in a wire is a one-way or a two-way current. In the first experiment you see a copper wire stretched between the poles of a powerful horseshoe magnet. When a one-way current is sent through the wire, it is pulled either up or down, like a fiddle or harp string being plucked by the finger. If, however, we send a two-way current through the wire, it moves alternately up and down, and vibrates just like a harp-string when plucked and left to itself.

The next experiment gives us, however, a more convenient method of ascertaining the presence in a wire of an alternating or two-way electric current. If two wire circuits are laid parallel to each other, and we send through one of these an electric current, then, in accordance with Faraday’s most notable discovery, we find that the beginning or the ending of the one-way current in the first wire gives rise at the moment to a transitory current in the second wire. If, however, we pass through the first wire, which we call the primary circuit, a two-way current, then, since this is, so to speak, continually beginning and ending, we have a similar alternating or two-way electric current produced in the secondary circuit.

This fact may be most neatly and forcibly illustrated by the employment of the following pieces of apparatus: An insulated wire is wound many times round a great bundle of iron wire, thus forming what is called an electro-magnet. Through this wire is passed a strong alternating electric current which reverses its direction 160 times a second.

Fig. 64.

Over the top of the electro-magnet we hold another coil of insulated wire, the ends of which are connected to a small electric glow lamp ([see Fig. 64]). When held near to the pole of the electro-magnet, we find the little lamp in the secondary coil lights up brilliantly, because there is created in that circuit a secondary or induced alternating electric current by the action of the other current in the primary or electro-magnet circuit. Thus we see that one alternating electric current can, so to speak, give birth to another in a second circuit held parallel to the first. In like manner this secondary current can give rise to a third or tertiary current, and the third to a fourth, and so on indefinitely.

We can always make use of this test to ascertain and prove the existence of an alternating current in any electric circuit. If we provide a coil of insulated wire, having its ends connected to a small incandescent lamp, and hold this lamp coil or secondary circuit near to and parallel with any other circuit in which we suspect the existence of an alternating electric current, and if the lamp in the secondary circuit lights up, then we can say with certainty that there is an alternating or two-way electric current in the first circuit.

Having, then, indicated briefly the effects which are produced by an electric current when it exists in a conducting circuit, and the way in which we can determine its presence and direction, we must pass on to discuss some other facts connected with its production.