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.