Changing the Current.—But we have still another thing to learn. If the two wires are not of the same thickness it would not prevent the current from leaping across, but another astonishing thing would result.
First, we shall use a wire B double the thickness of wire A. If now, we had an instrument to test the voltage and the amperage, it would be found that the voltage in B is less than that in A, and also that the amperage is greater.
Second, if the conditions are reversed, and the wire A is thicker than B, the latter will have an increase of voltage, but a lower ampere flow than in A.
Now this latter condition is just what is necessary to give a high tension. Voltage is necessary to make a current leap across a gap. By this simple illustration we have made an induction coil which may be used for making a high tension jump spark.
Construction of a Coil.—Two wires side by side do not have the appearance of a coil, and even though such an arrangement might make a high tension current, it would be difficult to apply. To put the device in such a shape that it can be utilized, a spool is made, as shown in [Fig. 50].
This spool A has a number of layers of thick, insulated wire B first wound around it, the layers being well insulated from each other, and the opposite ends brought out at one end or at the opposite ends, as shown at C, D. On this is a layer of finer wire, also insulated, this wire E having its terminals also brought out at the ends of the spool, and after the whole is thus wound, the outside of the coil is covered with a moisture proof material.
The Primary Coil.—The winding of thick wire is called the primary coil. The current from the battery or the electric generator is led to this inner coil.