coil. A coil of this kind is called a "solenoid." If two such solenoids are suspended horizontally from their own wires one behind the other, so that the axes are in the same line, with their ends half-an-inch apart, it will be found that on sending a current through them they will either repel or attract each other. If the direction of the current round the spiral of both coils is the same, there will be attraction; if the current direction in one solenoid is reversed, there will be repulsion. In both cases the force is feeble.
Now place into the paper tube of each solenoid an iron core. The force will now be very much increased. If the coils, instead of being suspended, be held fast, and the cores can slide easily within their paper tubes, it will be found that the cores themselves either come together or fly apart according to the relative direction of the current. Thus the presence of the iron not only increases the dynamic force of the current, but it also shifts the seat of this force from the wire to the iron. We are thus driven to the conclusion that the seat of the force, or at least part of it, is not in the wire itself, but in the space surrounding the wire.
The fact that an electric current produces mechanical forces acting through space
