Solenoid

—A spiral of conducting wire wound cylindrically so that, when an electric current pa es through it, its turns are nearly equivalent to a succe ion of parallel circular circuits, and it acquires magnetic properties similiar to those of a bar magnet. The lines of force must be thought of as closed loops linked with the current. The conductor conveying the current pa es through all the loops of force, and these are, so to speak, threaded or slung on the current line of flow. It will be readily inferred that since a solenoid of wire conveying a current attracts and repels b its extremities the poles of a magnet, two suc! spiral conductors conveying currents should — attract and repel each other. This is found to be the case. The lines of force form continuous closed curves running through the interior of the coil, and | i uing from one end and entering into the other end of the coil. A solenoid has north and south poles, and in fact po e es all the proper ee of an ordinary permanent magnet, with the important difference that the magnetism is entirely under control, for | it is found that under all circumstances the strength of the magnetic field of a solenoid is at every point proportional to the strength of the electric current pa ing through its coils; if the | current be increased, the magnetism is increased — in proportion also; and if the current be stopped, all trace of magnetism disappears.