Solenoids.—A solenoid consists of a spiral of conducting wire wound cylindrically so that, when an electric current passes through it, its turns are nearly equivalent to a succession of parallel circular circuits, and it acquires magnetic properties similar to those of a bar magnet.
Ques. What is the character of the lines of force of a solenoid in which a current is flowing?
Ans. The lines of force must be thought of as closed loops linked with the current. The conductor conveying the current passes through all the loops of force, and these are, so to speak, threaded or slung on the current-line of flow, as in fig. 116.
Ques. What is the distribution of the lines of force?
Ans. The lines of force form continuous closed curves running through the interior of the coil; they issue from one end and enter into the other end of the coil, as shown in fig. 117.
Ques. What are the properties of a solenoid?
Ans. A solenoid has north and south poles, and in fact possesses all the properties of an ordinary permanent magnet, with the important difference that the magnetism is entirely under control.