communicates to the iron core magnetic properties, and enables it to attract another piece of iron or steel called its armature; but, when the current ceases, the magnetism ceases also, and a spring—too weak to neutralize it—draws back the armature. It is shown in section at M, in Fig. 6. When the armature and the lever carrying it are discarded, and instead of them a jointed tongue of steel, as at P M, is inserted between the poles of the magnet, it will be unaffected by the current except when a change occurs in its direction. It is then called a polarized magnet. Its use will be explained a little further on. One
Fig. 6.
of the keys, K, in the diagram, is provided with a spring, which is in contact with the metal of the key when this latter is in its normal position, and maintains across the key a circuit including a portion of the battery b'. But when the key is depressed the spring comes in contact with a screw, to which another circuit is connected, applying the full strength of the battery to the line. The circuit across the key is never broken, because the spring remains in contact with the arm of the key until it begins to press against the screw. This key works the magnet M, which has its retractile spring so adjusted as to be overcome only by the full intensity of the current when the key is down. The other key, K', is for changing the direction of the current, and working the polarized magnet, P M. Its construction is such that, when not in use, one pole of the battery, the positive, for example, is in connection with the line, and the negative with the earth, necessitating the passage of the current through the line in the first place; but when the key is touched the negative pole is connected "to line" and the positive to earth, reversing the direction of the current, These reversals of direction operate, as has been said, the polarized magnet P M.
To revert to the illustration we made use of in describing the duplex, let the reader picture to himself a water-course in which both
