Page:Popular Science Monthly Volume 3.djvu/428

414 It revolves with the vertical shaft, and travels over a disk (D) pierced with as many holes as there are letters on the type-wheel, these holes being ranged in a circle round the base of the shaft, and at such a distance from the shaft that the extremity of the chariot passes exactly over them. In these holes are the upper ends of a set of pins (g), which are raised by putting down a set of keys (B N) resembling those of a piano. When the chariot passes over a pin which is thus raised, the piece v is lifted away from v', and the current from the battery, which previously passed from the pin through v and v'  to the earth, is now cut off from v', and passes through v to the electro-magnet, and thence to the line-wire.



This is the process for sending signals. We will now explain how a current thus sent causes a letter to be printed by the type-wheels at both the sending and receiving stations, the sending and receiving instruments being precisely alike.

The current traverses the coils of an electro-magnet (E, Fig. 13), beneath which is a permanent steel horseshoe magnet, having its poles in contact with the soft-iron cores of the electro-magnet. When no current is passing, the influence of the steel renders these cores temporary magnets, and enables them to hold the movable armature (p) against the force of an opposing spring. The current is in such a direction that it tends to reverse the magnetism induced by the steel. It is not necessary, however, that it should be strong enough to