Fig. 94.
The Magnet, M, is fully described in [App. 85]. M is held firmly to A by cord or wire, which should pass around it near the poles and at the curved part. The wire should pass through small holes in A, and be tied at the back. Wire nails driven into A at the sides of M will keep it from moving about. The wires from the magnet coils are led to two spring binding-posts, X and Y.
197. The Armature, C, is made of a narrow piece of thin iron, about 5½ × ¼ × ⅛ in. It may be made by bending up 3 or 4 thicknesses of tin into that shape. This is the part which will be attracted by M, when the current passes, and which will make the clicks by which the message can be read. (See telegraph alphabet.) There are many ways by which C can be held near M. The figure shows how it can be done entirely with 1-in. wire nails. At the right end of C two nails are driven into A above and below C. They are just far enough apart to allow the left end of C to be raised and lowered without binding; in other words, these nails make a pivot for C to swing upon, and they help to support it at the same time. The left end of C must not quite touch the poles of M when the current passes, because the residual magnetism would keep C from dropping back into place. To adjust the armature, pass the current through M, hold C so that it will not quite touch the poles, then drive in the upper nail, 2. Put another nail, 1, below C, so that M will not have to lift C more than ⅛ or 3⁄16 in. Try the nails in different positions until C quickly rises and falls when the circuit is closed and opened. A nail, 3, driven in front of C, will keep its right end in place. No springs are needed, as gravity acts upon C instantly, bringing it to the lowest position as soon as the current ceases to flow.
198. The Battery will depend upon how much you want to use the sounder. If just to show the principle of it, almost any cell of medium strength will do, like that of [App. 3], [4] or [5]. A dry battery will do, but if you use the sounder much, an open-circuit battery will soon use itself up. Where much work is needed of the battery use [App. 9].
Fig. 95.
The Key like [App. 119] is best. Push-buttons are handy where used only for experiments, and not for the actual sending of messages.