To make a model of the elaborate machines now employed, which not only receive but print messages in legible Roman characters, would be far beyond the scope of any amateur, necessitating the purchase of many costly parts and requiring a considerable knowledge of engineering to fit them satisfactorily together. But the instrument that can be made according to the following instructions, is exactly similar to those in use throughout many smaller offices the world over.
The telegraph is based upon the following principle. A current of electricity, passed through a hoop of wire within which a needle is suspended, will move that needle from left to right, or from right to left, according as the wires are attached to one or other of the poles of a battery. This simple apparatus is called a galvanometer. A telegraph instrument is nothing but a galvanometer adapted to convenient usage.
Before entering upon the method of construction, it would be as well to inform whoever undertakes this work that the utmost care is necessary in adjusting the various parts. The slightest fault will certainly spoil the whole instrument, and the least carelessness render all the labor expended utterly useless.
Now to describe the simplest way to make a receiver and transmitter.
Three pieces of wood will be required, a cigar-box furnishing the best material for the purpose; if you have not one at home, any tobacconist will let you have one for a few cents.
To commence with the base. Cut a piece of wood measuring 6 inches by 5 inches. In this two grooves must be made (D D and E) according to the measurements shown in A, [Fig. 1]. The width of these grooves must, of course, depend upon the thickness of the wood, for the dial and the upright will have to be fitted into them.
A.—Base of instrument.
B.