A Galvanometer Adapted
Like most of the inventions that have revolutionized the world, the electric telegraph is constructed upon the simplest principles.
As will be shown, an instrument for transmitting and receiving messages can easily be made at a very small cost by any one prepared to devote a little care to its construction; and when completed it will be found of considerable use and convenience.
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.
C.—Upright.
Fig. 1.
For the dial you will require a piece of wood similar to the base, but measuring 5 inches by 4 inches. One end of this should be rounded as shown in B, [Fig. 1]. Drill a hole (F) right through this dial, 3 inches from the bottom and 21⁄2 inches from either side. As a piece of wire will have to revolve easily in this hole, take great care to cut it cleanly. In the lower end of the dial cut out two notches (G H), each according to the measurements in B, [Fig. 1], i.e. 3⁄4-inch deep and 1⁄2-inch wide. Each notch must be 3⁄4-inch from its side of the dial.
The upright is very easily made, being simply a strip of wood 31⁄2 inches by 1 inch (C, [Fig. 1]). In this drill a hole (K) 3 inches from the end and 1⁄2-inch from either side, taking the same care as with the hole in the dial.
Having thus prepared the several pieces of the framework, the dial should be glued into its groove. A small square block of wood glued into the angle formed by the base and the dial will insure the latter’s being perfectly perpendicular. The upright will be fixed in its place later.
To proceed with the more delicate part of the work. Cut a piece of cardboard, fairly thin in texture, 5 inches long and 1 inch wide, and bend it into the shape of an oblong hoop. Upon the hoop or bobbin wind about half an ounce of fine cotton-covered copper wire, size No. 36, which can be obtained from any electrician. Wind this wire very carefully and evenly, but not tightly, leaving about 4 inches free at either end (A, [Fig. 2]).
The next thing to be made is a magnetized needle, which can be easily done as follows. Take a piece of ordinary watch spring, an inch long, and make it red hot. When in this condition drill a hole through the center, and when it is cooler round the ends, as in B, [Fig. 2]. Heat it once more to a red heat and then plunge into cold water to harden. The needle must now be magnetized by being drawn several times firmly across the pole of a strong magnet, care being taken that it is always drawn in the same direction.
The pointer (C, [Fig. 2]) should be made in a similar manner, except that it must not be magnetized, and should be at least twice as large as the needle. Through the hole in the magnetized needle pass a couple of inches of straight stout wire, and with a little solder, or even sealing-wax, fasten the needle as shown in B, [Fig. 2].
A.—Coil of wire.
B.—Magnetized needle.
C. Pointer.
Fig. 2.
All these parts can now be fixed together. In the middle of the broad side of the coil of wire you must push aside the strands in such a manner that holes can be made through the cardboard hoop. Pass the two ends of the thick wire, to which the needle is attached through these holes, so that the needle is then suspended within the coil. Take care to make these holes large enough to allow the axles of the needle to revolve freely, as the whole success of your apparatus depends upon the needle turning easily within the coil, which should now appear as in [Fig. 3].
Fasten the coil upon the back of the dial, one end of the protruding axle passing through the hole you have already made in the wood. Then glue the upright firmly in its groove, with the other end of the axle through its corresponding hole. Take great care that the needle can revolve freely within the coil. Fasten the pointer to the end of the axle passing through the dial, and in this latter drive two pins or wooden pegs (P P) on either side of the pointer, in order that it may only move within a limited arc (B, [Fig. 1]).
Fig. 3.—Revolving needle in coil.