A sensitive and well-made galvanometer may be used for a variety of purposes. Not only may it be used for the testing of resistances, both high and low, but by the use of a proper shunt may be used to measure strong currents, and by the use of a high series resistance may be used to measure the voltage of an electric circuit. An old telephone generator furnishes excellent magnets for the construction of such a galvanometer. A magnet which the writer secured from such a source measures 6 ins. in length and is made of steel, which is 1/2 in. by5/8 fin. The more powerful the magnet the better will be the results. Its dimensions may vary somewhat from the one used in the following paper, but the reader can easily modify his instrument to suit his needs.

Fig. 1. Details of Galvanometer. ( Not to scale. )

A bottomless box with a glass top will be required, mounted upon a base board, the whole being suited to be screwed to the wallas shown in Fig. 2. This box is 7 in. by 13 in. outside measurement, and 4 1/2 in. deep.

The base board should be 15 1/2 in. by 8 1/2 ins. The box is secured to the base board by two hasps, one on each side, two or three dowel pins helping to hold the box from slipping. This method of securing the box is adopted so that the case maybe easily removed, giving access to the working parts of the instruments inside. The magnet used being £ in. wide, two pieces of iron, shown at P, are made for pole pieces. These are 5/8 in. square and 17/8 in. long, and have bored in them two holes 1/8 in. in diameter, through which are to pass screws to secure them in place. Secure the magnet firmly to the base board, its poles being 9 1/4 in. from the

Fig. 2. General Arrangement.

bottom and at equal distances each side of the centre line. A block of wood at each side of the magnet, another at the bottom, and two clamps - one at each side - ought to secure the magnet firmly in place so that it cannot slip. Then screw the pole pieces into place, taking care that they rest firmly against the inner poles of the magnet. This will leave 1 7-16 in. of clear space between the poles, if the dimensions given have been followed. If the magnet used has dimensions differing from those given at M (Fig. 1), allowance will have to be made in the pole pieces so as to leave the proper space between them.

In the exact centre of this space is to be secured an iron cylinder, shown in Fig. 2, and also at C in Fig. 1. This is If in. long and J in. in diameter. It is to be fastened to the base board by a screw passing completely through it. This should leave a clear space of l-32nd in. on each side of the cylinder. It is well at this point to take a very small, sharp chisel and cut two grooves in the base board, these grooves being extensions backward of the spaces between the poles and the cylinder on each side. These grooves are necessary in order to allow the coil shown in Fig. 2 to swing freely in either direction without striking the back board.

Take next a piece of the thinnest copper procurable. It should be very thin in order to be light and to take up as little space as possible. From the sheet copper make a frame such as is shown at F (Fig. 1). It is rectangular in shape and measures 2 in. by 7/8 in. inside, and 2 1/2 in. by If in. outside. Its width is 1/4 in. As shown in the side view at K, it is a frame with the edges bent up so as to form a deep groove running around the face of the frame for holding a coil of fine wire. Where a frame overlaps it must be neatly soldered. At the corners the turned up edges will be cut away, but this will do no harm. Line the slot in this frame with a layer of thin but tough paper, fastened in place by shellac. This serves to insulate the frame, Then wind the slot full of No. 36 single silk-covered magnet wire.

The ends of this coil are left projecting, one at each end. Shellac the outer surface of the coil and set it aside to dry. Now make two little pieces shown at E (Fig. 1). They are made by taking a piece of thin copper, 1/4 in. by f in., and soldering to the centre a projecting wire of stiff brass 1/4 in. long. Flatten the outer end of the brass wire and drill a small hole through the flattened part. These little pieces are then bound on to the ends of the coil by a silk thread, so that the projecting wires form a spindle about which the coil may rotate. For this reason they may be so adjusted as to project from the exact centre of each end. Also care must be taken in bending them on to insulate them from the coils by slipping a piece of thin paper under them. Then the projecting ends or the coil are soldered to these little strips, one at each end, and the superfluous wire cut off.

Two pieces of brass should be made like those shown at B and also at II (Fig 1). As shown in Fig 2, these are to support the coil in position. The hole through B , therefore, should be f in. from the back side of the piece, and H should slide freely through B, but may be secured by a set screw. One of the pieces shown at H should be threaded and provided with a thumb nut as shown at T(Fig. 2). One end of If should be flattened and drilled, as were the ends of the projecting wires on the coil. Now procure some fine silk fibres, preferably of raw silk, and pass one end of the fibre through the hole in the upper wire spindle of the coil, securing it firmly by a drop of sealing wax. In a like manner secure a fibre to the lower spindle. Then , with a Tin place (Fig. 2) pass the fibre through the hole in T, pull it up until it is of the right length, and fasten with sealing wax. Do the same at the bottom and the coil will be suspended so as to swing freely in the space between the cylinder and the poles.

Current is led into and out of the coil by two very small, slender springs shown at the top and bottom. They are made from No. 36 (no finer) German silver wire, coiled around a small pencil so as to make a very weak spring. By carefully removing T and leaving the fibre slack, the ends of this coil may be soldered to Tand to the pivots of the coil. This process should be repeated at the bottom. A circular scale, made of a piece of white Bristol board, projects forward from the instrument and is bent so as to have the axis of the coil for a centre. The radius of the arc of this circle is 2 1/2 in. A pointer (shown in Fig. 1) is glued to the bottom of the coil, and its front end moves over the cardboard scale. This pointer is made by taking a strand from a broom and fitting a thin piece of copper at its outer end to serve as an indicator. The back end of the pointer projects beyond the coil and is counter-weighted with a small piece of lead, as shown at L.

Thus the silk fibres serve to suspend the coil in place so that it may swing freely, while the coiled springs encircling the fibre carry the current into and out of the coil, and also serve to bring the needle to zero after being deflected. Binding posts at the bottom are connected to the upper and lower suspensions as shown.

If the amateur is skilful he can improve the instrument by using two very fine hair springs in place of tha coiled German silver springs. These may be secured at a watchmaker's, and besides being more reliable, are not so stiff as the German silver springs, and therefore render the instrument more sensitive. - The American Telephone Journal.