This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
A galvanometer in which the moving part of the instrument is a permanent magnet controlled by the action of the earth's magnetic field and the magnetic effect of a current in a coil of wire, that usually surrounds the magnet, has the great disadvantage of having its indications changed, although the current itself may remain constant, due to a change in the strength of the magnetic field in which the instrument operates. The operation of instruments of the above type is satisfactory only in localities where there is a practically constant magnetic field for them to operate in, which it is almost impossible to have, due to the presence of permanent and electric magnets and magnetic materials such as iron and steel.
An instrument constructed as follows will not have the above disadvantage and its operation will be a great deal more satisfactory, as its indications will be practically independent of outside disturbances. In this instrument, the moving part is the coil carrying the current, and it moves in a permanent magnetic field so strong that other disturbing magnetic effects can be neglected. The coil is hung by means of a fine wire and the twist in this wire is the only force acting to bring the coil back to its zero position, after it has been deflected, and maintain it there.
The construction of the magnet and containing case for the instrument will be taken up first. Obtain a piece of Norway iron, 1/2 in. square and about 9 in. long. Bend this piece into the form shown in Fig. 1, and file off the inner edges until they are parallel and about 7/8 in. apart. Drill four 1/8-in. holes in the ends of this piece, two in each end, as indicated. This piece of iron is first tempered and then magnetized by placing it in contact with a powerful electromagnet. Cut a second piece from some soft iron with dimensions corresponding to those given in Fig. 2. Drill two 1/8-in. holes,
A and B, in this piece as shown in the sketch. This second piece is mounted between the poles of the magnet, as follows: Cut from some 1/32-in. sheet brass a piece similar to the one shown in Fig. 3. Drill the holes indicated and thread those designated by A, B, C, D, E, and F to take a 1/8-in. machine screw. Bend the upper end of the piece over at the point indicated by the dotted line until it is perpendicular to the lower part. The center of the hole in the projecting part K, when it is bent over, should be about 1/4in. from the outer surface of the main part of the piece. The small piece of iron is then fastened to the piece of brass with two round-headed screws that pass through the two holes in it and into the holes A and B in the brass piece. The magnet is mounted, also with small brass screws, so that the main part of the magnet and the piece of brass extend in opposite directions, as shown in Fig. 1. The assembled parts are then mounted on a wooden board, whose dimensions are given in Fig. 1, with three brass screws that pass through the holes G, H, and J, as shown.
Ill: The Permanent Magnet and Its Brass Support, and Their Position on the Base
The moving coil of the galvanometer is constructed as follows: Cut from some 1/8 -in. pine a piece 1 1/8 in. long and % in. wide. Cut two other pieces whose dimensions, except their thickness, are 1/4in. larger than the first piece. Then fasten these two pieces to the sides of the first, with three or four small screws through each of them, thus forming a small spool. Saw about 16 slots with a very fine saw in the edges of the projecting pieces and a short way into the edge of the center piece. Wind on this spool about 300 turns of No. 38 gauge silk-covered copper wire. Start with the terminal of the wire in the center of one end of the spool, with a few inches of free wire for making connections, and end up with the terminal in the center of the opposite end of the spool. A small thread is then passed through the slots under the coil and tied, thus serving to hold the various turns of wire together when the coil is removed from the form. The coil should be given a coat of shellac as soon as it is removed from the form.
Ill: Upper and Lower Connections to the Coil and Supports, and the Supports for Suspension
Two pieces must now be attached to the top and bottom of the coil to be used in making electrical connections and suspending the coil. Cut from some very thin sheet brass two pieces whose dimensions correspond to those given in Figs. 4 and 5. Drill a small hole in the center of each of these pieces. Bend the lower part of each piece over at the dotted lines L until it is perpendicular to the main portion of the piece. The bent-over portions of these two pieces are then fastened to the ends of the coil with some fine thread, making sure that they are in the center of the ends before they are fastened. The terminals of the coil are now soldered to these pieces. It would be best to place a sheet or two of thin paper between the brass pieces and the coil, to prevent any part of the coil, except the ends, from coming into contact with the brass pieces. Obtain a small piece of thin mirror and mount it with some glue, as shown by the dotted lines in Fig. 4.
The upper support for the suspension is shown in Fig. 6 and consists of a 1/8-in. threaded screw, A, that passes through the hole in the part K, Fig 3, and is provided with two lock nuts, B. The lower end of this screw should be slotted a short distance, and a small screw put through it, perpendicular to the slot, so that a wire can be easily clamped in the slot by turning up the screw. Next, take a piece of 1 32 -in. brass, as shown in Fig. 7, and bend it at the dotted line A until it forms a right angle. The hole B should be threaded to take a 1/8 -in. screw. The holes C and D are for mounting the piece on the back of the instrument. Slot the end of a 1/8 -in. screw, about 1/2in. long, and put a screw through the end as for the upper support for the suspension. This piece is mounted below the position the coil is to occupy, as shown by M, Fig. 1.
A case should be made for the gal-vanometer whose inside dimensions correspond to those of the piece N. Fig. 1, and whose depth is about 3/4 in. more than the thickness of that piece. Four pieces of wood can be fastened FIG.8 in the corners that will allow the case to slip just far enough on the piece N to make the edge of the case and the back surface of the piece N flush. Cut an opening in the front of this case, about 2 in. long and 1 in. wide, in such a place that the center of the opening is about level with the ends of the magnet. Fasten, back of this opening, a piece of thin glass with four small screws whose heads rest upon the edge of the glass. The interior of this case and all the parts should be given a coat of lampblack mixed with a little vinegar. Two small binding posts, O and P, are mounted on the upper end of the piece N and connected to the upper and lower supports for the suspension of the coil.
This galvanometer will work best, of course, when it is in an exactly vertical position and the following simple device, when attached to it, will allow it to assume this position independent of the level of the surface its base may rest upon. Cut from some 1/8-in. brass two pieces, 1/2 in. wide and 2 1/2 in. long. Drill a 1/8-in. hole in the center of each end of them, 1/4 in. from the end, and a 1/4-in. hole through the center of each. Bend these pieces to a 3/4-in. radius. Cut from some 1/2-in. hard wood a block, 1 1/4 in. square. Fasten the two pieces of brass to the wooden block with 1/8-in. screws, as shown in Fig. 8. One of these pieces is fastened to the upper end of the piece N, Fig. 1, so that the galvanometer will hang vertically. The other piece is fastened to a bracket from which the galvanometer is suspended. A suitable bracket for this purpose can be easily made. When the galvanometer is hung in this way, two binding posts are mounted on the bracket, and connected to the two on the galvanometer. In this way the galvanometer will not be disturbed when making connections.
The suspension is made as follows: Take a piece of small copper wire and roll it out flat. Solder one end of a piece of this wire in the hole in the piece of brass, with the mirror mounted on it. Fasten a piece of the same wire to the lower brass piece, attached to the coil. The upper piece of wire is then clamped in the end of the screw A, Fig. 6, so that the coil hangs perfectly free about the iron core. The lower piece of wire is bent around a small rod several times and its end fastened in the slot in the lower screw.
The deflection of the instrument is read by causing a beam of light from a lamp or candle to be reflected from the mirror to a scale located in front of the instrument. If the light from the lamp is allowed to shine through a small slit in a piece of dark paper, there will be a streak of light reflected upon the scale, instead of a spot.
To use this instrument in measuring larger currents than it will safely carry, connect it in parallel with another resistance which will carry the larger part of the total current. The galvanometer can be calibrated with this resistance, which is known as a shunt.