The Parts as They are Assembled to Make a Pocket Voltmeter for Direct Currents (Fig. 1)
Procure a piece of hard rubber or fiber, about 1/4 in. in thickness and of sufficient size to cut from it a disk, 2-1/2 in. in diameter. Make a small horseshoe magnet from a piece of the very best steel obtainable, and magnetize it to as high a strength as possible. This magnet is made of a piece of steel, 1/8 in. thick, about 3/8 in. in breadth, and of such length that the overall lengthwise dimension of the completed magnet will be about 1-7/8 in. and the distance between the inside edges of the ends a little greater than 1/2 in. Fasten the completed magnet to the base F by means of two or three straps, made from some thin brass, and small machine or wood screws.
Then cut from some 1/16-in. sheet brass a piece having the general appearance and dimensions shown at A, Fig. 2. Bend the ends of this piece over at right angles to the center portion along the dotted lines. Drill the hole at the upper end and thread it for a 1/16-in. machine screw. By means of a pointed drill, make a small recess at the lower end directly opposite the first hole. This small recess is to form the lower bearing for the shaft supporting the moving system, while a small recess cut centrally in the end of a screw, mounted in the upper hole, will form the upper bearing. The screw placed in the upper hole need be only about 3/16 in. long. The holes in the two wings are for mounting this piece upon the fiber base, as shown in Fig. 1.
The shaft for supporting the moving system is made of a piece of a hatpin. It is about 13/16 in. long and its ends are pointed so that they will turn freely in the bearings provided for them.
The armature is cut from a piece of 1/16-in. sheet steel. It is made about 3/4 in. long, 5/16 in. wide at the center, tapering to 1/8 in. at the ends. A hole is drilled in its center so that it may be forced onto the shaft. It is mounted so that its lower surface comes about 1/4 in. from the lower end of the shaft.
Then cut from some very thin brass a piece, that is to form the needle, 1/4 in. wide at one end and tapered to a point at the other, the total length being about 3 in. Drill a hole in the large end of this piece, the same size as the shaft and 1/2 in. from the end. This piece is not fastened to the shaft until some of the other parts are completed.
Details of the Supports for the Coil and for the Needle, or Pointer, Shaft (Fig. 2)
The spool upon which the winding is to be placed is made as follows: Procure a piece of very soft wrought iron, 1-1/4 in. long and 1/4 in. in diameter, to form the core. The ends of the spool are made of thin brass and are dimensioned as shown in Fig. 2, at B and C. The piece shown at B is to form the lower end of the spool, and is bent at right angles along the dotted line. The two holes at the lower edge are for attaching the end of the spool to the fiber base. The piece shown at C forms the upper end of the spool and at the same time a back upon which the scale of the instrument is mounted. The holes in the lower edge are threaded for small machine screws, as it will be necessary to fasten this piece to the base by means of screws that pass through the base from the under side, as shown in Fig. 1. Bend the upper and lower portion of the piece over at right angles to the center portion along the dotted lines. Make sure that the large hole in the center of each end piece is of such size that it will fit very tight on the end of the wrought-iron core. Force the end pieces onto the ends of the core a short distance, say, 1/16 in., and hammer down the edges of the core so that the end pieces cannot be easily removed. In fastening the ends to the core be sure that the parts that are to rest upon the base are parallel with each other and extend in opposite directions; also that the ends are at right angles to the core. Then insulate the inner portions of the completed spool with several thicknesses of onion-skin paper, or any good-quality, thin writing paper, and shellac. The winding will be described later.