The armature is a piece of soft steel, one and one-eighth inches long, and one-quarter of an inch thick. A one-sixteenth inch hole is bored, slightly above the centre of the armature, to receive the shaft. The centre of gravity is thus thrown below the centre of mass and the pointer attached to the armature will always return to zero if the instrument is level. The shaft is a piece of one-sixteenth inch bessemer steel rod, three-eighths of an inch long. The ends are filed to a sharp point as shown in the upper pert of Fig. 69. A small hole is bored in the top of the armature almost directly over the shaft as shown in Fig, 69 to receive the lower end of the pointer, which is a piece of No. 16 aluminum wire, three and one-quarter inches long. The other end of the pointer should be flattened slightly by tapping with a hammer and then filed into a shape resembling a spear head. [Illustration: FIG. 70.—A complete Voltmeter having the Scale at the top.] After all the holes have been bored and before the shaft and the pointer are in place, the armature must be tempered so that it will retain its magnetism. In order to temper the armature, heat it to a bright red and drop it immediately into a basin of strong salt water. It may then be magnetized by rubbing one end against the pole of a strong magnet. The bearings are formed by two 6-32 brass screws, one-half an inch long and having a small hole in the end to accommodate the end of the shaft. These screws pass through the upper flange of the bobbin from the opposite sides. The holes in the flanges should be slightly smaller than the outside of the screw so that the latter will fit snugly and "take hold" as if the wood were threaded. Figures 70 and 71 show two different methods of assembling and completing the instruments. In one, the bobbin is mounted on the base and the scale is at the top. In the other, this relation is just turned abound and the bobbin is at the top and the scale at the bottom. In the latter case the pointer must be attached to the bottom of the armature instead of the top. [Illustration: FIG. 71—An Ammeter so constructed that the Scale is at the bottom.] Figure 73 shows the shapes and dimensions of the wooden parts which compose the case. A glass front slides in two shallow grooves cut in the wooden sides, one-eighth of an inch from the front. Glue and brass screws should be used in putting the case together. Do not use iron or iron screws. [Illustration: FIG. 72.—Showing how the Armature, Shaft and Pointer are assembled for a Meter having the Scale at the bottom.]