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.]
The two binding posts connected to the terminals of the wire on the bobbin should be mounted on the base. A small, round-headed brass screw, long enough to pass all the way through the base will serve to level the instrument and bring the pointer exactly at zero. If a little brass strip is placed in the slot in the screw head and soldered there so as to form what is known as a "winged screw", the adjustment may be made with the fingers and without the aid of a screw driver.
The scale is formed on a piece of stiff white cardboard directly under the pointer. The scale is supported by gluing it to two small wooden blocks. The various values are marked on the scale with a pen and ink. The glass front should therefore not be put in place until the instrument has been calibrated.
To properly calibrate the meters, they must be compared with a suitable standard.
The zero value on the meters is normally in the centre of the scale. When a current is passed through the bobbins, the armature tends to swing around at right angles. But since the armature is pivoted above its centre of mass, the centre of gravity is displaced when it swings and exerts a pull in opposition to that of the bobbin. The amount of swing will be greater as the current is correspondingly stronger. The pointer will swing either to the right or to the left, depending upon the direction in which the current passes through the coil of wire on the bobbin.