Ammeters and voltmeters are really, in principle, galvanometers, the scales of which have been calibrated to read in amperes or volts, as the case may be.
The little meters described below are very simple but quite sensitive.
[Illustration: FIG. 68.—Details of the Bobbin.]
The wooden bobbin which holds the wire is shown in Figure 68. The exact dimensions are best understood from the illustration. The wood can be easily secured from an old cigar box. In laying out the work, scratch the lines on the wood with the point of a darning needle. Pencil lines are too thick to permit of accuracy in small work. The bobbin should be perfectly square and true when finished. Finish by rubbing with fine sandpaper and then give it a coat of shellac. Two bobbins will be required, one for the voltmeter and one for the ammeter. Do not use any nails in putting the bobbins together. Use strong glue only.
The bobbin for the ammeter should be wound full of No. 14 B. & S. Gauge double cotton covered magnet wire. The voltmeter requires much smaller wire. No. 37 B. & S. Gauge single silk covered wire will serve the purpose satisfactorily, but a finer size such as No. 38 or 40, is better. Such fine sizes will probably, however, prove more difficult for the experimenter to obtain. Sufficient wire should be used to fill the bobbin up. The wire should be wound on carefully in smooth even layers.
A small hole should be bored in one of the flanges, through which to pass the end of the wire when starting the first layer. About six inches of wire should be left at both ends of the coils to make connection to the terminals with. The whole winding should be given a coat of shellac when finished.
[Illustration: FIG. 69.—Details of the Armature, Bearings and Pointer.]
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.