The sharp-pointed sewing-needle, together with the magnetized spring, pointer, and counterweight, should slip down into the dents made in the strips and swing freely there. It may require a little filing and bending, but the work should be done patiently, because the proper working of the meter will depend upon having the needle swing freely and easily in its place. Fasten an upright board, four inches wide and one-quarter of an inch thick, to the base-board, back of the bobbin. Attach a piece of thick cardboard to the upright by means of small blocks, in such a position that the pointer swings very close to it but does not touch it. The meter is now complete, except for marking or calibrating the scale. The method of accomplishing this will be described farther on. [Illustration: Fig. 105.—The Completed Meter.] If the meter is wound with No. 36 B. & S. gauge wire it is a voltmeter for measuring voltage. If it is wound with No. 16 B. & S. gauge wire it will constitute an ammeter for measuring amperes. A Portable Voltmeter and Ammeter The bobbin upon which the wire is wound is illustrated in Figure 106. The wood is the Spanish cedar, of which cigar boxes are made. It should be one-eighth of an inch thick, and can be easily worked with a pocket-knife. 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 when finished must be perfectly true and square. The dimensions are best understood from the illustrations. In putting the bobbin together, do not use any nails. Use strong glue only. Two bobbins are required, one for the ammeter and one for the voltmeter. After completing the bobbins, sandpaper them and coat them with shellac. [Illustration: Fig. 106.—Details of the Bobbin.] The bobbin for the ammeter is wound with No. 14 B. & S. double-cotton-covered magnet wire. The voltmeter requires No. 40 B. & S. silk-covered wire. In both cases the wire should be wound carefully in smooth, even layers. A small hole is bored in the flange through which to pass the end of the wire when starting the first layer. After finishing the winding, about six inches of wire should be left at both ends to make connection with the terminals. The whole winding is then given a coat of shellac. A strip of passe-partout tape, one-half of an inch wide wound over the wire around the bobbin will not only protect the wire from injury, but also give the bobbin a very neat appearance. The armature is a piece of soft steel one inch long, one-eighth of an inch thick and three-eighths wide. A one-eighth-inch hole is bored one-sixteenth of an inch above the center for the reception of the shaft. The center of gravity is thus thrown below the center of the mass of the armature, and the pointer will always return to zero if the instrument is level.