The shaft is a piece of one-eighth-inch Bessemer steel rod, seven-sixteenths of an inch long. The ends are filed to a sharp knife-edge on the under side, as indicated in the figure. [Illustration: Fig. 107.—The Bobbin partly cut away so as to show the Bearing. Details of the Armature and Shaft.] A one-sixteenth-inch hole is bored in the top of the armature to receive the lower end of the pointer, which is a piece of No. 16 aluminum wire, four and one-half inches long. After the holes have been bored, the armature is tempered so that it will retain its magnetism. It is heated to a bright red heat and dropped into a basin of strong salt water. The armature is then magnetized by rubbing one end against the pole of a strong magnet. The bearings are formed by two strips of thin sheet-brass, three-sixteenths of an inch wide, and one and one-quarter inches long, bent and glued to the sides of the bobbin. In the illustration, part of the bobbin is represented as cut away. The center of the bearing is bent out so that the end of the shaft will not come in contact with the sides of the bobbin. The top of the center is notched with a file to form a socket for the knife-edges of the shaft. [Illustration: Fig. 108.—Completed Voltmeter.] The bobbin is glued to the center of a wooden base, seven inches long, four inches wide and three-quarters of an inch thick. The terminals of the coil lead down through two small holes in the base and thence to two large binding-posts. The wires are inlaid on the under side of the base, i.e., they pass from the holes to the binding-posts through two grooves. This precaution avoids the possibility of their becoming short-circuited or broken. The case is formed of two sides, a back and top of one-half-inch wood. It is six inches high, four inches wide, and two inches deep. A glass front slides in two shallow grooves cut in the wooden sides, one-eighth of an inch from the front. The case is held down to the base by four round-headed brass screws, which pass through the base into the sides. It is then easily removable in case it ever becomes necessary to repair or adjust the instrument. The meter and case, as illustrated in Figure 108, are intended for portable use and are so constructed that they will stand up. A small brass screw, long enough to pass all the way through the base, serves to level the instrument. If a little brass strip is placed in the slot in the screw-head and soldered 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. Where the instrument is intended for mounting upon a switch-board, it can be given a much better appearance by fitting with a smaller base, similar in size and shape to the top. The binding-posts are then mounted in the center of the sides.