The glass strip and its standards supporting the hot wire may then be fastened to the baseboard of the instrument by means of two round headed brass wood screws. Two rubber washers must be interposed between the glass and the wood. A piece of 3/32 inch brass 1/2 inch wide and 3 1/4 inches long is bent in the shape shown by F in Fig. 84. The upper end is bored and tapped to receive a thumbscrew similar to the one in the standard on the glass strip. Two brass springs 1/64 inch thick, 3/8 inch wide and 1 1/4 inches long are soldered or riveted at opposite ends of F in the positions shown in Fig. 85. The springs should project one inch from the upright. A small hook made from an ordinary pin is soldered to the outside end of each.
Fig. 85. Complete Movement.
The movement is shown in perspective by Fig. 85. G is a rectangle of very thin copper, 1/2 inch long and 1/4 inch wide, having a little projection 1/4 inch long bent in a curve so that it forms a sextant of a circle, of which the intersection of the diagonals of G would be the center.
The pointer is a piece of steel wire 5 inches long. It is slightly flattened by hammering so that it will retain its shape and not curl. About 1/2 inch is allowed to project through G and is weighted with a lead shot so as to partly counterbalance its weight.
Two loops of wire are fastened to the corners of G by tying them in holes which are bored there for that purpose. The wire is fine phosphor bronze .003-.005 of an inch in diameter, which is used for suspending the movements of delicate galvanometers. Pass the loops over the hooks on the springs and adjust until the pointer moves horizontally. Then fasten the wires permanently to the hooks by means of a small drop of solder.
Fig. 86. Side View of Hot Wire and Movement.
The movement is mounted in the position shown by Fig. 86. The silk thread tied to the eyelet runs to the little sextant and is cemented at the further end by means of a small drop of sealing wax. The scale is a piece of sheet copper or brass, covered with white paper and calibrated in degrees or made to read in amperes by connecting it in series with an ammeter and a source of direct current. A rheostat should be included in the circuit and the current varied so that various values may be marked off. All the different points must be located by sending an actual current of that value through the meter. An error is liable to result if any of the points are marked by guesswork, for the divisions grow smaller and smaller as they become farther away from zero. For example a position of the pointer corresponding to 0.1 of an ampere will not be half way between zero and 0.2 but will be nearer the 0.2 division.
The resistance of wires to high frequency currents is much higher than their resistance to constant currents. This would seem at first to indicate that our meter will give a higher reading for an equal current value, when used with a high frequency current after being calibrated with a direct current. But with wires of very small diameter such as No. 40 B. S. gauge there is almost no perceptible difference and consequently no error unless the frequency of the oscillations exceeds 1,000,000 per second, which is very unlikely with the "spark" method of wireless telegraphy.