High Frequency Apparatus Page 134

134 HIGH FREQUENCY APPARATUS up of brass strip and the lower bearing is passed through the base of the meter. The meter may be covered with a suitable case, preferably of wood if the instrument is to be used on high frequency currents. The mechanism is, of course, covered by the case which has an opening cut in it to show the scale. The latter should be made as the instrument is calibrated. If no standard meter is available for calibration, Own Spindle Fig 3 Not wire Shunt 110 Y Fig. 4 Fig. 3. The Meter pivot. Fig. 4.-Showing connections for calibration the values may be placed on the scale with fair accuracy if a bank of 16-candlepower carbon lamps is available on a 110-volt circuit. Each lamp takes approximately ½ amp. and by adding one lamp at a time and marking the scale in half ampere divisions, a fairly correct scale will be the result. Various shunts should be experimented with during the first tests in order that the correct one may be permanently connected across the binding posts of the meter. If the meter is to be read from 0 to 10 amp., a strip of German silver 3/8-in. wide and in about No. 28 to 30 gauge will be approximately correct. In testing, turn the current on in gradually increasing amounts until 10 amp. pass. If