Figs. 2,508 to 2,511.—Hoskins instruments. Fig. 2,508, voltmeter, small pattern; fig. 2,509, ammeter, large pattern; fig. 2,510, voltmeter, horizontal edgewise pattern; fig. 2,511, illuminated dial voltmeter.
Ques. Describe the rotary field type of induction instrument.
Ans. The parts are arranged similar to those of wattmeters, the necessary split phase being produced by dividing the current into two circuits, one inductive and the other non-inductive.
Fig. 2,512.—Hoskins instrument with case removed. It has a very short magnetic circuit which is composed of silicon steel, permitting low magnetic densities to be used.
Dynamometers.—This type of instrument is used to measure volts, amperes, or watts, and its operation depends on the reaction between two coils when the current to be measured is passed through them. One of the coils is fixed and the other movable.
Fig. 2,513.—Diagram of Siemens' dynamometer. It consists of two coils on a common axis but set in planes at right angles to each other in such a way that a torque is produced between the two coils which measures the product of their currents. This torque is measured by twisting a spiral spring through a measured angle of such degree that the coils shall resume their original relative positions. When constructed as a voltmeter, both coils are wound with a large number of turns of fine wire, making the instrument sensitive to small currents. Then by connecting a high resistance in series with the instrument it can be connected across the terminals of a circuit whose voltage is to be measured. When constructed as a wattmeter, one coil is wound so as to carry the main current and the other made with many turns of fine wire of high resistance suitable for connecting across the circuit.