Ques. Explain the operation of the Siemen's electrodynamometer.
Ans. In [fig. 646], when a current is passed through both coils, the movable coil is deflected against a stop pin, then the screw D is turned in a direction to oppose the action of the current until the deflection has been overcome and the coil brought back to its original position. The angle through which the pointer of the torsion screw was turned is directly proportional to the square root of the angle of torsion. To determine the current strength in amperes, the square root of the angle of torsion is multiplied by a calculated constant furnished by the makers of this instrument.
Fig. 653.--Thompson watt hour meter (type C-6). This form is furnished with side connections, the line wires entering at the left and the load wires at the right. Both sides of the system are carried through the meter in all sizes up to and including the 50 ampere size. In meters of larger ampere capacities, a voltage tap is used.
Ques. How is the electrodynamometer adapted to measure volts or watts?
Ans. 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. With this arrangement, the force between the two coils will be proportional to the product of amperes by volts, hence, the instrument will measure watts.
Fig. 654.--Interior view of Thompson watt hour meter (type C-6). Capacity: 5 to 600 amperes, two wire, and 5 to 300 amperes, three wire; 100 to 250 volts. The meter is supported by three lugs, the upper one of which is keyholed, and the lower right hand one slotted. This permits rapid and accurate levelling as the top screw can be inserted and the meter hung thereon approximately level. The right hand screw may then be placed in position and the meter adjusted as may be required before forcing the screw home.
Ques. Describe briefly the construction of the Thompson recording wattmeter.
Ans. It consists of four elements: 1, a motor causing rotation; 2, a dynamo providing the necessary load or drag; 3, a registering device, the function of which is to integrate the instantaneous values of the electrical energy to be measured; and 4, means of regulation for light and full load.