[Illustration: Fig. 641 to 642.--Multipliers for Western standard portable voltmeters. Multipliers are resistance boxes, the coils in which are highly insulated, and are adjusted so that the readings of the instrument may be multiplied by any desired constant. Multipliers are usually constructed so that the indications of the pointer, multiplied by 2, 5, 10, 20 or 50, will give the voltage of the circuit. By the use of multipliers the range of voltmeters may be increased to any practical limit.] [Illustration: Fig. 643.--Portable multiplier for portable voltmeter. A multiplier is used for increasing the readings of voltmeters, and consists of resistance coils placed in a portable case. A multiplier is connected in series with the voltmeter and must be adjusted for the instrument with which it is to be used, because the resistance coil must be a multiple of the voltmeter resistance. For instance, a multiplier with a value of 10, used with a 6 volt voltmeter or 521 ohms would measure about 5,215 ohms; one with a value of 40, would equal about 20,860 ohms. The multiplier 10 would give a total scale value of 60, and the multiplier 40, a total scale value of 240 volts to the 6 volt instrument. A multiplier is of considerable value in that it does away with the necessity of having a number of voltmeters of different ranges. The instrument here illustrated has a range of 150 volts.] Ques. What are multipliers? Ans. These are extra resistance coils which are connected in series with a voltmeter for increasing its capacity or readings. They are put up in portable boxes, and must be adjusted for each particular voltmeter as the resistance of a multiplier coil must be a multiple of the resistance of the voltmeter itself. Ques. What is an electro-dynamometer? Ans. An instrument for measuring amperes, volts, or watts by 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. [Illustration: Figs. 644 to 645.--Western standard portable shunts. The milli-voltmeters used in connection with these shunts read directly in amperes. Shunts of different capacities can be adjusted to the same instrument, and it can, therefore, be used to measure a current of 2,000 amperes with the same degree of accuracy as a current of 1 ampere. In selecting shunts of different capacities for use in connection with one instrument it should be considered that the higher ranges must be even multiples of the lower one in order to suit the same scale on the instrument.] Ques. Describe the Siemens' electro-dynamometer.