EXAMPLE.--What must be the resistance of a shunt for a galvanometer of 2,000 ohms resistance where only one fifth of the current is to pass through the galvanometer?
The multiplying power less one is
5 - 1 = 4
and the required resistance is
2,000 ÷ 4 = 500 ohms.
When it is essential that the total resistance of the circuit should not be altered by an alternation of the galvanometer shunt, a compensating box should be used which automatically inserts a resistance for each shunt in series with the shunted galvanometer to bring the total resistance up equal to the unshunted value. Thus the current in the main circuit is not altered.
CHAPTER XXVII
TESTING AND TESTING APPARATUS
The practical electrician frequently has to make tests of various kinds which require the rapid and accurate measurement of voltage, current and resistance. It is therefore essential that he understand the methods employed in testing and the operation of the instruments used.
Most tests are made with a galvanometer, and the devices, such as resistances, switches, etc., which are used in connection with the galvanometer may be obtained put up in a neat and substantial box together with the galvanometer, the combination being called a "testing set." Numerous forms of testing set are illustrated in this chapter.