If .1 of the current flowing, passed through the galvanometer and .9 through the shunt, then the current in the circuit would be ten times that through the galvanometer. Accordingly the current in the galvanometer must be multiplied by the multiplying power of the shunt to obtain the true value of the current in the circuit. In order to determine the resistance necessary to be used with a certain galvanometer, the resistance of the latter _is to be divided by the multiplying power desired less one_. 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. The construction, use, and operation of the various types of galvanometer have been explained in chapter twenty-six. Ammeters and voltmeters, which are simply special forms of galvanometer, and which are largely used are fully described in the preceding chapter. Pressure Measurement.--An electromotive force has been defined as that which causes or tends to cause a current; it is analogous to water pressure; potential difference corresponds to difference of level. The _total_ electromotive force of a circuit is independent of resistance or current, and cannot be limited to mean the fall of pressure between any two points, as for instance the terminals of a battery. If the pressure of a battery be two volts when measured on open circuit by a static voltmeter, there will still be two volts on closed circuit, but there will now be a loss of pressure through the internal resistance of the battery and the voltage across the terminals will be less than the _total_ voltage. The static voltmeter, never closing the circuit, actually measures the total voltage.