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Page 472 | Volume 2 | Chapter 27 | Testing and Testing Apparatus

Ques. What is the value of the International ampere as measured with the silver voltameter?

Ans. The International ampere is represented sufficiently well for practical use by the unvarying current which, when passed through a silver voltameter (as described above) deposits silver at the rate of .001118 gramme per second.

Fig. 546. - Single contact and short circuiting key. This key is intended especially for uae with D'Arsonval galvanometers in zero deflection methods.
Fig. 546.—Single contact and short circuiting key. This key is intended especially for use with D'Arsonval galvanometers in zero deflection methods. The key is connected in circuit with the galvanometer so that whenever the key is not depressed, the galvanometer is short circuited, and its oscillations quickly damped out by the currents induced in its coi1. The back end of the spring is held in a slot in a rubber block attached to the base.

Ohm's Law and the Ohm.—The various tests here described depend for their truth upon the definite relation existing between the electric current, its pressure, and the resistance which the circuit offers to its flow. This relation was fully investigated by Ohm in 1827. Using the same conductor, he proved not only that the current varies with the pressure, but that it varies in direct proportion.

Ohm's law has already been discussed in a previous chapter and the several ways of expressing it are repeated here for convenience:

1. Amperes = volts / ohms;



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