Hawkins Electrical Guide v. 07 (of 10) / Questions, Answers, & Illustrations, A progressive course of study for engineers, electricians, students and those desiring to acquire a working knowledge of electricity and its applications

by N. Hawkins

Fig. 2,559.—Diagram showing relative direction of current, magnetic flux, and motion of disc in Faraday disc, or mercury motor ampere hour meter.

Ques. Explain its operation.

Ans. The electric current, as shown in fig. 2,558, enters the contact C, passes through the comparatively high resistance mercury H to the edge of the low resistance copper disc D across the disc to the mercury H and out of contact C'. The magnetic flux cuts across the disc on each side from N to S, making a complete circuit through M and M'. The relative directions of the magnetic flux and the current of electricity as well as the resulting motion are shown in fig. 2,559. According to the laws of electromagnetic induction, if a current carrying conductor cut a magnetic field of flux at right angles, a force is exerted upon the conductor, tending to push it at right angles to both the current and the flux. When connected to an eddy current damper or generator which requires a driving force directly proportional to the speed of rotation, the mercury motor generator becomes a meter. The speed of such a meter is a measure of the current or rate of flow of the electricity through the motor element, and each revolution of the motor corresponds to a given quantity of electricity. Then, by connecting a revolution counter to this motor generator, a means is provided for indicating the total quantity of electricity in ampere hours that is passed through the meter.

Fig. 2,560.—Sectional view of Faraday disc or mercury motor ampere hour meter as made by Sangamo Electric Co. The illustration does not show the magnets and indicating mechanism.

Ques. How is the flux produced in the alternating current form of Faraday disc meter?

Ans. By the secondary current of a series transformer.

Frequency Indicators.—A frequency indicator or meter is an instrument used for determining the frequency, or number of cycles per second of an alternating current. There are several forms of frequency indicator, whose principle of operation differs, and according to which, they may be classed as