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

When the magnet is excited by alternating current, or interrupted direct current, the armature is set in vibration, and that gives a slight movement to the base plate at right angles to its axis, thereby affecting all the reeds, especially those which are almost in tune with its vibrations.

The reed which is in tune will vibrate through an arc of considerable amplitude, and so indicate the frequency of the exciting current.

Ques. For what use is the resonance type of frequency meter most desirable?

Ans. For laboratory use.

Fig. 2,567.—Westinghouse induction type frequency meter. The normal frequency is usually at the top of the scale to facilitate reading. The damping disc moves in a magnetic field, thus damping by the method of eddy currents. The standard meters are designed for circuits of 100 volts nominal and can be used for voltages up to 125 volts. For higher voltages, transformers with nominal 100 volt secondary should be used.

Ques. Describe the induction type of frequency meter.

Ans. It consists of two voltmeter electromagnets acting in opposition on a disc attached to the pointer shaft. One of the magnets is in series with an inductance, and the other with a resistance, so that any change in the frequency will unbalance the forces acting on the shaft and cause the pointer to assume a new position, when the forces are again balanced. The aluminum disc is so arranged that when the shaft turns in one direction the torque of the magnet tending to rotate it decreases, while the torque of the other magnet increases. The pointer therefore comes to rest where the torques of the two magnets are equal, the pointer indicating the frequency on the scale.

Fig. 2,568.—Langsdorf and Begole frequency meter. The operation of this meter is based on the fact that if an alternating pressure of E Volts be impressed on a condenser of capacity C, in farads, the current in amperes will be equal to 2π ~ EC, provided the pressure be constant. In construction, the scale is mounted on the same axis as the pressure coil, across the mains so as to render the instrument independent of variation of voltage. For a discussion of this meter, see Electrical Review, vol. LVIII, page 114.