Ans. It must overcome the friction of all the moving parts.
Ques. Is the friction constant?
Ans. No.
Figs. 2,531 to 2,533.—Diagram of electromagnetic circuit of Westinghouse induction type watt hour meter, and diagram showing rotation of field. The dotted lines show the main paths of the magnetic flux produced by the two windings, the directions, however, are constantly reversing owing to the alternations of the current in the coils. Denoting the shunt and series pole tips by the letters as shown, a clear statement of the relation of the fields for each quarter period may be given. The signs + and - represent the instantaneous values of the poles indicated. Thus, at one instant the shunt pole tips A, C, and A1 are maximum +, -, and +, respectively because the instantaneous value of the current is maximum, while the value of the series flux is zero. At ¼ period later the shunt current is zero, giving zero magnetic pressure at the pole tips, while the series current has reached a maximum value, giving maximum-and + at the pole tips B and D. At the next ¼ period the shunt current is again maximum, but in a direction opposite to what it was at the beginning, making the pole tips A, C, and A1 +, -, and +, respectively, while the series current again is zero, etc., the values for the complete cycle being given in fig. 2,533. It will be observed from the table that both the + and - signs move constantly in the direction from A1 to A, indicating a shifting of the field in this direction, the process being repeated during each cycle.
Ques. What provision is made to correct the error due to friction?
Ans. The meter is compensated by exciting an adjustable auxiliary field from the shunt or pressure circuit.
Ques. What is the construction of the generator?
Ans. In nearly all meters it consists of a copper or aluminum disc carried on the same shaft with the motor and rotated in a magnetic field of constant value.
Ques. How is the counter torque produced?