THE WATTMETER. 41 



The average mutual force would produce a steady deflection 

 of the movable coil and is measured by the angle through which 

 the torsion head is rotated to keep the two coils at right angles to 

 each other. 



If 9 is the angle through which the torsion head is rotated, the 

 power, P, is given by the equation 



where k is a constant which has to be determined experimentally. 



The unit of power the watt is equal to 10 7 ergs per second. 



We shall deal further with this in Chap. XVI I, which treats 

 of various methods of measuring power. 



PROBLEMS ON CHAPTER V. 



1. What energy is absorbed in five minutes by a coil of resistance 5 ohms 

 and self-induction 0.05 henry if 100 volts is applied between its terminals, the 

 frequency being 80 periods per second ? 



Answer. 228 x 10 9 ergs. 



2. What is the power given to a series circuit whose self-induction is 0.05 

 henry, capacity 1 microfarad, and resistance 10 ohms, when a P.D. of 100 volts 

 at a frequency of 50 periods per second is applied between its terminals ? 



Answer. 0.01 watt. 



