ELEMENTARY THEORY OY ELECTROMAGNETISM. 73 



rate at which the generator delivers work to the circuit, (b) 

 Find the dragging force in volts which is opposing the flow of the 

 current through the circuit due to the resistance of the circuit. 

 (c) Find the rate at which work is dissipated in overcoming the 

 resistance of the circuit, (d) Explain what is becoming of the 

 difference between (a) and (c). Ans. (a) 150 watts. (6) 30 

 volts, (c) 90 watts, (d) It is being stored as increasing kinetic 

 energy of the circuit. 



14. The field winding of a dynamo has 50 ohms resistance 

 and, approximately, 7.5 henrys of inductance. Assuming that the 

 current grows in the coil in accordance with equation (4) of Art. 36 

 calculate the time required for the current in the winding to 

 reach 2 amperes, when the winding is connected to a generator of 

 which the electromotive force is no volts. Ans. 0.359 second. 



15. A current has been left to die away in a circuit of 0.6 ohm 

 resistance and 0.05 henry inductance. Find the rate of change 

 of the current as it passes the values 100 amperes, 10 amperes, 

 and one ampere. Ans. 1,200 amperes per second, 120 

 amperes per second, 12 amperes per second. 



1 6. Find the approximate inductance in henrys of a cylindrical 

 coil 25 centimeters long, 5 centimeters mean diameter, wound 

 with one layer of wire containing 150 turns. Ans. 0.000222 

 henry. 



17. The choke coil of a lightning arrester consists of 50 turns 

 of wire wound in one layer on a cylinder of which the diameter is 

 15 centimeters and the length is 50 centimeters, (a) Calculate 

 the approximate inductance of this coil, (b) Calculate the ap- 

 proximate rate of increase of current in the coil at the instant 

 that a lightning discharge jumps across two centimeters of air 

 in preference to going through the coil. Ans. (a) o.oooin 

 henry, (b) 360,000,000 amperes per second. 



Note. The electromotive force required to strike across 2 centimeters of air 

 is approximately 40,000 volts. 



1 8. The field coil of a dynamo has 5,000 turns of wire and, 

 when a current of one ampere flows through the field winding, 



