158 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



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

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

 1 20 amperes per second, 12 amperes per second. 



108. 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. 



109. 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 1 5 centimeters and the length is 50 centimeters. () Calculate 

 the approximate inductance of this coil. ($) 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 pref- 

 erence to going through the coil. Ans. (a) o.ooom henry. 

 (<) 360,000,000 amperes per second. 



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

 approximately 40,000 volts. 



110. (a) Calculate the magnetic flux through the solenoid which 

 is specified in problem 1 1 1 when a current of 30 amperes flows 

 through it. (^) Calculate the value of the electrical momentum 

 of this current in flux-turns, (a) 4,440 maxwells, (fr) 666,000 

 flux-turns. 



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

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

 1,500,000 lines of force are produced through the field core. 

 Assuming that the flux is proportional to the exciting current, 

 find the inductance of the field coil in henrys. Ans. 75 henrys. 



112. A battery having an electromotive force of 10 volts and a 

 resistance of one ohm is connected to a coil of wire wound on an 

 iron core. The coil has 1,000 turns of wire and its resistance is 

 4 ohms. What is the current in the coil when the magnetic flux, 

 in the core is increasing at a rate of 500,000 lines per second ? 

 Ans. i ampere. 



