7. What is the resistance of a 20-watt tungsten lamp if the E.M.F. is 115 volts?
8. Find the resistance of a 40-watt tungsten lamp when the voltage is 115? How much heat will it produce per minute?
9. An Edison storage battery cell on a test gave a discharge of 30 amperes. The average voltage was 1.19. What was the resistance of the cell?
10. Eight storage cells are connected in series. Each has an E.M.F. of 1.2 volts and an internal resistance of 0.03 ohms. What will be the current flowing through a voltmeter having 500 ohms resistance in circuit with them?
(2) The Dynamo and the Motor
302. The Dynamo may be defined as a machine for transforming mechanical energy into the energy of electric currents by electromagnetic induction. Although electromagnetic induction was discovered in 1821, practical dynamos were not built for about 40 years or until between 1860 and 1870. The great development in the production and use of electric currents has come since the latter date. The principle parts of the dynamo are (a) the field magnet, (b) the armature, (c) the commutator or collecting rings, (d) the brushes. Fig. 288 shows several common methods of arranging the field coils and the armature.
Fig. 288.—Several methods of arranging the field coils and the armature of a dynamo.
