_C._ the _compound wound dynamo_. This dynamo has both shunt and series coils upon its fields. (See Fig. 287.) If more current is drawn into the main circuit with this dynamo, the series coils produce a stronger field compensating for the weaker field of the shunt coils, so that uniform voltage is maintained. The compound wound generator is therefore the one most commonly employed. Important Topics 1. Laws of electromagnetic induction (a) conditions, (b) E.M.F., (c) direction. 2. Devices, (a) magneto, (b) dynamo: series, shunt, compound. 3. Illustrations of the laws. Exercises 1. Under what conditions may an electric current be produced by a magnet? 2. Show how Lenz's Law, follows from the principle of conservation of energy. 3. A bar magnet is fixed upright with its north-seeking pole upward. A coil is thrust down over the magnet. What is the direction of the current induced in the coil? Explain. 4. In what two ways may a current be induced in a closed coil? 5. What method is employed in the magneto? In the dynamo? 6. What is the nature of the current produced in the armature coil of a magneto, that is, is it direct or alternating? Why? 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. [Illustration: FIG. 288.--Several methods of arranging the field coils and the armature of a dynamo.] [Illustration: FIG. 289.--A drum armature.]