We learn, therefore, that the greater the speed of the motor, the less the current passing through it. When an armature is set into rotation by sending a current from some outside source through it, its coils move through a magnetic field as truly as if the rotation were produced by a steam engine, as is the case in running a dynamo. An induced E.M.F. is therefore set up by this rotation. In other words, while the machine is acting as a motor it is also acting as a dynamo. The direction of the induced E.M.F. due to this dynamo effect will be seen, from Lenz's law or from a consideration of the dynamo and motor rules, to be opposite to the outside P.D., which is causing current to pass through the motor. The faster the motor rotates, the faster the lines of force are cut, and hence the greater the value of this so-called back E.M.F. If the motor were doing no work, the speed of rotation would increase until the back E.M.F. reduced the current to a value simply sufficient to overcome friction. It will be seen, therefore, that, in general, the faster the motor goes, the less the current which passes through its armature, for this current is always due to the difference between the P.D. applied at the brushes 500 volts in the case of trolley cars and the back E.M.F. When the motor is starting, the back E.M.F. is zero; and hence, if the full 500 volts were applied to the brushes, the current sent through would be so large as to ruin the armature through overheating. To prevent this motors are furnished with a starting box, consisting of resistance coils which are thrown into series with the motor on starting, and thrown out again gradually as the speed increases and the back E.M.F. rises.* Trolley cars are usually run by two motors which, on starting, work in series, so that each supplies a part of the starting resistance for the other. After speed is acquired, they work in parallel. This is a more economical method than starting-box control. * This discussion should be followed by a laboratory experiment on the study of a small electric motor or dynamo. See, for example, Experiments 48 and 49 of "Exercises in Laboratory Physics," by Millikan, Gale, and Davis. 362. The recording watt-hour meter. The recording watthour meter (Fig. 324) is the instrument which fixes our electric-light bills. It is essentially an electric motor containing no iron, so that the current through the armature A is proportional to the P.D. between the mains, while the current through the field magnets F is the current flowing into the house. Therefore the force acting between A and F, or the turning power on A (torque), is proportional to the product of volts by amperes; that is, it is proportional to the watts consumed. The rate of rotation is made slow by the magnetic drag due to the reaction between the magnets M and the current induced in the rotating aluminum disk D which rotates between the poles of the magnets. The recording dials, which are connected to the worm gear G, have therefore a speed which is proportional to the watts used, and their total rotation is proportional to the total energy, or watt hours, consumed. (Let the pupil examine the watt-hour meter in his home.) M To the To the M house -wires mains: FIG. 324. Interior of a watt-hour meter SUMMARY. A dynamo generates E.M.F. through mutual cutting of conductors and lines of magnetic force. It transforms mechanical energy into electrical energy. An electric motor is essentially the same in construction as a dynamo. It transforms electrical energy into mechanical work. The commutator of a dynamo is for the purpose of allowing the alternating current in the armature to come as a direct current into the external circuit. Back E.M.F. is always generated in a motor in accord with Lenz's law. A watt-hour meter is constructed to rotate at a speed proportional to the kilowatts, or power, used. It can, therefore, be calibrated in kilowatt hours (energy consumed). QUESTIONS AND PROBLEMS 1. (1) What are the essential parts of an alternating-current generator? (2) Describe its operation. (3) Does it create electrical energy? (4) Give reason for your answer. 2. What is the function (use) of the field magnet of a dynamo? Wood is cheaper than iron; why are not the field cores made of wood? 3. Will it take more work to rotate a dynamo armature when the circuit is closed than when it is open? Why? 4. Explain how an alternating current in the armature is transformed into a unidirectional current in the external circuit. 5. What is the essential difference in construction between a direct-current and an alternating-current dynamo? 6. Two successive coils on the armature of a multipolar alternator are cutting lines of force which run in opposite directions. How does it happen that the currents generated flow through the wires in the same direction? (Fig. 315.) 7. Explain the process of building up in a dynamo. 8. How would it affect the voltage of a dynamo to increase the speed of rotation of its armature? Why? to increase the number of turns of wire in the armature coils? Why? to increase the strength of the magnetic field? Why? 9. When a wire is cutting lines of force at the rate of 100,000,000 per second, there is induced in it an E.M.F. of one volt. A certain dynamo armature has 50 coils of 5 loops each and makes 600 revolutions per minute. Each wire cuts 2,000,000 lines of force twice in a revolution. What is the E.M.F. developed? 10. Make a diagram of the two wires leading out from a generator and indicate the usual manner of attaching commercial incandescent lamps to these wires. 11. Single dynamos often operate as many as 10,000 incandescent lamps at 110 volts. If these lamps are all arranged in parallel and each requires a current of .5 ampere, what is the total current furnished by the dynamo? What is the activity of the machine in kilowatts and in horse power? 12. How many 110-volt lamps like those of problem 11 can be lighted by a 12,000-kilowatt generator? 13. Why does it take twice as much work to keep a dynamo running when 1000 lights are on the circuit as when only 500 are turned on? 14. A direct-current generator operating at 500 volts furnishes to a factory a current of 50 amperes through a line having a resistance of 2 ohms. (1) How much power is developed by the generator? (2) At what voltage does the factory receive its current? 15. What does the commutator of a dynamo do? What is the purpose of the commutator of a motor? 16. Explain why a series-wound motor can run on either a direct or an alternating circuit. 17. An ammeter in circuit with a small motor indicates 7 amperes when the motor is starting and 3.5 amperes when the motor is running at full speed. Explain. 18. If the pressure applied at the terminals of a motor is 500 volts, and the back pressure, when running at full speed, is 450 volts, what is the current flowing through the armature, its resistance being 10 ohms? 19. An electric motor developed 2 H.P. when taking 16.5 amperes at 110 volts. Find the efficiency of the motor. (One horse power = 746 watts.) 20. The resistance of the wire connecting a generator to a motor is .05 ohm; the generator can deliver 200 amperes. (1) What is the fall in potential between the generator and the motor? (2) How many watts are expended in sending the current through the wire? 21. An electric motor having an efficiency of 85 per cent develops 3 H. P. when connected to a 220-volt circuit. How much current flows through the motor? 22. Name two uses and two disadvantages of mechanical friction; of electrical resistance. PRINCIPLE OF THE INDUCTION COIL AND TRANSFORMER 363. Currents induced by varying the strength of a magnetic field. Let about 500 turns of No. 28 copper wire be wound around one end of an iron core, as in Fig. 325, and connected to the circuit of a galvanometer G. Let about 500 more turns be wrapped about another portion of the core and connected into the circuit of two dry cells. When the key K is closed, the deflection of the galvanometer will indicate that a temporary current has been induced in one direction through the coils; and when it is opened, an equal but opposite deflection will indicate an equal current flowing in the opposite direction. The experiment illustrates the principle of the induction coil and the transformer. The coil p, which is connected to the source of the current, is called the primary coil, and the coil s, in which the currents G FIG. 325. Induction of current by magnetizing and demagnetizing an iron core are induced, is called the secondary coil. Causing lines of force to spring into existence inside of s (in other words, magnetizing the space inside of s) has caused an induced current to flow in s; and demagnetizing the space inside of s has also induced a current in s in accordance with the general principle stated in § 349, that any change in the number of magnetic lines of force which thread through a coil induces a current in the coil. We may think of the lines as always existing as closed loops (see Fig. 262, p. 275) which collapse upon demagnetization to mere double lines at the axis of the coil. Upon magnetization one of these two lines springs out, cutting the encircling conductors and inducing a current. 364. Direction of the induced current. Lenz's law, which, it will be remembered, followed from the principle of conservation of energy, enables us to predict at once the direction of the induced currents in the above experiments; and an observation of the deflections of the galvanometer enables us to verify the correctness of the predictions. Consider first the |