[Illustration: FIG. 299.--View of a one-half horse-power motor.] If the current in the armature is reversed (in on the left and out on the right), the lines of force will crowd the armature around in the opposite direction or _counter clockwise_. The rotation of the armature will also be reversed if, while the current in the armature is unchanged in direction, the poles of the magnet are changed thus reversing the magnetic field. The motorman of a street car reverses the motion of his car by reversing the direction of the current in the _armature_ of the motor. [Illustration: FIG. 300.--The frame and electromagnet (at left), front bracket and brush holder (at right) of the motor shown in Fig. 299.] [Illustration: FIG. 301.--The armature of a motor.] =306. Practical motors= have many coils upon the armature with a corresponding number of segments upon the commutator. A large number of coils and commutator segments enables some one of the coils to exert its greatest efficiency at each instant, hence a steady force is provided for turning the armature which causes it to run smoothly. Fig. 299 represents a 1/2 horse-power motor ready for use while Fig. 300 shows the frame and poles and the front bracket and brush holder, and Fig. 301 represents the armature. Important Topics 1. The dynamo, four essential parts, action (a) for alternating currents, (b) for direct currents. 2. The electric motor: (a) essential parts, (b) action. Exercises 1. _Why_ is an alternating current produced in the armature of a dynamo? 2. _How_ is this current produced? Give careful explanations. 3. What is the result of Lenz's law as applied to the dynamo? 4. Apply the first two laws of electromagnetic induction to the dynamo. 5. What is the power of a dynamo if it produces 40 amperes of current at 110 volts? 6. How much power must be applied to this dynamo if its efficiency is 90 per cent.? 7. A motor takes 10 amperes of current at 220 volts; what is the _power_ of the current in _watts_? If this motor has an efficiency of 95 per cent., how many horse-power of mechanical energy can it develop? 8. Explain why reversing the current in the armature of a motor reverses the direction of rotation. 9. Find the cost of running a washing machine using a 1/2-horsepower motor 2 hours if the cost of the electricity is 10 cents a kilowatt hour. 10. A 1/8-horse-power motor is used to run a sewing machine. If used for 3 hours what will be the cost at 11 cents a kilowatt hour? (3) THE INDUCTION COIL AND THE TRANSFORMER