In Fig. 163 the ends of the coil are joined to the two halves of a cylinder. These halves, X and Y, are insulated from each other and from the axis. The current flows from X onto the brush A, through some external circuit where it does work, and thence back through brush B onto Y. By the time that Y gets around to A the direction of the current in the loop has reversed, so that it passes towards Y; but it still enters the outside circuit through A because Y is then in contact with A. This device is called a _commutator_, and it allows a constant or direct current to leave the machine. In regular machines there are many loops of wire and several segments to the commutator. The rotating coils are wound upon an iron core, so that the lines of force, in passing from one pole to the other, will meet with as little resistance as possible. The coils, core, and commutator, taken together, are called the _armature_. The magnets which furnish the field are called the _field-magnets_. These are electromagnets, the current from the dynamo, or a part of it, being used to excite them. There are many forms of dynamos, and many ways of winding the armature and field-magnets, but space will not permit a discussion of them here. (See "Things a Boy Should Know About Electricity.") _=477. The Electric Motor.=_ Experiments have shown that motion can be produced by the electric current in many ways. The galvanoscope may be considered a tiny motor. _An electric motor is a machine for transforming electric energy into mechanical power._ While the electric motor is similar in construction to the dynamo, it is opposite to it in action. Motors receive current and produce motion. The motion is a rotary one, the power being applied to other machines by means of belts or gears. [Illustration: Fig. 164.] =EXPERIMENT 200. To study the action of the telephone.= =478. Directions.= (A) Join the ends of coil H (Fig. 164) to the astatic galvanoscope. Move magnet M back and forth in front of the soft iron core, while H is held in position. Watch the needle. Imagine that vibrations in the air caused by the voice are strong enough to give M a slight motion to and fro, and you can see how a current would be sent through the galvanoscope by speaking against M. _=479. The Telephone=_ is an instrument for reproducing sounds at a distance, and electricity is the agent by which this is generally accomplished. The part spoken to is called the _transmitter_, and the part which gives the sound out again is called the _receiver_. Sound itself does not pass over the line. Although the same apparatus may be used for both transmitter and receiver, they are generally different in construction. [Illustration: Fig. 165.]