_A transformer_ is an induction coil employed usually for lowering electric pressure, but it may also be used for raising the same, in which case it is sometimes called a _booster_. _A compensator_ is a transformer which works automatically. [Illustration: FIG. 231.] [Illustration: FIG. 232.] _Ammeters_ record _the quantity of current flowing through the circuit, in amperes_. _Voltmeters_ record _the pressure_ or strength of the current in _volts_. An _Ampere_ is an electric current which would pass through a circuit whose resistance is one ohm under an electro-motive force of one volt. A _Volt_ is an electro-motive force of sufficient strength to cause _a current of one ampere to flow against a resistance of one ohm_. The _ampere_ is the unit for calculations relating to _the quantity_ or volume of a current; the _volt_ is the unit for calculating the _pressure_ or strength of the current. _The action of the electric current_ in producing rotation in an electric motor is really quite simple. While many electrical problems are comparatively complicated, the principal elements in the operation of electric motors may be readily understood. The fundamental fact in this connection is the relation between an electric current and a magnet. If a piece of round bar iron be surrounded by a coil through which an electric current passes, it becomes a magnet. In Fig. 233 the passage of a current through the coil of wire around the iron bar in either direction, _renders the iron a magnet_, with all its well-known properties. It will attract iron, and the space surrounding it becomes magnetic. Iron filings will arrange themselves in the direction shown by the dotted lines in the figure. One end of the magnet is the North or positive + pole and the other the South or negative - pole. If a wire, such as _CD_, be moved past either pole of the magnet, there will be a tendency for current to flow in the wire either from _C_ to _D_ or _D_ to _C_, according to the character of the pole past which it is moved, and to the direction of the movement. If the ends of the wire _CD_ are joined by a conductor, so that there is a complete circuit, a current of electricity will flow through this circuit. This circuit may be a simple wire, as shown by the line _CEFD_, or it may be the wire coils on machines enabling the current to produce mechanical work, or it may be electric lamps producing light. The indispensable feature is that there shall be a complete unbroken circuit from _C_ to _D_ for the current to flow, no matter how complicated or how long this circuit may be.