_=475. Transformers=_, like induction coils, are instruments for changing the E. M. F. and strength of currents. There is very little loss of energy in well-made transformers. They consist of two coils of wire on the same core; in fact, an induction coil may be considered a transformer. If the secondary coil has 100 times as many turns of wire as the primary, a current with an E. M. F. of 100 volts can be taken from the secondary coil, when the E. M. F. of the current passing through the primary is 1 volt; but the _strength_ (amperes) of the secondary current will be but one-hundredth that of the primary current. By using the coil of fine wire as the primary, the E. M. F. of the current that comes from the other coil will be but one-hundredth that in the fine coil. It will have 100 times its strength, however. Continuous currents from cells or dynamos must be interrupted, as in induction coils, to be transformed from one E. M. F. to another. Transformers are now largely used in lighting and power circuits, etc. (See "Things A Boy Should Know About Electricity.") _=476. The Dynamo.=_ We saw in the Exps. of Chapter XXV. that currents of electricity can be generated in a coil of wire (closed circuit) by rapidly moving it through the field of a magnet. As shown by the experiments, this can be accomplished in many ways. The dynamo is a machine for doing this on a large scale, the coils being given a rotary motion in a very strong magnetic field; and as the number of lines of force that cut the coil is constantly changing, there is a current in the coil as long as power is applied, and this current is led from the machine by proper devices. _The dynamo is a machine for converting mechanical energy into an electric current, through electromagnetic induction._ If a loop of wire (Fig. 161) be so arranged on bearings at its ends that it can be made to revolve, a current will flow through it in one direction during one-half of the revolution, and in the opposite direction during the other half, it being insulated from all external conductors. Such a current inside of the machine would be of no value; it must be led out to external conductors. Some sort of sliding contact is necessary to connect a revolving conductor with a stationary one. [Illustration: Fig. 161.] [Illustration: Fig. 162.] Fig. 162 shows the ends of a coil joined to two rings, X, Y, which are insulated from each other, and which rotate with the coil. Two stationary pieces of carbon, A, B, called _brushes_, press against the rings, and to these are joined wires which complete the circuit, and which lead out where the current can do work. The arrows show the direction of the current during one-half of a revolution. The rings form a _collector_, and this arrangement gives an alternating current. [Illustration: Fig. 163.]