A transformer is a simple device composed of two coils of wire wound on a closed core of iron. The coil into which is sent the inducing current is the primary. That in which the current is induced is the secondary coil. The change in voltage between primary and secondary coils vary as the number of turns of wire which compose the coils. The house circuit may be stepped down from the customary 110 volts to a voltage such as is furnished by a single dry cell, or a battery of cells. In principle, the action of the transformer is the same as that of the induction coil, a detailed explanation of which will be found in any text-book of physics. Each impulse of current in the primary coil of the transformer magnetizes its core and the magnetism thus excited induces a corresponding current in the secondary coil. Since alternating current in the primary coil constantly changes the polarity of the core, each change of magnetism induces current in the secondary coil. Small transformers are frequently used for operating doorbells, annunciators, etc., in place of primary batteries. These transformers are also used to supply current for lighting low-power tungsten lamps that cannot be used with the ordinary voltages employed in house lighting. The primary wires of the transformer are attached to the service wires in the house and from the secondary wires voltages are taken to suit the desired purposes. [Illustration: FIG. 250.--Doorbell transformer.] [Illustration: FIG. 251.--Details of doorbell transformer.] Fig. 250 shows such a transformer with the cover partly broken away to expose the interior construction. The wires from house mains _MM_ lead the current to the primary coil _P_ which is a large number of turns of fine wire wound about a soft-iron core. The induced current in the secondary coil _S_ is taken from the contact points 1, 2, 3 and 4. The construction of the transformer coils shown in Fig. 250 indicates the primary wires at _LL_ of Fig. 251. The wires of the primary coil are permanently attached to house wires. The reactive effect of the magnetism in the coil permits only enough current to flow as will keep the core excited. This is a step-down transformer and the secondary coil contains fewer turns of wire than the primary coil. Since the voltage induced in the secondary coil is determined by the number of turns of wire in action, this coil is so arranged that circuits formed by attachment with different contacts give a variety of voltages. The numbers on the front of Fig. 250 correspond to those of Fig. 251. The coils between contact 1 and the others 2, 3 and 4, represent different number of turns of wire and in them is induced voltages corresponding with the number of turns of wire in each.