Fig. 250.—Doorbell transformer.
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.
The Recording Wattmeter.
—To determine the amount of electricity used by consumers, each circuit is provided with some form of wattmeter. These meters might be more correctly called watt-hour meters since they register the watt-hours of electrical energy that pass through the circuit.
Fig. 252.—Recording watt meter.