Such a high voltage is hard to insulate, and would kill most people coming into contact with the lines, and is otherwise dangerous.
Before the current enters a house, therefore, some apparatus is necessary, which is capable of reducing this high pressure to a value where it may be safely employed.
This is the duty performed by the "transformer" enclosed in the black iron box fastened on the top of the electric light poles about the streets.
If a transformer were to be defined it might be said to be a device for changing the voltage and current of an Alternating circuit in pressure and amount.
The word, alternating, has been placed in italics because it is only upon alternating currents that a transformer may be successfully employed. Therein, also, lies the reason why alternating current is supplied in some cases instead of direct current. It makes possible the use of transformers for lowering the voltage at the point of service.
Many boys possessing electrical toys and apparatus operating upon direct current only, have bemoaned the fact that the lighting system in their town furnished alternating current. Very often in the case of small cities or towns one power-house furnishes the current for several communities and the energy has to be carried a considerable distance. Alternating current is then usually employed.
Fig. 176.—Alternating Current System for Light and Power.
The illustration shows the general method of arranging such a system. A large dynamo located at the power-house generates alternating current. The alternating current passes into a "step-up" transformer which raises the potential to 2,200 volts (approximately). It is then possible to use much smaller line wires, and to transmit the energy with smaller loss than if the current were sent out at the ordinary dynamo voltage. The current passes over the wires at this high voltage, but wherever connection is established with a house or other building, the "service" wires which supply the house are not connected directly to the line wires, but to a a "step-down" transformer which lowers the potential of the current flowing into the house to about 110 volts.
In larger cities where the demand for current in a given area is much greater than that in a small town, a somewhat different method of distributing the energy is employed.