It has already been explained that wires offer resistance to an electrical current, and that some of the energy is lost in passing through a wire because of this resistance. Small wires possess more resistance than large ones, and if small wires are to be used, in order to save on the cost of the transmission line, the loss of energy will be greater, necessitating some method of partially reducing or overcoming this fault. In order to explain clearly how the problem is solved, the electric current may for the moment be compared to a stream of water flowing through a pipe. [Illustration: Fig. 175.—Comparison between Electric Current and Flow of Water.] The illustration shows two pipes, a small one and a large one, each supposed to be connected to the same tank, so that the pressure in each is equal, and it is clearly apparent that more water will flow out of the large one than out of the small one. If ten gallons of water flow out of the large pipe in one minute, it may be possible that the comparative sizes of the pipes are such that only one gallon of water will flow out of the small one in the same length of time. But in case it should be necessary or desirable to get ten gallons of water a minute out of a small pipe such as _B_, what could be done to accomplish it? The pressure could be increased. The water would then be able better to overcome the resistance of the small pipe. This is exactly what is done in the distribution of electric currents for power and lighting. The pressure or potential is increased to a value where it can overcome the resistance of the small wires. But unfortunately it rarely happens that electrical power can be utilized at high pressure for ordinary purposes. For instance, 110 volts is usually the maximum pressure required by incandescent lamps, whereas the pressure on the line wires issuing from the power-house is generally 2,200 volts or more. 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.