The balancers serve the purpose of balancing the voltage on each side, and they are machines capable of acting either as motors or dynamos. In order to comply with Board of Trade regulations, electric appliances of all kinds intended for ordinary domestic purposes, including lamps, and heating and cooking apparatus, are supplied with current at a pressure not exceeding 250 volts. In a system such as we are describing, all these appliances are connected between the mid-wire and one or other of the outer wires, thus receiving current at 220 volts. In practice it is impossible to arrange matters so that the lamps and other appliances connected with the positive side of the system shall always take the same amount of current as those connected with the negative side, and there is always liable to be a much greater load on one side or the other. If, for instance, a heavy load is thrown on the negative side, the voltage on that side will drop. The balancer on the positive side then acts as an electric motor, drives the balancer on the negative side as a dynamo, and thus provides the current required to raise the voltage on the negative side until the balance is restored. The working of the balancers, which need not be described in further detail, is practically automatic. Electric motors, for driving electric trams or machinery of any kind, are connected between the outer wires, so that they receive the full 440 volts of the system. In any electric supply system the demand for current does not remain constant, but fluctuates more or less. For instance, in a system including an electric tramway, if a car breaks down and remains a fixture for a short time, all cars behind it are held up, and a long line of cars is quickly formed. When the breakdown is repaired, all the cars start practically at the same instant, and consequently a sudden and tremendous demand for current is made. In a very large tramway system in a fairly level city, the fluctuations in the demand for current, apart from accidents, are not very serious, for they tend to average themselves; but in a small system, and particularly if the district is hilly, the fluctuations are very great, and the current demand may vary as much as from 400 to 2000 amperes. Again, in a system supplying power and light, the current demand rises rapidly as the daylight fails on winter afternoons, because, while workshop and other motors are still in full swing, thousands of electric lamps are switched on more or less at the same time. The power station must be able to deal with any exceptional demands which are likely to occur, and consequently more current must be available than is actually required under average conditions. Instead of having generating machinery large enough to meet all unusual demands, the generators at a station using continuous current may be only of sufficient size to supply a little more than the average demand, any current beyond this being supplied by a battery of storage cells.