Electric currents supply light for our homes and factories, heat for all kinds of electric devices, power for industrial purposes, and so on. Both the transmitting and receiving of radio communications depend on electricity. The flow of electrical charge can be used to great advantage power distribution, for power can be generated wherever suitable and used wherever required, even hundreds of kilometres away from the point of generation.
There are many thousand ways of using electric circuits and we are certainly unable to list them here.
In fact, whether our problem is to flash a minute flashlight or to supply electrical energy to a great factory, we make use of an electric circuit. One must know beforehand whether the current flowing through the circuit is sufficient for performing the required work. In addition to that it is necessary to know how to control the flow of the electric current so that it were suitable for running the machinery, without being too great or damaging.
Suppose we want a flashlight to be operated with a dry battery. Passing a wire from the positive terminal of the battery to the lamp and another wire back from the lamp to the negative terminal of the battery makes a completed or closed circuit. If the battery voltage is great enough to force sufficient current through the circuit, the flashlight will burn. It is certainly impossible to know whether the lamp will light properly unless we know the amount of current.
But first we have an even simpler problem of control, that of switching the current "on" and "off". Unless we break the circuit in some manner, the flashlight will be almost useless, because it will burn all the time instead of its burning only when needed. Besides, the lamp would burn out in no time. To break the circuit we cut either wire and insert a push button. This device may consist of two brass strips which touch each other only when the button is pressed. At other times, that is to say, when the button is not pressed, the two brass strips do not touch, so that the circuit is open and no current flows. The push button is a form of switch. Switches of various designs are generally used for opening or closing circuits. We use them, for instance, to turn the light on or off. The circuit principles would not be changed, of course, by replacing the battery with some other source of electricity or by replacing the electric lamp with any other device using electric energy.