The water contained within the tank represents its pressure at the outlet of the pipe. The current in volts, generated in a battery or dynamo, represents its pressure on an outlet or conductor wire; and both represent the force behind their respective conductors. The valve, or faucet, at the end of the pipe plus the friction in the pipe would represent the resistance to the flow of water, while the resistance-coils or other mediums plus the size of the wire, or conductor and switch, would regulate the flow of electric current. The flow of water in a pipe under certain pressure would represent its gallons per minute or hour, while with electricity its flow in a wire or other conductor would represent its amperage. It is to govern the flow of current that resisting mediums are employed. The resistance of electric current is measured in ohms, and it is with this phase that we are interested in this chapter. If there is only a small resistance put in the path of a current, then it requires but a small pressure or voltage to send it through the wires or circuit. This is easily understood by the boy who has experimented with small incandescent lamps in which short pieces of carbon-filament are contained. It requires the pressure of a few volts only to send the current through the carbon; but for the large carbon-filaments, measuring ten or twelve inches in length, from one hundred to five hundred volts may be necessary. The ordinary house lamps require one hundred and ten volts and half an ampere to give sixteen candle-power. It is easily understood, then, that it requires a high pressure or voltage to force the current through the resisting carbon-filament, or across the space from one carbon to the other in the arc-lamps used for street lighting. The shorter and larger the conducting wires the less the resistance, and consequently the lower the voltage or pressure necessary to force it. Contrariwise the longer and finer the conducting wares, the greater the resistance. As copper is the best commercial conductor of electric currents, it is in universal use, and in it the minimum of resistance is offered to the current. Iron wire is a poorer conductor, and is not used for high voltage (such as trolleys or transmission of power), but is confined to telegraph and telephone lines and low-pressure work. German-silver wire, one of the poorest conductors, is not used for lines at all, but is employed solely as a resisting medium for controlling current. Ohm’s Law This is the fundamental formula expressing the relations between current, electro-motive force, and resistance in an active electric circuit. It may be expressed in several ways with the same result, as follows: 1. The current strength is equal to the E. M. F. (electro-motive force) divided by the resistance. 2. The E. M. F. (electro-motive force) is equal to the current strength multiplied by the resistance.