As already explained the volt is the unit of electric pressure which causes current to be sent through any circuit. The electric circuits of houses are intended to be under constant voltage--commonly 110 or 220--but the voltage may be any amount for which the generating system is designed. Independent lighting systems such as are used in house-lighting plants--to be described later--commonly employ 32 volts of electric pressure. Opposed to the effect of the volts of electromotive force is the resistance of the circuit, which is measured in ohms. Resistance has been called electric friction; it expresses itself as heat and tends to diminish the flow of current. Every circuit offers resistance depending on the length, the kind and the size of wire used. Since the wires of commercial lighting systems are made of copper, it can be said that the resistance of the circuit increases as the size of the conducting wire decreases. In large wires the resistance is small but as the size of the wire is reduced the resistance is increased. A long attachment cord of a flat-iron, may offer sufficient resistance to prevent the iron from heating properly. The ampere is the unit which measures the amount of current. The amperes of current determine the rate at which the electricity is being used in any circuit. The wires of a house must be of a size sufficient to carry the necessary current without heating. Any house wire which becomes noticeably warm is too small for the current it carries and should be replaced by one that is larger. The watt is the unit of electric quantity. The quantity of electricity being used in any circuit is the product of the volts of pressure and amperes of current flowing through the wires. The amount of current--in amperes--sent through the circuit is the direct result of the volts of pressure; the quantity of electricity is therefore the product of these two factors. A 25-watt lamp on a circuit of 110 volts uses 0.227 ampere of current. 25 watts = 110 volts × 0.227 amperes. Ten such lamps use 10 × 0.227 amperes = 2.27 amperes. The product of 110 volts and 2.27 amperes is 250 watts. In order to express quantity of energy, it is necessary to state the length of time the energy is to act and originally the watt represented the energy of a volt-ampere for one second. For commercial purposes this quantity is too small for convenient use and the hour of time was taken instead. The watt of commercial measurement is the watt-hour and in the purchase of electricity the watt is always understood as that quantity. Even as a watt-hour the measure is so small as to require a large number to express ordinary amounts and a still larger unit of 1000 watt-hours or the kilowatt-hour was adopted and has become the accepted unit of commercial electric measurement. Just as a dollar in money conveniently represents 1000 mills so does a kilowatt of electricity represent a convenient quantity.