The reason for two kinds of electricity at all is that each, though obeying the larger laws, has its own peculiar habits and good points. For example, alternating current can be carried long distances at high pressure (high voltage) and side-tracked by a transformer to a little home and the pressure very simply reduced. In other cases the pressure can be very simply increased. Therefore in country districts one is very prone to see A. C. in vogue. The same amount of current, whether D. C. or A. C., is used for lighting, etc. A. C. is not used for electro-plating, etc., or for storage batteries. This is a good point to remember if you have storage batteries to supply for bells, etc., and your current is A. C. You will have to have your batteries charged from a plant which makes D. C. or install a small “converter.” If you attempt to use the A. C. you will burn out your plates. But how is electricity measured? How, in other words, do we know how much we use and how can we test our bills? The following paradigm will give the electric measures translated into more familiar terms of water measurement: Volt Pressure Ampere Rate of flow of current per second Watt Fraction of horsepower (H.P.) Kilowatt (1000 watts) 1¹⁄₃ H.P. Resistance Friction (as water resists the sides of a pipe.) Ohms (the unit of Friction (as water resists the sides of a measuring resistance). pipe). The volt takes its name from Volta, an Italian scientist; the ampere from a Frenchman, the ohm from a German, the watt from an Englishman. We hear most about volts and watts. Voltage is found by multiplying the ohms by the amperes. The volt is the pressure that makes electricity flow through the wire, and the friction of resistance to its flow is measured by the ohm. The amount of work a given number of amperes will do at a certain voltage (pressure) is known as watts. So if by chance you ever need formulæ here is a little one for your card catalogue: Ohms×amperes=volts. Volts÷ohm=ampere. Volts×ampere=watts. 1 Kilowatt=1000 watts. 1000 watts=1¹⁄₃ H. P. The next thing which is necessary for the householder to know is how to compute costs of electrical usage. The amount of electric power used, for example, by the electric light is measured in watts. Look on any incandescent bulb and you will see thereon the number of watts--usually around 50 or 60. In order to know how many watts a light consumes, divide the number of watts it consumes by 1000 to reduce it to a something of a kilowatt. Then multiply this result by the number of hours the lamp has been lit by the kilowatt to get the kilowatt hour of electricity. The kilowatt hour, of course, multiplied by the rate per kilowatt hour in your locality will give you the cost. The rate is always figured on the kilowatt hour.