A flow of water is measured in gallons per minute. An electrical current is measured in coulombs per second. Thus the coulomb is the electrical counterpart of the gallon. But in this particular we differ slightly in our methods of talking of water and electricity. Gallons per minute or per hour is the invariable term in the former case, but in the latter we do not speak of coulombs per second, although that is what we mean, for we have a special name for one coulomb per second, and that same is ampere. One ampere is one coulomb per second, two amperes are two coulombs per second, and so on. There is no recognised term to denote the resistance which a water-pipe offers to the passage of water through it, but in the similar case with electricity there is a term specially invented for the purpose, the ohm. Legally it is the resistance of a column of mercury of a certain size and weight. A rough idea of it is given by the fact that a copper wire a sixteenth of an inch thick and 400 feet long has a resistance of about one ohm. The three units--the volt, ampere and ohm--are so related that a pressure of one volt acting upon a circuit with a resistance of one ohm will produce a current of one ampere. A current can do work; when it lights or heats your room or drives a tramcar it is doing work; and the rate at which a current does work is found by multiplying together the number of volts and the number of amperes. The result is in still another unit, the watt. And 1000 watts is a kilowatt. Finally, to crown the whole story, a kilowatt for one hour is a Board of Trade unit. So for every unit which you pay for in the quarterly bill you have had a current equal to 1000 watts for an hour. To give a concrete example, if the pressure of your supply is 200 volts, and you take a current of five amperes for an hour, you will have consumed one B.T.U. Perhaps it will give added clearness to this explanation to tabulate the terms as follow:-- _Volt_ = The unit of pressure, analogous to "pounds per square inch" in the case of water. _Coulomb_ = The measure of quantity, analogous to the gallon. _Ampere_ = The measure of the "strength" of a current, meaning one coulomb per second. _Watt_ = The unit denoting the power for work of any current. It is the result of multiplying together volts and amperes. _Kilowatt_ = 1000 watts. _Board of Trade Unit_ = A current of one kilowatt flowing for one hour. [Illustration: _By permission of Dupont Powder Co._ A FINE CROP Celery grown on soil tilled by dynamite.--_See_ p. 24]