In surveying for water-power, in Chapter III, we found that the number of gallons or cubic feet of water alone did not determine the amount of power. We found that the number of gallons or cubic feet multiplied by the distance in feet it falls in a given time, was the determining factor--pounds (quantity) multiplied by feet per second--(velocity). [Illustration: Showing the analogy of water to volts and amperes of electricity] The same is true in figuring the power of electricity. We multiply the _amperes_ by the number of electric impulses that are created in the wire in the course of one second. The unit of velocity, or pressure of the electric current is called a _volt_. Voltage is the pressure which causes electricity to flow. A volt may be likened to the velocity in feet per second of water in falling past a certain point. If you think a moment you will see that this has nothing to do with quantity. A pin-hole stream of water under 40 pounds pressure has the same velocity as water coming from a nozzle as big as a barrel, under the same pressure. So with electricity under the pressure of one volt or one hundred volts. One volt is said to consist of a succession of impulses caused by _one wire cutting 100,000,000 lines of magnetic force in one second_. Thus, if the strength of a magnet consisted of one line of force, to create the pressure of one volt we would have to "cut" that line of force 100,000,000 times a second, with one wire; or 100,000 times a second with one thousand wires. Or, if a magnet could be made with 100,000,000 lines of force, a single wire cutting those lines once in a second would create one volt pressure. In actual practice, field magnets of dynamos are worked at densities up to and over 100,000 lines of force to the square inch, and armatures contain several hundred conductors to "cut" these magnetic lines. The voltage then depends on the speed at which the armature is driven. In machines for isolated plants, it will be found that the speed varies from 400 revolutions per minute, to 1,800, according to the design of dynamo used. [Illustration: Pressure determines volume of flow in a given time] Multiplying amperes (strength) by volts (pressure), gives us _watts_ (power). Seven hundred and forty-six watts of electrical energy is equal to one horsepower of mechanical energy--will do the same work. Thus an electric current under a pressure of 100 volts, and a density of 7.46 amperes, is one horsepower; as is 74.6 amperes, at 10 volts pressure; or 746 amperes at one volt pressure. For convenience (as a watt is a small quantity) electricity is measured in _kilowatts_, or 1,000 watts. Since 746 watts is one horsepower, 1,000 watts or one kilowatt is 1.34 horsepower. The work of such a current for one hour is called a _kilowatt-hour_, and in our cities, where electricity is generated from steam, the retail price of a kilowatt-hour varies from 10 to 15 cents.