The science of controlling forces is so well understood and figured out that it becomes a simple mechanical proposition to adapt the various types of controllers to any form of power that may be employed. The tremendous force stored within the mechanism of a great transatlantic liner is governed by the twist of a man’s wrist. The locomotive that will pull a hundred cars loaded with coal, representing a weight of thousands of tons, is started and stopped by a short lever that is drawn in one direction or the other by a man’s hand. Great forces of all kinds are quite as easily controlled as the supply of gas through a jet--by simply turning the key that lets out so much as may be required, no matter what the pressure is back of the flow. This same principle applies to electricity, but the means of governing it is vastly different from the methods employed for other forces. Electricity is an unknown and unseen force, coming from apparently nowhere and returning to its undiscovered country immediately upon the completion of its work. The flow of steam, water, liquid air, gas, and compressed air through pipes is governed by a throttle or cock, which allows as much or as little to pass as may be required; and if the joints, unions, and couplings in the pipes are not absolutely tight there will be a leakage. Electricity is controlled by resistance in its passage through solid wires, rods, or bars, and cannot be confined within a given space like water, nor held in tanks or pipes as a vapor or gas. It is invisible, colorless, odorless, and occupies no apparent space that can be measured; it is the most powerful and terrible and yet docile force known to man, doing his bidding at all times when properly governed and regulated. In some respects, electricity can be compared to water stored in a tank--for instance, if you have a tank of water containing fifty gallons at an elevation of twenty-five feet, and a pipe leading down from it, the pressure of the water at the outlet of the pipe will be a given number of pounds. Now if the tank were double the size the pressure at the outlet of the pipe would be proportionately greater. Now if you have a battery made up of a number of cells they will develop a given number of volts, and if the number of the cells be doubled the voltage will be correspondingly increased. Or if you have a dynamo giving a certain number of volts, that number may be increased by doubling the size.