Let us illustrate its application by an example. The water wheel is started and is spinning the dynamo at its rated speed, say 1,500 r.p.m. Two heavy wires, leading from brushes which collect electricity from the revolving armature, are led, by suitable insulated supports to the switchboard, and fastened there. They do not touch each other. Dynamo mains must not be permitted to touch each other _under any conditions_. They are separated by say four inches of air. Dry air is a very poor conductor of electricity. Let us say, for the example, that dry air has a resistance to the flow of an electric current, of 1,000,000 ohms to the inch--that would be 4,000,000 ohms. How much electricity is being permitted to escape from the armature of this 110-volt dynamo, when the mains are separated by four inches of dry air? Apply Ohm's law, C equals E divided by R. E, in this case is 110; R is 4,000,000; therefore C (amperes) equals 110/4,000,000--an infinitesimal amount--about .0000277 ampere. Let us say that instead of separating these two mains by air we separated them by the human body--that a man took hold of the bare wires, one in each hand. The resistance of the human body varies from 5,000 to 10,000 ohms. In that case C (amperes) equals 110/5,000, or 110/10,000--about 1/50th, or 1/100th of an ampere. This illustrates why an electric current of 110 volts pressure is not fatal to human beings, under ordinary circumstances. The body offers too much resistance. But, if the volts were 1,100 instead of the usual 110 used in commercial and private plants for domestic use, the value of C, by this formula at 5,000 ohms, would be nearly 1/5th ampere. To drive 1/5th ampere of electricity through the human body would be fatal in many instances. The higher the voltage, the more dangerous the current. In large water-power installations in the Far West, where the current must be transmitted over long distances to the spot where it is to be used, it is occasionally generated at a pressure of 150,000 volts. Needless to say, contact with such wires means instant death. Before being used for commercial or domestic purposes, in such cases, the voltage is "stepped down" to safe pressures--to 110, or to 220, or to 550 volts--always depending on the use made of it.