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18 RICHARDSON'S BLUEBOOK OF PROJECTION
IIP or H.P. Thus 100 volts times 10 amperes equals 1,000 watts, or 1,000 -*■ 746 = 1.34 HP.
In a. c. circuits this rule applies only with certain modifications.
(54) You will learn later that the necessary diameter of wire used is governed by the number of amperes and not by the voltage. A No. 8 wire, for example, can carry only 50 amperes without overloadbut it may carry that amperage at low or high voltage. (55) Now 50 amperes at 100 volts pressure would be only five thousand (50 X 100) watts, or less than seven HP whereas 50 amperes at 1,000 volts pressure would be 50,000 watts or more than 67 HP. (56) To convey 67 HP at 100 volts would require No. 4 rubber-covered wires, expensive in themselves and expensive to install.
(57) A. c. may be generated at ordinary commercial voltage, say 110, and merely by passing it through a relatively cheap machine (transformer) having no moving parts, requiring practically no attention at all and with about 95 percent efficiency, the voltage may be raised to any desired pressure. It may then be conveyed to the point where it is desired to use it, and there by passing through a similar machine (transformer) having a different winding, the pressure may again be reduced as low as may be desired, usually 110 volts. Then too, there are advantages in certain types of a. c. motors. Broadly speaking, the reason why a. c. is so generally used is the economy in transportation, though there are many minor reasons.
Alternating Current Action
(58) As has been said, a. c. flows first in one direction, then flows in the opposite direction for an exactly equal space of time. These changes in direction of flow are called alternations. (59) Two alternations (the current starting from horizontal line 0, Fig. 4, following line BC to point 2) form one complete "cycle. " (60) The number of cycles per second are termed the frequency of the current.
The frequency of commercial current varies. (61 ) It