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14 HANDBOOK OF PROJECTION FOR
The second reason is that since wattage, which is the measure of electrical power, is the product of volts multiplied by amperes, the size of a wire necessary to convey a given number of horsepower will be much less at high voltage, thus : Suppose we wish to transmit 10,000 watts (746 watts equal one horsepower) at 100 volts. Since volts times amperes equals watts, it follows that watts divided by volts equals amperes; hence, to transmit 10,000 watts at 100 volts would require 10,000-^-100=100 amperes, and the power transmitted would be about 13.5 horsepower. But if the voltage were 1,000 instead of 100, then only 10.000-^-1,000=10 amperes would be required to convey the required 10,000 watts of power.
To put it another way, 100 amperes at 100 volts represents exactly the same wattage (horsepower) as does 10 amperes at 1,000 volts. To carry 100 amperes requires a No. 3 wire, whereas 10 amperes can be carried by a No. 16 wire; and since a No. 3 wire is .22942 and No. 16 wire .050820 of an inch in diameter, it is readily seen that with high voltage a given wattage (horsepower) can be conveyed on very much smaller wires than could be used were a lower voltage employed. See Hawkins' Electrical Guide, No. 4, page 997, for further details.
Third, still another factor entering into the matter is the fact that once the current has been generated and commuted into D. C, its pressure (voltage) cannot be raised or lowered except by the use of expensive machines having moving parts, thus requiring more or less constant attention, whereas it is quite practicable to attach a very simple device known as a "transformer" (See page 544), which has no moving parts and therefore requires practically no attention, to A. C. lines at any desired point, the action of which will be to raise the voltage to any desired pressure which it is commercially possible to insulate, or to lower it to any required voltage.
It therefore follows that A. C. may be generated at relatively low voltage, "stepped up" by means of transformers to any required pressure, transmitted for long distances over relatively small wires and again "stepped down" to commercial pressures at destination. Or power for commercial purposes may be generated at high voltage, which may be "transformed" to a voltage to suit any commercial requirement at any desired point along the lines.
There are other reasons why A. C. is more desirable for general commercial use than D. C., but those named are perhaps the chief ones.