=437. Transmission of Electric Power.=--A field of peculiar usefulness for a.-c. currents is in the economical transmission of electric power. This fact is due to the following reasons: (_a_) The loss of electrical power in a transmission line is due to the production of heat; the heat produced being proportional to _I²R_, or to the _square_ of the _current intensity_. Any lessening of the current flow required to transmit a given power will therefore increase the efficiency of transmission. (_b_) In order to employ a small current in transmitting a large amount of power, we must use a very high e.m.f. Such high electromotive forces, say from 60,000 to 100,000 volts, can be obtained only by the use of a.-c. transformers, since it is not practicable to build a direct current generator capable of producing 60,000 volts. In large power transmission systems, a.-c. generators are used to produce powerful alternating currents. The e.m.f. is then stepped up to a suitable voltage (2300-100,000) by transformers and sent over transmission lines to the various places where the power is to be used; at these places suitable transformers "step-down" the e.m.f. to a convenient or safe voltage for use. (See Fig. 442 of a transmission line and Fig. 438 of a large power transmission system, and Fig. 439 of an a.-c. generator and power plant.) [Illustration: FIG. 438.--Diagram of an alternating current high tension power system. (_A_) Alternator, (_Tu_) water turbine, direct connected to alternator, (_E_) exciter, (_T_{1}_) step-up transformers in power station, (_T_{2}_) step-down transformers in substation, (_M_) motor, (_L_) lamps, single-phase, three-wire system, (_T_{3}_) step-down transformers delivering three-phase current to rotary converter (_R_) which delivers direct current to the trolley line.] =438. Power Factor.=--The _power factor_ is a matter of interest and importance in the use of a.-c. machines. Its meaning and use may be learned from the following explanation: In a direct current circuit, watts equals volts times amperes. In an alternating current circuit, this equation is true only when the current is "in step" with the voltage, that is, only when there is no _inductance_ or _capacity_ in the circuit. If current and voltage are out of step, _i.e._, if there is _lag_ or _lead_ (see Fig. 434), the product of volts and amperes gives only the _apparent power_, the ratio between true and apparent power depending on the amount of lag or lead. This ratio is called the power factor. In an a.-c. circuit, then, the power equation is: watts = volts × amperes × power factor, or power factor = true power/apparent power. The product of volts and amperes is the _apparent power_ and is called volt-amperes in distinction from the true power or watts. Therefore the following is true: power factor = true watts/volt-amperes.