Form Factor.—This term was introduced by Fleming, and denotes the ratio of the virtual value of an alternating wave to the average value. That is
| virtual value | .707 | |||||
| form factor | = | = | = | 1.11 | ||
| average value | .637 |
Ques. What does this indicate?
Ans. It gives the relative heating effects of alternating and direct currents, as illustrated in figs. 1,239 and 1,240.
That is, the alternating current will have about 11 per cent. more heating power than the direct current which is of the same average strength.
If an alternating current voltmeter be placed upon a circuit in which the volts range from +100 to -100, it will read 70.7 volts, although the arithmetical average, irrespective of + or-sign, is only 63.7 volts. If the voltmeter be connected to a direct current circuit, the pressure necessary to give the same reading would be 70.7 volts.
Figs. 1,239 and 1,240.—Relative heating effects of alternating and direct currents. If it takes say five minutes to produce a certain heating effect with alternating current at say 63.7 average volts, it will take 33 seconds longer with direct current at the same pressure, that is, the alternating current has about 11 per cent. more heating power than the direct current of the same average pressure. The reader should be careful not to get a wrong conception of the above; it does not mean that there is a saving by using alternating current. When both voltmeters read the same, that is, when the virtual pressure of the alternating current is the same as the direct current pressure, the heating effect is of course the same.
Ques. What is the relation between the shape of the wave curve and the form factor?
Ans. The more peaked the wave, the greater the value of its form factor.