The factors given in this table, multiplied by the resistance to direct cur rents, will give the resistance to alternating currents for copper conductors of circular cross-section.
Mutual Induction. When two or more circuits are run in the same vicinity, there is a possibility of one circuit inducing an electro motive force in the conductors of an adjoining circuit. This effect may result in raising or lowering the E. M. F. in the circuit in which a mutual induction takes place. The amount of this induced E. M. F. set up in one circuit by a parallel current, is dependent upon the cur rent, the frequency, the lengths of the circuits running parallel to each other, and the relative positions of the conductors constituting the said circuits.
Under ordinary conditions, and except for long circuits carrying high potentials, the effect of mutual induction is so slight as to be negligible, unless the conductors are improperly arranged. In order to prevent mutual induction, the conductors constituting a given circuit should be grouped together. Figs. 35 to 39, inclusive, show five arrangements of two two-wire circuits; and show how relatively small the effect of first induction is when the conductors are properly arranged, as in Fig. 38, and how relatively large it may be when im
properly arranged, as in Fig. 39. These diagrams are taken from a publication of Mr. Charles F. Scott, entitled Polyphase Trans mission, issued by the Westinghouse Electric & Manufacturing Company.
Line Capacity. The effect of capacity is usually negligible, except in long transmission lines where high potentials are used; no calculations or allowance need be made for capacity, for ordinary circuits.
Calculation of Alternating-Current Circuits. In the instruction paper on "Power Stations and Transmission," a method is given for calculating alternating-current lines by means of formulee, and data are given regarding power factor and the calculation of both single-phase and polyphase circuits. For short lines, secon,iary wiring, etc., how ever, it is probably more convenient to use the chart method devised by Mr. Ralph D. Mershon, described in the Am,'rican Electrician of June, 1897, and partially reproduced as follows: