. American telephone practice . ce set up by current flowingin the disturbing wire. About every wire carrying a current thereis a field of force, or magnetic whirl, consisting of closed lines 160 AMERICAN TELEPHONE PRACTICE. of force surrounding the conductors. Such a condition is repre-sented in Fig. 140. If the current is a continuous one, the linesof force will not vary after being once set up, and the telephonewire lying in this field will not be affected. If the current in thedisturbing wire is fluctuating, the number of lines of force in thisfield will vary; or, by a clearer way of expre
. American telephone practice . ce set up by current flowingin the disturbing wire. About every wire carrying a current thereis a field of force, or magnetic whirl, consisting of closed lines 160 AMERICAN TELEPHONE PRACTICE. of force surrounding the conductors. Such a condition is repre-sented in Fig. 140. If the current is a continuous one, the linesof force will not vary after being once set up, and the telephonewire lying in this field will not be affected. If the current in thedisturbing wire is fluctuating, the number of lines of force in thisfield will vary; or, by a clearer way of expressing it, the field offorce will expand and contract accordingly. This expansion andcontraction of the field will cause its lines of force to cut the tele-phone wire, and will by the laws of electromagnetic induction causecurrents to flow in the latter. If the current in the disturbing wireis an alternating one, the field of force around it will be establishedin one direction, destroyed and established in the reverse direction,. FIG. 140.—MAGNETIC LINES AROUND A CONDUCTOR. and again destroyed, with every complete cycle of the is the condition for a maximum disturbance in the telephone wire. Electrostatic induction may be explained by reference to , where a grounded telephone line is shown running parallelwith a disturbing wire, which we will say is carrying an alternatingelectric current. The disturbing wire will receive from its sourceof current alternate positive and negative charges of electricity, andits potential will pass from a maximum in one direction throughzero to a maximum in the other, and again through zero to themaximum in the first direction during each cycle. Consider the condition when the potential of the disturbing wireis zero. No charge will then be induced on the telephone wire, so TELEPHONE LINES. 161 that its potential will also be zero, unless subject to other charge on the disturbing wire then becomes, we will say, p
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