60 PROCEEDINGS OF THE AMERICAN ACADEMY 



by induction from one wire to another, unless the two wires between 

 which induction is produced run parallel and very near to each other 

 a long distance. This distance generally exceeds the distance at 

 which the ordinary Bell telephone ceases to transmit articulate speech. 

 The effects which have usually been attributed to induction on tele- 

 phone circuits are due to the earth connections and to imperfect insu- 

 lation. Indeed, there would be no trouble from induction if telephone 

 wires were enclosed in a cable ; for a consideration of Eq. (1) will make 

 it evident that the telephonic messagess transmitted over one wire, on 

 account of the feeble currents which produce them, would have no 

 practical effect upon the neighboring wires enclosed with it. 



Since the transmission of the time signal service of Harvard College 

 Observatory through all the telephone circuits in Boston and Cam- 

 bridge is evidently not due to induction, but to tapping, so to speak, 

 the earth at points which are not at the same electric potential, it was 

 an interesting question to study the extent of the equipotential sur- 

 faces formed around the grounds of the time service circuit at Cam- 

 bridge and in Boston. In this survey I was greatly assisted by 

 Mr. G. H. Francis and Mr. H. C. French. 



I speedily discovered that the time signals could be distinctly heard 

 in a field an eighth of a mile from the Observatory, where one ground 

 of the time circuit is located. The method of exploration consisted 

 in running a wire five or six hundred feet along the grass, grounding 

 its ends in moist earth, and including a telephone in the circuit. On 

 completing the circuit through the telephone and the ground the 

 evidence of an electrical current was plainly apparent from the ticking 

 which making and breaking the circuit produced in the telephone, and 

 the signals of the Observatory clock were distinctly heard. At the 

 distance of a mile from the Observatory and not in the direct line 

 between the Observatory and the Boston office, the time signals were 

 obtained by tapping the earth at points only fifty feet apart. At a 

 distance of five hundred feet directly behind the Observatory, no points 

 five hundred feet apart could be found which were not practically at 

 the same potential. The survey was carried to a distance of a mile 

 behind the Observatory grounds with negative results. At points one 

 mile from the central line between the Observatory and the Boston 

 office the time signals could not be heard on the trial wire of six hun- 

 dred feet. This was to be expected, since the trial wire should have 

 its length increased as the distance from the grounds of the battery 

 increases, in order to permit of one end of the wire touching a point 

 of much higher potential than the other. 



