304 TRANSATLANTIC TELEPHONING. 



no groping in the dark, no lucky discovery. Different forms of appa- 

 ratus were tried, to be sure, but all were based upon the results of the 

 mathematical analysis. The last form has the advantage that it is an 

 exact representation of a standard telephone line with coils arranged 

 to be inserted every mile at pleasure. 



Fig. ."» (PI. I) is a general view of apparatus which Dr. Pupin has 

 used in his experiments. On the left is seen one of the 50-mile sec- 

 tions of this line. Other sections are seen in full or in part, in front 

 and on the left and right. The line proper is contained in the large 

 case standing on top of the frame, and consists of tin-foil strips of 

 such width and length as to have the resistance and in such relation as 

 to have the capacity of the standard line. This makes an artificial 

 line, having all the characteristics of the standard telephone line, 

 except the length. This line is subdivided into 50 sections, each 

 equivalent to one mile of standard line. The ends of these are seen 

 in the maze of wire going from the case to the frame below. On this 

 frame are the 50 coils, which may be included in the line or left out of 

 it by removing or inserting plugs. With the coils out of circuit, tele-* 

 phone conversation is distinct up to 30 miles, can he barely made out at 

 100, and is absolutely unrecognizable at 110 miles. Introducing the 

 coils, the conversation becomes again perfectly distinct and continues 

 so through all the sections, equivalent to 250 miles of line. 



Fig. 6. (PI. 1) shows a larger view of one of these 50-mile sections, 

 and in the foreground a small generator of alternating currents of 600 

 periods per second, corresponding to about the average frequency of 

 the vibrations of the human voice. This generator was used for pro- 

 ducing electric waves for comparing the actual with the theoretical 

 results. From the resistance and capacity of the artificial line, and 

 the resistance and self-induction of the coils, the velocity of trans- 

 mission, and the length of the electric waves corresponding to 600 

 periods per second, were computed. 



The first computation gave for the wave length about 26 miles. A 

 measurement of the wave length gave about 18 miles. Here was a 

 wide discrepancy between the theoretical and measured lengths, too 

 great a discrepancy to be ascribed to any ordinary error of measure- 

 ment. And since, of course, the wave length determines the proper 

 distance which should be between these coils in actual work, the mat- 

 ter was one of prime importance. All the measurements of resist- 

 ance, capacity, and self-induction were repeated without finding any 

 errors. The generator was thoroughly studied to see that it really 

 gave a frequency of 600 periods. All the apparatus for measuring 

 the wave lengths was subjected to a rigid examination and analysis, 

 to see if any error could be introduced there. The computations were 

 reviewed again and again, with always the same result of 26 miles 

 for the wave lengths. Means were provided for maintaining a per- 



