﻿of Electricity on Iron Wires. 435 



produced by replacing copper by iron does not exceed 2 

 per cent. The difference in length between a copper and an 

 iron circuit of the same period would be very small with 

 circuits a half wave-length long ; but this difference would 

 be three times as great with circuits three half wave-lengths 

 long, and there might be a cumulative difference that would 

 become measurable b}^ the use of circuits of still greater 

 length. To examine this question, a copper wire (diameter 

 0*1201 centim.) was used as the secondary circuit in fig. 2. 

 The sides were taken 15 centim. long, and gradually lengthened 

 to 875 centim., and bolometer-readings taken for each addition, 

 the exploring terminals being always at the ends L and N. 

 The result is shown graphically by the upper curve in fig. 4, 

 PI. XII. The critical points in the curve are the results 

 of many separate determinations. The unsteadiness of the 

 spark on the vibrator made the determinations somewhat 

 laborious, though a single series of observations would locate 

 a maximum very closely. After this had been done, the space 

 of about a metre, including the maximum point, was worked 

 over forward and back ; the constancy of the spark was 

 insured by choosing a convenient point of reference, as 

 already described. 



An examination of the curve shows four maxima occurring 

 when the sides of the rectangle were 45, 306, 562*5, and 

 818 centim. long. The additions of wire for the successive 

 maxima after the first were 261, 256*5, and 255*5. These 

 should be half wave-lengths. The last two agree well, but 

 the first differs from the average of the last two (256 centim.) 

 by 5 centim. The sides were fixed at 818 centim. and the 

 wave-form, fig. 3 (PI. XII.), was obtained. From this the half 

 wave-length was found to be 255*6 centim., and the total 

 length of the circuit seven half wave-lengths. By fixing the 

 sides of the rectangle at 562*5 centim. and 306 centim., a 

 similar investigation showed the circuits to be respectively 

 5 and 3 half wave-lengths long. 



An explanation of the fact that the distance between the first 

 and second maxima w r as anomalously large may possibly be 

 this: for the first maximum the sides of the rectangle were but 

 45 centim. long, so that the effect of the closed end was rela- 

 tively great and the maximum appeared earlier than it other- 

 wise would, but when the rectangle was 300 centim. long, 

 the influence of the closed end became relatively small, and 

 the second and future maxima came in the normal positions. 

 Besides, in the first case the capacity was largely local, while 

 in the others it was mainly distributive. This same effect 

 appeared in eveiy case, and seemed a constant phenomenon. 



