ST. JOHN. — WAVE LENGTHS OF ELECTRICITY. 237 



a half wave length long, a second resonating circuit ought to be fouud 

 by increasing each side of the rectangle by a half wave length, making 

 the circuit 3 half wave lengths long, and a third when the circuit is 

 5 half wave lengths long, and so on.* This is evident from a con- 

 sideration of the accompanying diagram. O is the center of a central 

 segment and the points marked 1, 3, 5, are always nodes. 







O 



O 



Fig. 8. 



From the results that he obtained, Bjerknes concluded that the 

 change of period, if there was such a change, by the use of iron in 

 place of copper, could not exceed two 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 

 3 half wave lengths long, and there might be a cumulative difference 

 that would finally become measurable by the use of circuits of still 

 greater length. This theory was tested in the following way. A 

 copper wire (diameter 0.1201 cm.) was used as the secondary circuit 

 in Figure 5. The sides were taken 15 cm. long, and then gradually 

 lengthened to 875 cm., and bolometer readings observed for each 

 addition. The results are shown graphically by the upper curve in 

 Figure 4 of the Plate. The critical points in the curve are the re- 

 sults of many separate determinations. The unsteadiness of the spark 

 in the vibrator made the determinations somewhat laborious, though a 

 single series of observations would locate a maximum very closely. 

 After this had been done, a space of about a meter including the 

 maximum point was worked over forward and back, changing its length 

 2.5 to 5 centimeters at a time in the region of the maximum. To assure 

 the steadiness of the spark during such a series of observations, some 

 convenient length of circuit was chosen as a point of reference, and 

 observations taken before and after the series ; if these showed that 

 the activity of the spark was practically the same, the rendings of the 



* J. J. Thompson, Recent Researches in Electricity and Magnetism, § 297. 



