318 St. John — Wave lengths of Electricity on Iron Wires. 



were retained. A maximum point was found when KL was 

 818 cm . The sides were fixed at this length, and the form of 

 the wave was obtained by sliding the exploring terminals 

 along the wire, and taking bolometer observations for each 

 position. The result is shown in fig. 3, of the plate. The 

 critical points were determined several times, and the steadi- 

 ness of the spark assured by choosing, as before, a point of 

 reference. The curve shows three minima at 240, 496, and 

 752 cm , starting from these give half wave lengths of 255, 

 256, and 256 0m , with an average of 255'6 cm . The third mini- 

 mum at 752 cm was determined with care, as it was to be used 

 as a basis for calculating' the half wave length. An error in 

 determining the position of this minimum would be divided 

 by three, since the distance from to this minimum was three 

 half wave lengths. The total length of the circuit was seven 

 half wave lengths, and it was the equivalent of one-fourth of 

 a wave length from the third minimum to the end. The 

 actual distance to the end was 818 — 752 = 66 cm . 127*8-66= 

 61-8 cm = the equivalent of the capacities in centimeters of wire. 



A comparison of the curve (fig. 1, of plate) obtained from 

 the plain wire circuit with the curve (fig. 3, of plate) obtained 

 when capacities were fixed On the free ends shows a quite 

 satisfactory agreement, which tends to create confidence in 

 both methods. The half wave length by the first is 254 - 3 cm , 

 and by the second it is 255*6, values which differ by about 

 one-half of one per cent. There is a marked difference, as 

 was to be expected, in the form of the curve next the free 

 ends. When end capacities were used, the accumulation of 

 charge seemed largely confined to them out of reach of the 

 exploring terminals, while with the plain wire it seemed 

 distributed over a greater distance. In each case the effect of 

 the ends was to make the curve depart from the normal form 

 along the free wire. 



The theory of my investigations rests upon the principle of 

 electrical resonance. The sides of the rectangle KLMJNT 

 (fig. 2a) were shortened to a few centimeters in length, so 

 that it could be safely assumed that the period of the secondary 

 was considerably shorter than that of the vibrator. The 

 exploring terminals were kept at LN and bolometer observa- 

 tions taken for each small addition to the length of the sides 

 KL and MIST. When best resonance was found with the 

 shortest length of the secondary circuit that gave a maximum, 

 it was assumed that the secondary had the same period as the 

 vibrator, and that its equivalent length was a half wave length, 

 its actual length depending upon the effect due to the free 

 ends. The occurrence of resonance is a very marked phe- 

 nomenon even with a vibrator that damps as rapidly as the 



