294 Prof. J. J. Thomson. [June 20, 



Fig. 2. 



the Rate of Propagation of Electrical Disturbances along it," £ Roy. 

 Soc. Proc.,' vol. 46, p. 1). Two wires RS, VW, of equal length, had 

 the ends S and W fastened to the poles of a spark micrometer, while 

 the other ends, R, Y, could slide along the wires LT, MU respectively. 

 At first R was placed at T, and V was moved until the sparks in the 

 micrometer were as small as possible ; suppose that « was the position 

 of V when this was the case. T and « will be at the same potential. 

 The end Y was now kept fixed at «, and R moved until the sparks 

 again became as faint as possible ; suppose that /3 was the position of 

 R when this was the case, then /3 and at, and therefore /3 and T, are at 

 the same potential ; so that, since T is a place of maximum potential, 

 /3T equals a wave-length. 



By starting from ft and proceeding further up the wire we can get 

 another determination of the wave-length. 



Since from the nature of the case other conductors besides the two 

 disks were in the field, the capacity of the condenser was in excess of 

 the value given by the formula S/4<7rt, where S is the area of one of 

 the plates and t the distance between them ; but this is the only part 

 of the capacity which is increased when the slab of dielectric is inter- 

 posed between the plates. The capacity when the disks were 2 cm. 

 apart was determined by the tuning-fork method given in Maxwell's 

 " Electricity and Magnetism," vol. 2, p. 385. and was found to be 



