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XV . The Velocity of Electric Waves. 

 By John Tkowbeidge and William Duane*. 



THE direct determination of a velocity implies the measure- 

 ment of a length and of a time. So far as we know, no 

 direct measurement of the velocity of electric waves has been 

 hitherto published. A number of experiments have been 

 performed on the velocity of single pulses of electricity 

 travelling over long metallic circuits, by Wheatstone, Fizeau 

 and Gounelle, Siemens, Blondlotj, and others ; but the results 

 obtained are very discordant. Blondlot's method is not open 

 to the objections that have been raised against some of the 

 others ; for his electric pulses theoretically were short, and 

 had steep wave fronts. Blondlot J also described a method of 

 determining the velocity of a system of electric waves. He 

 measured the distance between the nodes on a metallic circuit, 

 but, in order to get the time, he calculated the period of oscilla- 

 tion of another circuit that was in resonance with the first. 

 His determination, therefore, was not a direct measurement. 



Early last year the following direct method of measuring 

 the velocity of electric waves suggested itself to us : — Increase 

 the size, and if necessary change the shape of a Hertz vibrator 

 until the period of oscillation is sufficiently long to be deter- 

 mined by photographing the spark ; measure the length of 

 the waves induced in a secondary circuit tuned to resonance 

 with the vibrator ; and the quotient of the wave-length by 

 the time of a complete oscillation will be the required velocity. 



The first apparatus experimented with was that used by 

 Mr. St. John in investigating the peculiarities of waves along 

 iron wires. For a detailed description of this apparatus see 

 the Philosophical Magazine for November 1894. It has been 

 assumed by certain writers that the reaction between the 

 circuits, arranged in the manner employed by Mr. St. John, 

 is, to a large extent, what would be called in the older theory 

 of electricity electromagnetic rather than electrostatic. 

 Certain phenomena, however, which appeared in the early 

 part of our investigation seemed to point to the view that the 

 greater portion of the action was electrostatic ; and we there- 

 fore concluded to arrange the apparatus so that the reaction 

 should be wholly electrostatic, thinking by this means to ob- 

 tain a more powerful oscillation and a more regular wave 

 than with the electromagnetic arrangement described and 



* Communicated by the Authors. 



t Comptes Rendus, 1891, tome xiii. p. 638. 



X Ibid. 1894, tome cxvii. p. 543. 



