224 Velocity of Electric Waves. 



With the exception of three preliminary trials, which gave 

 values differing from the mean by 10 per cent, or by 12 per 

 cent., these are the only determinations we have made. In 

 some cases the waves in the circuit were just as good with 

 the spark-gap as without it. In others there was a decided 

 wave-formation when sparks occurred, but the node was not 

 quite so well-marked. For this reason, and since it did not 

 appear to make any difference in their length, the waves 

 usually were measured without the spark-gap. As the sparks 

 were quite regular, the difference in the bolometer readings 

 must have been due to Faraday tubes that were reflected from 

 the spark-gap without forming a spark and reversing them- 

 selves. The variation in the number of revolutions of the 

 mirror per second is due to the fact that different cells were 

 used to drive the motor on different occasions. 



As an example of the data taken to ascertain the position 

 of the node we give the following table. The top line con- 

 tains the distances of the bolometer terminals from a pair of 

 arbitrary fixed points on the circuit : — 



Distances from fixed points 

 Bolometer deflexions 



20 cm. 



40 cm. 



60 cm. 



4-3 



4-0 



4-3 



4-5 



4-1 



4-4 



4-5 



4-0 



4-2 



Average deflexions .... 4*43 4*03 4 # 3 



From these deflexions the position of the node was estimated. 

 It appears from the best results that we have obtained, that 

 the velocity of short electric waves travelling along two 

 parallel wires differs from the velocity of light by less than 

 •2 per cent, of its value. It has been shown theoretically, 

 that the velocity of such waves travelling along a single wire 

 should be the velocity of light approximately. Our results, 

 therefore, in a certain sense confirm the theory to an accuracy 

 within their probable error. Theoretically, too, the velocity 

 should be approximately equal to the ratio between the two 

 systems of electrical units. The average of the best measure- 

 ments of this ratio is 3*001, which is nearer the average 

 velocity obtained by us than the velocity of light. 



Jefferson Physical Laboratory, 

 Cambridge, U.S. 



