130 Prof. J. J. Thomson on the Velocity of 



upon the relative lengths of the wires BL ? BM. If these 

 wires are both surrounded by air, the sparking-distance is a 

 minimum when the lengths are equal. This is the case what- 

 ever the material or cross section of the wires. This is also 

 the case if one of the wires is placed between two large zinc 

 plates at a small distance apart ; though in this case the elec- 

 trostatic capacity of one of the wires is very much greater 

 than that of the other. 



If (p cosnt represents the potential of B, I, V the lengths of 

 the wires BL, BM respectively, v and v' the velocities of 

 transmission of electric impulses along them, then the poten- 

 tials of L, M are respectively 



(f) cos nt <£ n cos nt 



nl nl' 



cos— cos— T 



v v 



These potentials will be equal, and the sparks therefore have 

 a minimum length, when l/v = l'jv f . When v = v', 1 = 1', which 

 agrees with the effects observed when both wires were sur- 

 rounded by air. But if v and vf are different, then the lengths 

 of the circuits for minimum spark-length will be unequal, and 

 the ratio of the two lengths will be the ratio of the velocities 

 of transmission of electric impulses along the wires. We have 

 thus a method of rinding the rate of transmission of electric 

 waves along different wires*. 



In order to compare the velocity along a wire surrounded 

 by air with that along one surrounded by paraffin or sulphur, 

 one of the wires BM was placed inside and insulated from a 

 brass tube connected with the earth at each end. The tube 

 was rilled either with paraffin which was poured in in a melted 

 condition and allowed to set, or with sulphur melted down so 

 as to form a solid mass. The length of the circuit BL could 

 be altered by successively adding pieces of wire each \ of a 

 metre in length. The length of the wires when the spark- 

 length was least are given in the following Table, where the 

 different numbers refer to experiments made on different 



days : — Wire surrounded Wire surrounded 



by paraffin. by air. 



4 metres. 5, 5J metres. 



4 „ 5-5 „ 



4 „ 6 „ 



4 „ 5 



Mean . . 4 metres. 5*4 metres. 



* I have found since this paper was in type that exactly the same me- 

 thod was used by von Bezold twenty years ago to prove that the velocity 

 of transmission of electricity along- wires is not affected by the material 

 or diameter of the wire (PoggendorfFs Annalen, cxl. p. 541). 



