Electric Waves along Wires. 653 



More recently Gutton *, in comparing the velocities of electric 

 waves along wires (diam. "11 cm.) and in air, found within 

 the errors of observation (about 1 in 200) that the velocities 

 are the same. 



St. Johnf showed the dependence of the velocity on the 

 magnetic permeability of the substance of the wire by using 

 iron and copper wires of the same diameter, and measuring 

 the wave-lengths, corresponding to a certain frequency, along 

 the wires. His results verified what had previously been 

 demonstrated by J. J. Thomson, that the magnetic properties 

 of iron are called into play under extremely rapid alternations 

 of the magnetizing forces. 



St. John also used copper wires, of different diameters, and 

 found, for oscillations of the same period, the wave-length 

 along wire of diameter "03915 cm. to be more than 4 percent. 

 less than that along *1201 cm. wire. The half wave-lengths 

 which he obtained for the same frequency along different 

 wires were as follows : — 



Copper (*1201. centim. diameter) 255*8 centim. 

 (♦0884 „ ; , ) 252-2 „ 



(•07836 „ „ ) 251-6 „ 



('03915 „ „ ) 244-8 „ 



The parallel wires were 30 centim. apart. 



Prof. J. J. Thomson suggested the use of the Rutherford 

 detector J in a measurement of the velocities along wires of 

 various diameters and materials, and it was with this object 

 in view that the following experiments were made. 



Preparatory to attempting the measurement of the relative 

 velocities of the waves along wires of different diameters. 

 Lecher's system of wires was set up, and the exhausted tube, 

 as used by by him, replaced by a wire bridge with a loop at 

 the middle for the reception of the detector-needle§. The 

 .system then consisted of two large rectangular zinc plates, to 

 which were attached by rods the brass balls between which 

 sparking occurred. Parallel to and distant 5 or 6 centimetres 

 from these plates were two smaller ones S and S' (fig. 1). 

 To the centres of these were attached the two ends of a copper 

 wire SKK'S', the two portions AK and A'K' of the wire 

 being parallel and about 7*8 centimetres apart. Connecting 

 corresponding points of the parallel wires was a movable 

 bridge BB J of the same material as the wires. 



* Gutton, Comptes JRendus, cxxviii. p. 1508 (1899). 

 t St. John, Phil. Mag. [5] vol. xxxviii. p. 425 (1894). 

 X Rutherford, Phil. Trans. 189 a, p. 1 (1897). 

 § .Rutherford, Phil. Trans. 1897. 



