248 VELOCITY OF LIGHT IN AIR. 



electrostatic units of electricity, it will be interesting to compare 

 some recent determinations of this ratio. These we give in the 

 following table. Since the determinations are affected by any error 

 in the standard of resistance, we have corrected the results, first, on 

 the supposition that the B. A. ohm = *987 true ohms (Lord Rayleigh's 

 result), and secondly, on the supposition that the B. A. ohm = '989 true 

 ohms, which is essentially assuming that the legal ohm represents the 

 true value. 



Ratio of Electromagnetic and Electrostatic units of Electricity in millions of 



meters and seconds. 



Date. As published. B. A. ohm = '987. B.A. ohm = '989. 



Ayrton & Perry,* 1878 298'0 296'1 296'4 



Hockin.t 1879 298 "8 296 "9 297 '2 



Shida, * 1880 299 '5 295 '6 296 "2 



Exner, 1882 30M (?) 291'7(?) 292'3(?) 



J. J. Thomson, || 1883 296 '3 296 "3 296 '9 



Klemeni&,U 1884 301'88(?) 301'88(?) 302*48 (?) 



These numbers are to be compared with the velocity of light in air, in 

 millions of meters per second, for which Professor Newcomb gives 

 299*778. Of the electrical determinations, that of J. J. Thomson 

 appears by far the most worthy of confidence. That of Klemencic 

 the only one as great as the velocity of light was obtained by the 

 use of a condenser with glass, a method which would presumably 

 give too great a ratio. Exner's value is obtained from the mean of 

 three determinations, one of which differed from the others by about 

 three per cent. If we reject this discordant determination, the mean 

 of the other two would give when corrected for resistance 2944 and 

 295'0. If we set aside the determinations of Exner and Klemenc'ic', 

 the remaining four, which represent three different methods, are very 

 accordant, the mean being nearly identical with the result of J. J. 

 Thomson, and about one per cent, less than the velocity of light. 



Professor Michelson's experiments on the velocity of light in carbon 

 disulphide afford an interesting illustration of the difference between 

 the velocity of waves and the velocity of groups of waves a subject 

 which is treated at length in an appendix to the second volume of 

 Lord Rayleigh's Theory of Sound. If we write V for the velocity 

 of waves, U for that of a group of waves, L for the wave-length, and 

 T for the period of vibration, 



L 

 "T' 



For purposes of numerical calculation, it will be convenient to 

 transform these formulae by the use of X for the wave-length in 



* Phil. Mag., (5), vol. vii, p. 277. t Report Brit. Assoc., 1879, p. 285. 



%Phil. Mag., (5), vol. x, p. 431. Sitzungsberichte Wien. Akad., vol. Ixxxvi, p. 106. 



\\PhU. Trans., vol. clxxiv, p. 707. IF Sitzungsberichte Wien. Akad., vol. Ixxxix, p. 298. 



