

Date. 



Ayrton & Perry,* 



1878 



Hockin,f 



1879 



Shida4 



]880 



Exner,§ 



1882 



J. J. Thomson, || 



1883 



Klemencic,^" 



1884 



B.A. ohm=-987. 



B.A. ohm=-989. 



296-1 



2964 



296-9 



297-2 



295-6 



296-2 



291-t (?) 



292-3 (?) 



2963 



2969 



301-88 (?) 



30248 (?) 



Chemistry and Physios. 63 



of the slit formed by reflection from the revolving mirror. No 

 trace of such iridescence could ever be seen. Professor Michelson 

 made an experiment, in which a red glass covered one-half the 

 slit. The two halves of the image — the upper white, the lower 

 red — were exactly in line. 



Since Maxwell's electromagnetic theory of light makes the 

 velocity of light in air equal to the ratio of the electromagnetic 

 and electrostatic units of electricity, it will be interesting to com- 

 pare 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. 

 ohni=r'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. 

 As published. 

 298-0 

 2988 

 299-5 

 301-1 (?) 

 296-3 

 301-88 (?) 



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"V78. 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 ob- 

 tained from the mean of three determinations, one of which dif- 

 fered 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 294'4 and 295 - 0. If we set aside 

 the determinations of Exner and Klemencic, 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 differ- 

 ence 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, IJ for that of a group of waves, 

 L for the wave-length, and T for the period of vibration, 



T' tf(L-i)' 



*Phil. Mag., 5, vii. p. 277. f Report Brit. Assoc, 1879, p. 285. 



X Phil. Mag., 5, x, p. 431. § Sitzungsberichte Wien. Acad., Ixxxvi, p. 106. 



I Phil. Trans., clxxiv, p. 707. \ SitzuDgsberichte Wien. Acad., lxxxix, p. 298. 



