312 E. B. Rosa — Ratio of the Units of Electricity. 



high one, and was less steady in its action in proportion to the 

 current; consequently the single observations were less reli- 

 able, but this alone does not account for the nearly uniform 

 difference. 



In view of the uncertainty as to the cause of the variations 

 it is difficult to determine how best to combine the results. 

 The weight of each single value of v in the table is determined 

 by considering the number of observations from which it is 

 calculated, the uniformity of the separate observations, the 

 steadiness of the spot, etc. If we give to groups II and III 

 one-half the weight of I and IV in proportion to the sum of 

 the weights of the separate values we have as the mean for the 

 fast fork 2-9994, and for the slow fork 3-0023. Giving now- 

 double weight to the results of the fast fork on account of 

 their greater accuracy and uniformity, we have as a mean of all 

 v-3-000J f ,Xl0 10 cm. per sec. 



Again, if it be found that the cause suggested is the true 

 cause of the excess of groups II and III then those groups 

 should be thrown out and we should have 2*9982 and 3-0014: as 

 the means, which combined as before would give for the mean 

 of all v=*2'9993 x 10 10 cm. per sec. These values are based 

 upon the value -98664 for the British Association Unit. 



It is proposed to resume this investigation next winter, when 

 more perfect insulation can be obtained, and several improve- 

 ments in the details of the apparatus will be made. The 

 smaller ball of the condenser will then be used also, and the 

 cause of the difference in the values given by the two forks 

 will be studied. Although we cannot yet say whether v is 

 greater or less than 300,000,000 meters per second, it seems 

 certain that it is within a tenth per cent of this number, and it 

 is hoped in the continuation of this investigation to narrow 

 considerably further the range of uncertainty. 



For convenience of reference the following values of v and 

 of the velocity of light as found by different observers are 

 added, the values of v being corrected to the value '98664 for 

 the B. A. Unit : 



v, ratio of the units. 

 1 1856. Weber & Kohlrausch 3-107 x 10 10 

 2 1869. W. Thomson & King 2-808 " 



3 1868. Maxwell ...2-842 " 



4 1872. M.'Kiehan 2'S96 " 



6 1879. Ayrton& Perry 2'960 " 



6 1880. Shida 2-955 " 



7 1883. J. J. Thomson 2963 " 



8 1884. Klemencic 3-019 " 



9 1888. Himstedt 3-009 " 



1889. W. Thomson 3 004 " 



Johns Hopkins University, Baltimore, June 15, 1889. 



1 Weber & Kohlrausch, Electrodyn. Maasbestim., Abb. der Konigl. - Sachs. 

 Gesellschaft der Wissensch., vol. v, p. 219, 1856. 



2 King:, Report of the Committee on Electrical Standards. 1869. 



3 Maxwell. Phil. Trans., 1868, p. 643. 4 Dugald M'Kichan, Phil. Trans., 1879. 



5 Ayrton & Perry, Jour. Soc. Tel. Engineers, 1879, p. 126. 



6 Shida, Phil. Mag.. V, x, p! 431. 7 J. J. Thomson, Phil. Trans., 1883, p. 707. 

 8 Klemencic, Wien. Ber. lxxxiii, 88. 9 Himstedt, Wied. Ann., No. 9, 1888. 



Velocity of light. 



1879^ Michelson 2-9991x10' 



1882. Michelson... 2-9985 ,: 



1882. Newcorab 2-9986 " 



and 2-9981 " 



1874. Cornu.. 2-9850 " 



1878. Cornu 3-0040 " 



1880-81. Young & Forbes. 3-0138 " 



