Ratio of the Electromagnetic to the Electrostatic Unit. 331 



fork should give higher values for v, which means a lower 

 value for the capacity, than the high fork, is rather unex- 

 pected and not fully understood. The low fork gave only a 

 quarter the current given by the high one, and was less steady 

 in its action in proportion to the current ; consequently the 

 single observations were less reliable, 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 deter- 

 mined by considering the number of observations from which 

 it is calculated, the uniformity of the separate observations, 

 the steadiness of the spot, &c. 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 doable weight to the results of the fast fork, on 

 account of their greater accuracy and uniformity, we have, as 

 a mean of all, 



v = 30004 x 10 10 cm. per second. 



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 would give for the mean of all 



v = 2*9993 x 10 10 cm. per second. 



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 

 improvements 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 metres 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: — 



