450 Comparison of Electrostatic with Electromagnetic Force. [Jane 18, 



in Sir W. Thomson's electrometers, and its position was observed by means 

 of a microscope directed on a graduated glass scale attached to the disk. 

 In this way its position could be adjusted to the thousandth of an inch, 

 while a motion of much smaller extent was easily detected. 



An exactly similar coil was placed at the other end of the torsion- 

 balance, so as to get rid of the effects of terrestrial magnetism. 



It was found that though the suspended disk and coil weighed about 

 half a pound, a very slight want of equality between the opposing forces 

 could be detected, and remedied by means of the micrometer. 



The difference of potential between the disks was maintained by means 

 of Mr. Gassiot's great battery. To measure this difference of potential, it 

 was made to produce a current through Mr. Willoughby Smith's re- 

 sistance-coil, and the primary coil of the galvanometer shunted with a 

 variable resistance. 



The current in the coils was maintained by a Grove's battery, and was 

 led through the secondary coil of the galvanometer. 



One observer, by means of the micrometer-screw, altered the distance 

 of the disks till the suspended disk was in equihbriura at zero. At the 

 same time the other observer altered the shunt, till the galvanometer- 

 needle was also in equilibrium. The micrometer reading and the re- 

 sistance of the shunt were then set down as the results of the experi- 

 ment. 



The mean of twelve satisfactory experiments, at distances varying from 

 •25 to '5 inch, gave for the ratio of the electromagnetic to the electrostatic 

 unit of electricity — 



v=: 2/- 79 Ohms, or B. A. units. 

 =277,900,000 metres per second. 

 = 174,800 statute miles per second. 

 This value is considerably lower than that found by MM. Weber and 

 Kohlrausch by a different method, which was 310,740,000 metres per 

 second. Its correctness depends on that of the B. A. unit of resistance, 

 which, however, cannot be very far from the truth, as it agrees so well with 

 Dr. Joule's thermal experiments. 



It is also decidedly less than any estimate of the velocity of light, 

 of which the lowest, that of M. Foucault, is 298,000,000 metres per 

 second. 



In a note to this paper the author gave his reasons, in as simple a form 

 as he could, for believing that the ratio of the electrical units, and the ve- 

 locity of light, are one and the same physical quantity, pointing out the 

 difference between his theory and those of MM. Biemann and Lorenz, 

 which appear to lead to the same conclusion. 



