43() REPORT— 1869. 



the dynamometer is set up, care is taken to neutralize the effects of the 

 earth's magnetism by a large number of magnets fixed at a great distance 

 from the coils. If the adjustment of the magnets is perfect, there is no 

 alteration of the position of the spot of light when the current is reversed 

 through the coils by the battery-key. Up to the present time (May 1868) 

 various causes have prevented the obtainment of as satisfactory results as the 

 method described above allows us to expect. Eleven sets of experiments, 

 made at various dates, from March 10 to May 8 of the present year, have in- 

 dicated values for v, of which the greatest was 292x10% the smallest 

 275-4x10'*, and the mean 282-5x10" centimetres per second. Sir W. 

 Thomson intends to continue the investigation, hoping to attain much greater 

 accuracy. 



[P.S. Nov. 1869. — A new form of absolute electrometer has now been 

 completed and brought into use, with good promise as to accuracy and con- 

 venience. A glass jar constituting the " Leyden battery" contains within it 

 the " absolute electrometer " proper, the " idiostatic gauge," and the " re- 

 plenisher." One observer can use it effectively ; although it is more easily 

 worked by two, one maintaining constant potential in the Leyden jar by aid 

 of the idiostatic gauge and the replenisher, and the other attending to the ab- 

 solute electrometer fmain balance and micrometer screw). The main balance, 

 giving electric weighing in known weights, is as steady and as easily used as 

 any of the " attracted disk " electrometers, whether portable or stationary, 

 described in previous Reports.] 



Errata in Plate VI. 



For Dy Scale read D.\Tiamometer Scale. 



,, Ideostatic read Idiostatic. 

 Add a connexion between outside of Leyden battery and one terminal of the neighbouring 



Electrometer key. 



Experiments on the Value of v, the Hatio of the Electromagnetic to the Elec- 

 trostatic Unit of Electricity. By J. Clekk Maxwell. 



The experiments consisted in observing the equilibrium between the elec- 

 trostatic attraction of two disks, at a certain difference of potential, and the 

 electromagnetic repulsion of two coils traversed by a certain current. For 

 this purpose one of the disks, with one of the coils at its back, was attached 

 to one arm of a torsion-balance, while the other, with the other coil at its 

 back, was capable of being moved to various distances from the suspended 

 disk by a micrometer screw. Another coil, traversed by the same current 

 in the opposite direction, was attached to the other arm of the torsion- 

 balance, so as to do away with the effect of terrestrial magnetism. 



The fixed disk was larger than the suspended disk, and the latter, when 

 in its zero position, had its surface in the same plane as that of a "guard- 

 ring," as in Sir W. Thomson's electrometers. Its position and motion were 

 observed by means of a microscope, directed to a graduated glass scale, con- 

 nected with the disk. When the microscope was adjusted so that the 

 image of the zero line on the glass scale coincided with the cross wires of the 

 microscope, the very smallest motion of the scale could be easily detected, 

 so that the observations were very rapid. The disk was brought to zero by 

 the tangent screw at the top of the suspension-wire, and its equilibrium was 

 always observed at zero. The equilibrium, when the electrical forces were 

 applied, was always unstable. This electrical balance was made by Mr. 



