410 MR. DTJGALD M‘KICHAN ON THE DETERMINATION OF THE NUMBER 
Professor Clerk Maxwell has pointed out * a probable source of error in these expe- 
riments of MM. Weber and Kohlrausch. A neglect, in the measurement of the capa- 
city of the condenser, of the phenomenon of “ electric absorption,” the nature of which 
was not then well understood, would lead to an overestimation of the electrostatic capa- 
city of the condenser, and consequently to a value of v which would be too great. 
In the same paper Professor Clerk Maxwell describes a direct comparison of electro- 
static with electromagnetic force in which no absolute measurement of either was made. 
The ratio of electrostatic and electromagnetic units of quantity was determined by a 
direct experiment, in terms of the British-Association Unit of Resistance, by a balance 
of the electrostatic attraction of two disks maintained at a great difference of potentials 
and the electromagnetic repulsion of two circular coils attached to the attracting 
disks. The mean value of v derived from this comparison was 288-0 x 10 8 centims. per 
second. 
In the experiments to be described, the method employed was that of obtaining a 
common measure of electromotive force. The electromotive force produced by a battery 
can be measured directly in absolute electrostatic units ; its electromotive force can also 
be obtained in electromagnetic measure from the current which it produces in a circuit 
of known resistance. 
The measure of electromotive force from one point to another being defined (in the 
two systems) as the work done by electrical forces during the passage of unit quantity 
of electricity from the one to the other, v, the ratio of the units of quantity in the two 
systems, is also the inverse ratio of the units of electromotive force. 
For this comparison three independent measurements are required — the determi- 
nation of the resistance of the given circuit in absolute measure, the electrostatic 
measure of electromotive force produced by a given battery, and the electromagnetic 
measure of the current maintained in the circuit by this electromotive force. 
The resistance of the circuit was obtained, according to the usual method, from a 
comparison with resistance-coils derived from standards the resistance of which had 
been experimentally determined in absolute measure by observations completed in 1864 
at King’s College, for the Committee of the British Association, by Professors J. Clerk 
Maxwell, Balfour Stewart, and Fleeming Jenkin. Accordingly, whatever corrections 
may hereafter be discovered to be applicable to the absolute value of the unit of resist- 
ance must be applied to the value of v here determined. It is hoped that Professor 
Maxwell will soon be able to accomplish the new determination of the absolute resist- 
ance of the standard coil which he proposes to make in the Cavendish Laboratory, 
Cambridge University. 
The electromotive force of the battery, which consisted of a series of Daniell’s cells, 
was measured electrostatically or weighed by means of Sir William Thomson’s New 
Absolute Electrometer. A description of this instrument will be found in a “ Reprint 
of Papers on Electrostatics and Magnetism” by Sir William Thomson, §§ 364-367. 
* Philosophical Transactions, 1868, p. 651. 
