644 
ME. CLEEK MAXWELL ON A DIEECT COMPAEISON 
in terms of the resistance of a known standard coil ; and this expression will be inde- 
pendent of the magnitude of our standards of length, time, and mass. 
In the experiments here described no absolute measurements were made, either of 
length, time, or mass, the ratios only of these quantities being involved ; and the velo- 
city determined is expressed in terms of the British Association Unit of Resistance, so 
that whatever corrections may be discovered to be applicable to the absolute value of 
that unit must be also applied to the velocity here determined. 
A resistance-coil whose resistance is equal to about 28*8 B. A. units would represent 
the velocity derived from the present experiments in a manner independent of all par- 
ticular standards of measure. 
The importance of the determination of this ratio in all cases in which electrostatic 
and electromagnetic actions are combined is obvious. Such cases occur in the ordinary 
working of all submarine telegraph-cables, in induction-coils, and in many other artificial 
arrangements. But a knowledge of this ratio is, I think, of still greater scientific 
importance when we consider that the velocity of propagation of electromagnetic dis- 
turbances through a dielectric medium depends on this ratio, and, according to my cal- 
culations*, is expressed by the very same number. 
The first numerical determination of this quantity is that of Weber and KoHLRAUSCHf, 
who measured the capacity of a condenser electrostatically by comparison with the capa- 
city of a sphere of known radius, and electromagnetically by passing the discharge from 
the condenser through a galvanometer. 
The Electrical Committee of the British Association have turned their attention to 
the means of obtaining an accurate measurement of this velocity, and for this purpose 
have devised new forms of condensers and contact-breakers; and Sir William Thomson 
has obtained numerical values of continually increasing accuracy by the constant improve- 
ment of his own methods. 
A velocity which is so great compared with our ordinary units of space and time is 
probably most easily measured by steps, and by the use of several different instruments ; 
but as it seemed probable that the time occupied in the construction and improvement 
of these instruments would be considerable, I determined to employ a more direct method 
of comparing electrostatic with electromagnetic effects. 
I should not, however, have been able to do this, had not Mr. Gassiot, with his usual 
liberality, placed at my disposal his magnificent battery of 2600 cells charged with cor- 
rosive sublimate, with the use of his laboratory to work in. 
To Mr. Willoughby Smith I am indebted for the use of his resistance-coils, giving a 
resistance of more than a million B. A. units, and to Messrs. Forde and Fleeming 
jenkin for the use of a galvanometer and resistance-coils, a bridge and a key for double 
contacts. 
Mr. C. Hockin, who has greatly assisted me with suggestions since I first devised the 
* “ A Dynamical Theory of the Electromagnetic Eield,” Philosophical Transactions, 1865. 
t Pogg. Ann. Aug. 1856, Bd. xcix. p. 10. 
