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XI. Determination of the Number of Electrostatic Units in the Electromagnetic Unit 
made in the Physical Laboratory of Glasgow University. By Dugald M‘Kichan, 
M.A. Communicated by Sir William Thomson, F.R.S. &c. 
Received April 15, — Read May 15, 1873. 
The object of the following paper is to describe observations made to determine the 
relation between the fundamental units in the electromagnetic and electrostatic systems 
of absolute measurement. 
These observations were made at intervals from 1870 to 1872, first in the Physical 
Laboratory of the Old College, Glasgow, and afterwards in that of the New College. 
In the former series of experiments I had the advantage of the assistance of the late 
William Leitcii, of Glasgow*. The latter series I made chiefly with the assistance of 
George A. Hill, M.A., now Assistant Professor of Natural Philosophy in Harvard 
College, U. S. 
A summary is also given of the results of similar experiments made by Mr. William 
F. King in the Old College in 1867 and 1868. 
The two systems of absolute electrical measurement are founded on the fundamental 
units of time, mass, and space applied to the observed effects of electricity at rest and 
electricity in motion, the one system having a definite numerical relation to the other.. 
The number of electrostatic units in the electromagnetic unit of quantity, from the 
dimensions of the unit of quantity in the two systems, is expressible as a velocity, and 
is usually designated by the symbol v. 
In the Report of the British Association’s Committee on Electrical Measurements, 
1863, five possible methods are enumerated for experimentally determining v. It is 
necessary and sufficient that we should obtain a common electromagnetic and electro- 
static measure of some electric quantity, current, resistance, electromotive force, or 
capacity. 
The first numerical determination of v had been made by Weber and KoHLRAUSCHf, 
w r ho had employed the method last indicated : they derived the value of v from a 
common measure of capacity. The capacity of a condenser was measured electro- 
statically by comparison with the capacity of a sphere of known radius, and electro- 
magnetically by passing the discharge from the condenser through a galvanometer. The 
value of v derived from their experiments was 310’74xl0 8 centimetres per second. 
* Mr. Leitch afterwards went out to introduce Sir William Thomson’s Siphon-recorder at the various stations 
of the Eastern Telegraphs. He died on his way home, at Alexandria, after he had completed his work. 
f Pogg. Ann., Aug. 1856, Band xcix. p. 10. 
MDCCCLXJIII. 3 I 
