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XXVI. On a Method of making a Direct Comparison of Electrostatic with Electro- 
magnetic Force ; with a Note on the Electromagnetic Theory of Light. By J. Clerk 
Maxwell, F.B.8S. L. &E. 
Received June 10, — Read June 18, 1868. 
There are two distinct and independent methods of measuring electrical quantities with 
reference to received standards of length, time, and mass. 
The electrostatic method is founded on the attractions and repulsions between electri- 
fied bodies separated by a fluid dielectric medium, such as air ; and the electrical units 
are determined so that the repulsion between two small electrified bodies at a considerable 
distance may be represented numerically by the product of the quantities of electricity, 
divided by the square of the distance. 
The electromagnetic method is founded on the attractions and repulsions observed 
between conductors carrying electric currents, and separated by air ; and the electrical 
units are determined so that if two equal straight conductors are placed parallel to each 
other, and at a very small distance compared with their length, the attraction between 
them may be represented numerically by the product of the currents multiplied by the 
sum of the lengths of the conductors, and divided by the distance between them. 
These two methods lead to two different units by which the quantity of electricity is 
to be measured. The ratio of the two units is an important physical quantity, which 
we propose to measure. Let us consider the relation of these units to those of space, 
time, and force (that of force being a function of space, time, and mass). 
In the electrostatic system we have a force equal to the product of two quantities of 
electricity divided by the square of the distance. The unit of electricity will therefore 
vary directly as the unit of length, and as the square root of the unit of force. 
In the electromagnetic system we have a force equal to the product of two currents 
multiplied by the ratio of two lines. The unit of current in this system therefore varies 
as the square root of the unit of force ; and the unit of electrical quantity, which is that 
which is transmitted by the unit current in unit of time, varies as the unit of time and 
as the square root of the unit of force. 
The ratio of the electromagnetic unit to the electrostatic unit is therefore that of a 
certain distance to a certain time, or, in other words, this ratio is a velocity ; and this velo- 
city will be of the same absolute magnitude, whatever standards of length, time, and 
mass we adopt. 
The electromagnetic value of the resistance of a conductor is also a quantity of the 
nature of a velocity, and therefore we may express the ratio of the two electrical units 
MDCCCLXVII1. 4 T 
