[From the Philosophical Transactions, Vol. CLVin.] 



XXXVI. On a Method of Making a Direct Comparison of Electrostatic with 

 Electromagnetic Force; with a Note on the Electromagnetic Theory of Light. 



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 be-, 

 tween electrified 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. 



