SCIENCE AND PRACTICE. 335 



them to natural units, or at least to units of universal 

 application. 



The equation wanting is that of force an expression for 

 the relation of the deflecting power of a circular current 

 on the poles of a magnet needle, the conductor being at 

 all points equidistant from the same. If the current form 

 a circle whose radius is r, round the magnetic pole as its 

 centre, the deflecting force will be directly proportional to 

 the length, L, of the conductor, to the magnetic intensity, m, 

 of the pole, and to the intensity, I, of the current, and 

 inversely to the square of the distance of the pole from 

 the conductor. Hence the force is 



Defining each of the electrical magnitudes as certain units 

 determined by existing units of time, mass, and space, we 

 have, according to the foregoing, the following expressions of 

 their mutual relations : 



The unit of intensity will be produced in the circuit of 

 unit resistance, by the unit of electro-motive force. 



This unit of intensity will convey the unit quantity through 

 the same circuit in the unit of time. 



The same unit of intensity will produce the equivalent 

 effect of the unit of work in the unit of time. 



Lastly, the unit of intensity circulating in a conductor of 

 unit length, will exert the unit force upon the unit pole at 

 an unit distance. 



80 far the elements of the theory. To put all this into 

 practice, and determine these units, is a work of much patience 

 and expense. 



The unit of intensity is the easiest to determine. The cir- 

 cular conductor, and short magnet needle suspended in its 

 centre, which are conditions considered in (4, forms a simple 

 tangent galvanometer. 



When a current, I, moves in the circuit L, and deflects 

 the needle through an angle degrees from the plane of the 

 magnetic meridian, we know that the force with which one 



