182 JAMES CLEKK MAXWELL 



is measured by the current flowing through the wire, 

 and is obtained in the system of units usually adopted 

 by multiplying the current by 4?r. This last result then 

 gives us one of the necessary relations, that between 

 the magnetic force due to a current and the strength 

 of the current 



Again, consider a steady current flowing in a 

 conductor of any form or shape, the total How of 

 current across any section of the conductor can be 

 measured in various ways, and it is found that at any 

 time this total How is the saihe for each section of the 

 conductor. In this respect the How of a current re- 

 sembles that of an incompressible fluid through a 

 pipe ; where the pipe is narrow the velocity of How 

 is greater than it is where the pipe is broad, but the 

 total quantity crossing each section at nny given 

 instant is the same. 



Consider now two conducting bodies, two spheres, 

 or two flat plates placed near together but insulated. 

 Let each conductor be connected to one of the poles 

 of the battery by a conducting wire. Then, for a very 

 short interval after the contact is made, it is found 

 that there is a current in each wire which rapidly dies 

 away to zero. In the neighbourhood of the balls 

 there is electric force; the balls arc s;iid to be charged 

 with electricity, and the lines of force aro curved lines 

 running from one ball to the other. It is found that 

 the balls slightly attract each other, and the space 

 between them is now in a ditlerent condition from what 

 it was before the balls were charged. According to 

 Maxwell, Electric Displacement has been produced 

 in this space, and the electric displacement at each 



