292 



ELEMENTARY LESSONS ON [CHAP. v. 



Fig. 120. 



and cd are movable round O as a centre. There 

 will be repulsion between a and d and between c 

 and b, while in the 

 other quadrants there 

 will be attraction, a 

 attracting c, and b at- 

 tracting d. 



The foregoing laws 

 may be summed up in 

 one, by saying that two portions of circuits, how- 

 ever situated, experience a mutual force tending 

 to set them so that their currents flow as nearly 

 in the same path as possible. 



(iv.) The force exerted between two parallel portions 

 of circuits is proportional to the product of the strengths 

 of the two currents, to the length of the portions, and 

 inversely proportional to the distance between them. 



333. Ampere's Table. In order to observe these 



Fig. 



attractions and repulsions, Ampere devised the piece of 

 apparatus known as Ampere's Table, shown in Fig. 121, 



