MUTUAL ACTION OF CURRENTS. 693 



After the edge has been removed the face of the copy may be 

 polished with a small brush dipped into a little l whitening/ which 

 has been rubbed to a soft pulp with a few drops of spirit of wine. 



Whitening (or ' Spanish white ') is sold by oil and colourmen. 



50. Action of galvanic currents upon each other. 

 Ampere's laws. Two conductors traversed by galvanic 

 currents exert a mutual action upon each other which 

 depends on the relative position of the conductors and 

 the direction of the currents. In order to observe the 

 effects produced by this mutual action, one at least of 

 the conductors must be easily movable. If a fixed 

 conductor, through which a current passes, be brought 

 into the vicinity of such a movable conductor, which 

 is parallel to the first and is traversed by a current 

 which flows in the same direction, attraction takes 

 place, and the movable conductor approaches to the 

 fixed. But if the current in the first conductor 

 flows in a direction opposite to that of the current in 

 the movable conductor, repulsion takes place, and the 

 movable conductor recedes from the fixed. Finally, if 

 the currents cross each other, the movable conductor 

 turns about its axis of motion until it is parallel to the 

 fixed and both currents flow in the same direction. 



The laws which regulate these effects of currents 

 upon each other, are called Ampere's laws, and may be 

 enunciated thus : 



I. Two parallel currents in the same direction attract 

 one anotJier. 



II. Ttvo parallel currents in contrary directions repel 

 one another. 



III. Two intersecting currents tend to become parallel 

 and to be in the same direction. 



