286 LECTURES TO SCIENCE TEACHERS. 



the force exerted by one current on another, or by one 

 portion of a current on another portion of the same 

 current. There are on the table two instruments which 

 act on this principle. Professor Guthrie's may be regarded 

 as one of this class. The same current passes round two 

 pairs of small pieces of iron, and it is the action of one 

 part of the current on another which really gives us 

 the indication. Here, again, is an apparatus where the 

 electro-dynamic action of the current is applied more 

 simply, and without the intervention of the bits of iron. 

 We have a circular coil of wire, to begin with, through 

 which the current passes ; then instead of suspending a 

 magnet at the centre of the coil, there is another coil 

 suspended inside, and the two coils are so set that when 

 no current is passing, the planes of the separate portions of 

 the wire are at right angles to each other. The wire in 

 the fixed coil is in a plane parallel to the length of the 

 table, but in the suspended coil the wire is wound almost 

 at right angles to that direction. When it is properly 

 adjusted, the directions of winding the two coils are at right 

 angles to each other, but when a current passes through 

 both of them there is a tendency in the two coils to set 

 parallel to each other. If they are at right angles to 

 begin with, a force which would tend to set them parallel, 

 will cause a deflection, and the amount of this deflection, 

 combined with a knowledge of the force resisting the 

 displacement, and of the relative lengths and positions of 

 the two parts of the conductors which act upon each other, 

 gives us again the means of ascertaining the strength of the 

 current. When we employ this principle the statement 

 which involves the definition of the unit current may be 

 put in this way : Unit current placed around the circum- 

 ference of a circle of unit area exerts upon an equal current, 

 enclosing an equal area in a plane at right angles to the 

 first, a couple whose moment multiplied by the cube of the 

 distance between the areas surrounded by the currents 

 equals unity when the distance is very great. In order to 

 make the statement of the mechanical force simple, you 

 must imagine that the two currents are at a very great 

 distance from each other, as compared to the size of 

 the area surrounded by either of them; the force ex- 

 erted by one upon the other diminishes as the distance 



