244 PLEASANT WA YS TN SCIENCE. 



current. This needle is inside the coil^ the needle seen turns 

 on the same axis, which projects through the coil. 



If, then, the observer at the station B have a magnetic 

 needle suitably suspended, round which the wire from the 

 battery at A has been coiled, he can tell by the movement of 

 the needle whether a current is passing along the wire in 

 one direction or in the other ; while if the needle is at rest he 

 knows that no current is passing. 



FIG. 2. 



Now suppose that p and N, Fig. 2, are the positive and 

 negative poles of a galvanic battery at A, and that a wire passes 

 from P to the station B, where it is coiled round a needle 

 suspended vertically at n, and thence passes to the negative 

 pole N. Let the wire be interrupted at a b and also at c d. 

 Then no current passes along the wire, and the needle n 

 remains at rest in a vertical position. Now suppose the 



FIG. 3. 



points a b connected by the wire a b, and at the same moment 

 the points c d connected by the wire c d, then a current 

 flows along P a b to B, as shown in Fig. 2, circuiting the coil 

 round the needle n and returning by d c to N. The upper 

 end of the needle is deflected to the right while this current 

 continues to flow ; returning to rest when the connection is 

 broken at a b and c d. Next, let * b and a d be simultane- 



