TUB TELKTHONK. 



of coaroluuon returns into iteelf. Round such a circuit an electric 

 my JIM, and will flow if there is an electromotive force to drive it. 



lest a li of magnetic force, such a line as you see here made 



by prinkling iron filing* on a sheet of paraffin paper. This line, as 

 r also tint showed, ia a line returning into itself, or, as the mathema- 



woald my. it ia a closed curve. 



v if there are two closed curves in space, they must either embrace 

 Doctor so aa to be linked together, or they must not embrace each other. 

 If the line of force as well as the circuit were made of wire, and if it 

 he copper circuit, it would be impossible to unlink them without 

 cutting one or other of the wires. But the line of force is more like one of 

 Milton'* spirit*, which cannot 



"In their liquid texture mortal wound 

 Receire, no more than can the fluid air." 



Now, if the copper circuit or the lines of force move relatively to each 

 other, then in general some of the lines of force which originally embraced the 

 circuit will CIQMQ to embrace it, or else some of those which did not embrace 

 it will become linked with it, 



For every line of force which ceases to embrace the circuit there is a 

 certain amount of positive electromotive force, which, if unopposed, will generate 

 a current in the positive direction, and for every new line which embraces 

 the circuit there is a negative electromotive force, causing a negative current. 



In Bell's telephone the circuit forms a coil round a small core of soft iron 

 fcatened to the end of a steel magnet. Now lines of magnetic force pass more 

 freely through iron than through any other substance. They will go out of 

 their way in order to pass through iron instead of air. Hence a large proportion 

 of the lines of force belonging to the magnet pass through the iron core, and, 

 therefore, through the coil, even though there is no iron beyond the core, so 

 that they have to complete their circuit through air. 



Hut if another piece of soft iron is placed near the end of the core it 

 will afford greater facilities for lines which have passed through the core to 

 complete their circuit, and so the lines belonging to the magnet will crowd 

 till closer together to take advantage of an easy passage through the core 

 and the iron beyond it. If then the iron is moved nearer to the core, there 

 will be an increase in the number of such lines, and, therefore, a negative 

 current in the circuit If it is moved away there will be a diminution in the 



