Chap. IV.] 



INDUCTION COILS. 



39 



of electricity, circulating round a wire wound on a 

 piece of soft iron, converts the iron into a temporary 

 magnet, and that on removal of the current the iron 

 speedily loses its magnetism. Thus, if the two coils be 

 used as shown in Fig. 20, and if, in the centre of the 

 primary coil, there be placed a core formed of a piece 

 of soft iron or of a bundle of iron wires, as soon as the 

 current is established in the primary coil the core 

 becomes magnetised, and by this means the inductive 

 action of the primary on the secondary coil is intensi- 

 fied. It is also needful to note that the nearer the 

 inducing current is, the more considerable becomes 

 the induced current, and the farther removed the 

 inducing current the feebler is the induced. These 

 are the general principles adopted in the construction 

 of induction coils 

 made for the pur- 

 pose of generating 

 induced currents 

 capable of being 

 employed for prac- 

 tical purposes. The 

 first successfully to 

 construct such a 

 coil was Ruhmkorff; 

 and hence the appa- 

 ratus is called RUHMKORFF'S COIL. It consists of a 

 hollow cylinder, on which is coiled a copper wire about 

 two to three millimetres in diameter, and about 40 to 

 50 yards in length. As already noted, the wire must 

 be carefully insulated. Inside the cylinder is a core 

 of soft iron, either in one piece or consisting of a 

 bundle of iron wires. This primary coil is inclosed in 

 a cylinder of insulating material, glass, or caoutchouc. 

 On the outside of the insulating case is wound the 

 secondary coil, each turn being carefully insulated from 

 another. This coil is made of wire much thinner than the 



Fig. 22. Buhrnkorff's Coil. 



