60 



ELECTRO-MAGNETIC 



of these instruments; but their principal difference consists in 

 the means adopted for breaking the battery contact, and so 

 giving rise to the secondary current. The form of apparatus 

 which appears to us to be both simple and manageable, is that 

 represented in the accompanying diagram. 



Fig. 13 



Fig. 14, D represents the Electro-magnetic coil, the construc- 

 tion of which will now be readily understood. It consists of two 

 coils of insulated copper wire ; the one internal, which, by means 

 of the binding screws, can be connected with a Galvanic battery, 

 as seen in the figure. This is called the primary coil. The other, 

 or secondary coil, is placed externally, and is in connection with 

 the four binding screws, marked respectively 1, 2, 3, and 4. In 

 the centre of the primary coil is placed a bundle of fine iron 

 wires, (seen at B,) for the purpose of increasing the intensity of 

 the effect, as already explained. Fig. 13 is a Smee's battery, 

 connected with the Electro-magnetic coil. The moment that 

 battery contact is made, the electricity circulating through the 

 bundle of iron wires renders them magnetic, and they immediately 



