2 ELECTRICAL MACHINERY 



i 



Healing Effect. When a current is flowing through a 

 conductor heat is always generated in the conductor and it 

 may get very hot. The filament of an incandescent lamp 

 and the electric welding machine, e.g., utilize this effect of 

 the electric current. If the conductor is large in cross- 

 section and th'e current not large, the heat generated may 

 be very small, but no matter how large in cross-section the 

 conductor may be or how small the current, some heat is 

 always generated. The law expressing the rate at which 

 the heat is developed will be given in a following para- 

 graph. 



Chemical Effect. In a city where electric trolley lines 

 run through districts where iron water or gas mains are 

 laid, it is often found that the iron pipes become disin- 

 tegrated after a few years, due to the action of leakage cur- 

 rents from the trolley lines. The iron of which the pipes are 

 constructed is made to combine with acids in the surround- 

 ing soil and is carried away; the action is called electrolysis. 

 In a copper-plating establishment current is passed through 

 a bath containing a solution of some copper salt and the 

 copper is taken from the solution and deposited upon one 

 of the metallic plates of the bath. These two actions 

 illustrate the chemical effect of a current. 



Physiological Effect. If a current of sufficient magnitude 

 is sent through a living human being death is caused almost 

 instantaneously. If the current is smaller the result may 

 not be fatal, but severe muscular contractions occur, giving 

 what is called an " electric shock." This is an example of 

 the physiological action of an electric current. 



2. Electromotive Force. In the previous paragraph 

 the effects of an electric current were discussed but nothing 

 was said as to how such a current might be produced. 



Generation of e.m.f. in a Primary Cell. If two metal 

 plates, one of zinc and the other of copper, are partly 

 immersed in a solution of sulphuric acid and the outside 

 ends of the two plates are joined together by a wire, an 



