4 ELECTRICAL MACHINERY 



This machine which is able to continually maintain its 

 terminals at a difference of electric pressure while current 

 flows from one to the other, is said to generate an e.m.f. 

 and is called an electric generator. Again, it is to be noticed 

 that the machine generates an e.m.f. whether the con- 

 necting wire is used or not, i.e., whether current is flowing 

 or not. The machine generates an e.m.f. if it is capable of 

 sustaining a current between its terminals when they are 

 connected by some conductor. 



E.M.F. Generated by a Thermo-couple. If two strips of 

 dissimilar metals are joined together and this junction is 

 heated (the free ends of the strips being kept cool) the 

 combination generates an e.m.f. This may be ascertained 

 by connecting together the cool ends of the strips by a wire 

 and testing for the presence of a current 'in the wire. Such 

 a combination is called a thermo-couple. 



Nature of e.m.f. We may say in general that any 

 device which is capable of sustaining an electric current 

 through a closed circuit (of which circuit the device itself 

 is a part) generates an electromotive force. With our 

 present limited knowledge of electricity it is useless to try 

 to give an exact definition of electromotive force because 

 we know nothing about the nature of the force. In the 

 same way we cannot describe in simple terms the force of 

 gravitation. We only know that there is such a force as 

 gravitational force and that, as a result of the action of 

 this force, any two bodies in the universe are mutually 

 attracted toward one another and if free from the action 

 of other forces will move toward one another. Just as 

 gravitational force tends to produce motion in material 

 bodies, so electromotive force tends to produce motion of 

 minute electrical charges existing in all conducting bodies; 

 these electrical charges in motion constitute an electrical 

 current. 



3. Conductors Insulators Resistance. We have in 

 the previous paragraphs referred to bodies which freely 



