CHAPTER I. 



THE ELECTRIC CURRENT: ITS CHEMICAL EFFECT. 



1. The electric current. When a wire is connected to the 

 terminals of an ordinary battery, certain phenomena are produced 

 and an electric current is said to flow through the wire. A wire 

 in which an electric current is flowing is sometimes called an 

 electric wire for brevity. The production of an electric current 

 always requires a generator such as a battery or a dynamo. The 

 path of the current is usually a wire and it is termed the electric 

 circuit. If the path is complete, leading out from the generator 

 and returning to it without break or interruption, the circuit is 

 said to be closed ; otherwise, the circuit is said to be open. A 

 steady electric current always flows in a closed circuit, that is, in 

 a circuit which goes out from the generator and returns to it, and 

 the current ceases to flow when the circuit is broken. 



Certain substances such as metals and salt solutions may form 

 portions of an electric circuit. Such substances are called electri- 

 cal conductors. Other substances, such as glass, hard rubber, air, 

 and dry wood, cannot form a portion of an electric circuit, that 

 is, the electric current cannot flow through them to any appreci- 

 able extent. Such substances are called insulators* 



Energy must be supplied to an electric generator (chemical 

 energy in the case of a battery, mechanical energy in the case of 

 a dynamo), and this energy reappears in various parts of the 

 electric circuit through which the current flows. Thus, energy 

 reappears as heat in an electric lamp and as mechanical work in 

 an electric motor. 



The magnetic effect of the electric current. When an electric 

 wire is held above a compass and parallel to the compass needle, 

 the compass needle is deflected. When an electric wire is 



* All substances conduct the electric current more or less. See Art. 14. 

 2 I 



