SOME SPECIAL MUTUAL CHANGES 87 



of matter forming the circuit may dimmish the stress, and so 

 much may be added as to render it imperceptible. This, how- 

 ever, will not readily happen unless the matter added is a bad 

 conductor by reason of its nature or thinness. If any portion 

 of the circuit is much changed in temperature, the stress 

 varies, a rise in temperature generally causing a diminution. 



All these phenomena may be exhibited by using a galvano- 

 meter as well as by using an electrometer. 



We observe, then, that different kinds of matter have dif- 

 ferent effects upon the stress shown by an electric circuit. If 

 that portion of the circuit which is called the cell or battery 

 remains unaltered, the stress obtained will vary with the 

 quality, dimensions, quantity, and temperature of the rest of 

 the circuit. When the dimensions, quantity, and temperature 

 are kept the same, then the stress varies with the nature of 

 the matter employed. When the stress is large, the matter is 

 said to have good conductivity, and the value of the stress 

 varies with the degree of conductivity. In practical applica- 

 tion the term 'resistance' is more general. The greater the 

 stress, the lower is the resistance. The value of resistance will 

 therefore be the reciprocal of that of conductivity, and the use 

 of either will lead to the same result. It is easy to see that 

 both terms have their origin in the hypothesis of a current. 



It is important to note that the relations existing between 

 the stress and the matter in a circuit contained in it are also 

 observed to exist between the rate at which the process of 

 electric equilibrium takes place and the matter by which it 

 is effected. Hence we may regard an electric circuit as caused 

 by a system in which a state of equilibrium might readily 

 occur but for some process by which it is constantly disturbed. 



60. Change of Temperature in a Substance forming part of 

 an Electric Circuit. In addition to the stress which is shown 

 either by the movement of a magnet or the index of an electro- 

 meter, it will be found that the whole of an electric circuit 

 changes in temperature. The change will vary in different 

 portions of the circuit, according to the nature and dimensions 

 of the substances comprising it. 



This change may be shown by using a thermometer, which, 



