SOME SPECIAL MUTUAL CHANGES 81 



which is commonly described as having a current of electricity 

 flowing through it. This description has been so long used that 

 it has become a familiar term, but subsequent investigation 

 will show how far it is suitable or correct. 



The nature of the material used for bringing about the 

 equilibrium has an im- 

 portant bearing. Under 

 ordinary circumstances 

 air lies between the two 

 bodies, but in this case 

 equilibrium is indefinitely 

 retarded. The same will 

 be the case with glass, 



ebonite, and all insulators or non-conductors ; but with con- 

 ductors it takes place readily, and still more readily when 

 the conductors are relatively large. 



Two processes by which electric equilibrium may be brought 

 about are shown above (fig. 26). 



55. An Electric Circuit, Conditions necessary for. If 

 strips of copper and zinc are placed in hydrogen sulphate and 

 put in metallic connection with one another outside the liquid, 

 changes may be observed in the liquid and also in the zinc 

 employed. Investigation will prove these changes to be per- 

 manent. When they cease, the system is no longer able to 

 exhibit the electric properties of which it has already been 

 shown to be possessed. 



The same statement holds good for any of the other elec- 

 tric systems or cells. Co-existent with these changes there is 

 one which is not apparent, but which may be perceived by 

 special means. The space in the neighbourhood of the system 

 is in a certain condition afterwards to be investigated, and we 

 may speak of the bodies constituting such a system as an 

 ' electric circuit.' 



The quantity of matter in a circuit may be increased 

 without any alteration in the nature of the properties acquired, 

 provided the additional matter inserted is conducting matter ; 

 but the magnitude of the effects, characteristic of such a 

 system, may be shown to vary with any change either in the 



