1-26 PHYSICAL SCIENCE 



small to be investigated by means of a galvano- 

 meter. By the aid of an electrometer, however, 

 or by the use of some form of gold leaf electro- 

 scope, the passage of electricity may be detected, 

 and the amount of the current determined. 



The quadrant electrometer consists of a light 

 but rigid strip of aluminium or silvered paper, 

 suspended horizontally by a fine quartz fibre. 

 This strip is kept permanently charged with 

 electricity, and is therefore deflected when other 

 charges are given to brass quadrants which 

 surround it. By the rate at which the deflection 

 diminishes, it is possible to estimate the rate at 

 which the charge on the quadrants, and on any 

 conductor connected with them, disappears or 

 increases. 



Still simpler and yet more sensitive is the gold 

 leaf electroscope, in which a thin strip of gold leaf 

 is attached to a brass plate, and charged with 

 electricity. Owing to the repulsive forces between 

 portions of the same charge, the gold leaf is 

 repelled from the plate and stands out at an 

 angle. By observing through a microscope the 

 rate at which the leaf falls, we can determine the 

 rate at which its charge leaks away. 



Whichever apparatus be adopted, the natural 

 leak, due to the apparatus itself and the air 

 surrounding it, must first be determined, and 

 subtracted from the leakage afterwards found 

 under the influence of an ionizing agency. 



In the last chapter we have seen that the 

 properties of conducting solutions have been 

 successfully co-ordinated and explained on the 

 hypothesis that the passage of a current is effected 

 by the motion of charged particles called ions. 



