QUANTITY OF ELECTRIFICATION 25 



of great importance, may be verified with great exactness by 

 connecting the can to earth until the charged body is inserted. 

 The induced like electrification on the outside of the can is 

 discharged and the leaves remain together. They are then in the 

 most sensitive position, and the sensitiveness may be still further 

 increased bv approach of the side plates. If there were any 

 change in the effect on the leaves due to change of position of the 

 inside body it would now be noted most easily. 



Here we have a definite effect due to a charged body, which 

 may be easily observed, and one which is consistent. For on 

 successive withdrawals and insertions the same divergence of the 

 leaves is obtained, and the exact position of the charge within the 

 can is unimportant. We may, therefore, use this effect to compare 

 two different electrifications as to quantity. If two charged bodies 

 inserted successively, produce the same divergence, they are to be 

 regarded as having equal charges. If the divergence is not the 

 same the greater divergence corresponds to the greater charge. 

 Again, if two oppositely electrified bodies are inserted in the can at 

 the same time, they tend to send opposite electrifications into the 

 leaves and so to neutralise each the effect of the other. If the 

 leaves do not diverge at all, the charges are to be regarded as equal 

 in amount though opposite in kind. We have thus defined what 

 we mean by equal charges on different bodies, whether like or 

 opposite in kind, and \\e can now state the fundamental law of the 

 production and disappearance of electricity, viz. : 



The t:co electrification* ti/icat/Jt r.r/V hi equal amounts. If any 

 amount of one u/i/H'tirx or di^ippetir.^ an equal amount of the- other 

 appear* or d'tMi/tjtetir.f at the same tune. 



The truth of this law may be tested by the following simple 

 experiments with the gold-leaf electroscope and can. 



Electrification by friction. If a rod of sealing-wax and a 

 rod of glax> are rubbed together within the can the leaves do not 

 diverge. They are both electrified, however, for on withdrawing 

 OIK- of them the effect of the other is at once evident. But since, 

 before the withdrawal of one, the leaves did not diverge, it follows 

 that the two kinds of electrification are produced in equal 

 amounts, one on the sealing-wax and the other on the glass. 

 \V uiav u^e other pairs of substances, and in every case we shall 

 find that the two bodies rubbed together are electrified oppositely 

 with equal amount^. 



Electrification by conduction. If we first lower an insulated 

 charged conductor into the space inside the can, touching the can to 

 earth meanwhile so that the leaves remain together and in the 

 portion mo>t sensitive to variation of charge, and if we then 

 lower a >< (oiid iiiMilated uncharged conductor into the can till it 

 toiichc.s tin- fir-t. ire know that conduction takes place from the 

 first to the MTond. Hut the electroscope #ives no indication; 

 then the total amount of electrification remains the same, though 



