WHAT IS ELECTRICITY? — HEYL 231 



This law has been the subject of some very searching experiments. 

 We may operate within a large conducting cube, such as was built 

 by Faraday at the Eoyal Institution ; perform within it all the usual 

 electrical experiments; excite a glass tube by rubbing it with fur; 

 draw sparks from an electrical machine; and yet a sensitive gold- 

 leaf electroscope connected to the cube will remain undisturbed. It 

 seems impossible to create or destroy an electric charge without a 

 compensating creation or destruction of an equivalent charge of the 

 opposite sign. 



And yet the era of thought which has not hesitated to question the 

 conservation of energy can hardly be expected to respect this elec- 

 trical principle; and, in fact, this law has been brought under fire 

 from several quarters. If these points of order are sustained, they 

 will have an important bearing on future answers to the question, 

 Wliat is electricity? 



It is well to remember in this connection that all the experiments 

 upon which is based the law of conservation of electric charge have 

 started with neutral bodies. The glass tube and the fur were at first 

 neutral, but exhibited equal and opposite charges after being rubbed 

 together; the electrical machine was at first neutral, but on being 

 operated its two sides became equally and oppositely charged. 



Suppose a chemist should announce that as a result of the analysis 

 of several thousand neutral salts he had come to the conclusion that 

 acid and basic radicals existed in equal amounts in nature ; we would 

 likely think him ignorant of such syntheses as that of the acid radical 

 cyanogen (CN) from its elements in the electric arc. But is there 

 any laiown electric analogue of such a synthesis or its reverse disso- 

 ciation ? No ; nothing that we have so far been able to produce in the 

 laboratory ; yet if we imagine some race of children of the gods who 

 could play with planets as we with pith balls, something of this kind 

 might come to their notice. 



Among the phenomena of atmospheric electricity there is an un- 

 solved mystery. Many fruitless attempts have been made to explain 

 it consistently with the principle of conservation of electrical charge. 

 Continual failure has led more than one physicist to look for the 

 explanation in a slight departure from this principle, and it has been 

 shown that a departure so slight as to be beyond laboratory detection 

 would yet, on the large scale, solve this mystery. The difficulty in 

 question is to account for the negative charge of the earth. 



Our earth is not a neutral body. Its entire surface is negatively 

 charged to such an amount that there exists near the surface ft 

 potential gradient of 150 volts per meter. The conductivity of the 

 atmosphere is small, but not zero; and because of this conductivity 

 and the potential gradient there is a continual conduction of negative 



