ELECTRICITY 



2843 



ELECTRICITY 



Electricity. A demonstration of electrical experiments made before Queen Elizabeth by William Gilbert of Colchester 

 (1540-1603), an English pioneer in electrical discovery 



After the painting by A. Aeland Hunt 



as they exerted a force of re- 

 pulsion on a glass rod which had 

 been rubbed with silk or on a 

 stick of resin which had been 

 rubbed with flannel. Quite early 

 in the eighteenth century electric 

 machines capable of producing 

 fairly intense effects, such as spark 

 and physiological shock, were de- 

 signed. They took the form of balls, 

 cylinders, or disks of sulphur or 

 glass rotated by hand and rubbed 

 by the dry palm of the operator or 

 by a pad of silk, flannel, or rubber, 

 coated with a metal ama'gam. 



Electricity's Dual Nature 

 It was Du Fay who apparently 

 was the first to make the postulate 

 that electrification is the result of 

 an inequality in the amounts of 

 two " electric fluids " or " electri- 

 cities " which a body possesses, an 

 excess of positive fluid producing 



Eositive electrification (similarly 

 sr negative), equality of amount 

 resulting in neutrality or absence of 

 " charge." Franklin, in America, 

 maintained a one-fluid theory, 

 neutrality corresponding to the 

 possession of a normal amount of 

 the fluid on the part of the body, 

 while positive and negative effects 

 are the result of excess over or de- 

 fect under this amount. As a mat- 

 ter of fact, while the main results 

 of the science can be expressed 

 readily in terms of either theory, 

 recent research on the structure 

 of the atom rather emphasises the 

 dual nature of electricity. 

 Much more important than 



either of these speculations was the 

 discovery of induction and the 

 development of condensers. Modern 

 text-books make a point of present- 

 ing the concept of potential to the 

 student's mind at an early stage. 

 There is no doubt in any teacher's 

 mind of the difficulty experienced 

 in acquiring correct ideas concern- 

 ing this concept, probably due to 

 the fact that, as human beings, we 

 possess no sense corresponding to 

 that by which we appreciate tem- 

 perature (the analogous concept 

 in the science of Heat) ; and so 

 great use is made of analogies in 

 such presentation. For example, 

 the notion of pressure is appealed to, 

 and the flow of electricity from one 

 conductor to another at a different 

 " potential" is likened to the flow of 

 gas along a tube from one flask to 

 another at different pressures. 



The Leyden Jar 



All these analogies, however, 

 break down in one important par- 

 ticular. The mere juxtaposition of 

 a flask of gas at great pressure does 

 not affect the pressure of gas in a 

 neighbouring receptacle. But the 

 presence of an electrified body has 

 a very marked influence on the 

 potentials of all neighbouring in- 

 sulated conductors. The first dis- 

 covery of this fact is due to Von 

 Guericke in the 17th century; but 

 its application to the manufacture 

 of condensers, i.e. conductors which 

 can retain a relatively enormous 

 charge at a moderate potential, 

 did not begin until the middle 



of the eighteenth, when the so- 

 called Ley den jar was accidentally 

 discovered by Musschenbroek and 

 Cunaeus while endeavouring, by 

 means of a chain depending from a 

 machine, to electrify water con- 

 tained in a bottle which rested on 

 the observer's hand. 



Development of Electrokinetics 



This discovery was the starting 

 point for the development of the 

 condensers, which play such an 

 important part in the induction 

 coils, telegraphic, telephonic, and 

 wireless apparatus of to-day, and 

 also of the influence or induction 

 machines of the Voss type, which 

 have completely displaced the old 

 frictional machines. The middle 

 of the 18th century also witnessed 

 Franklin's famous investigations 

 on atmospheric electricity, his 

 identification of lighting with the 

 electric spark, and the subsequent 

 discovery that even in fine weather 

 there is a progressive difference of 

 potential between the air and the 

 earth's surface with increase of 

 altitude. In the 18th century were 

 laid the foundations of the mathe- 

 matical theory, due to the dis- 

 covery of the inverse square law of 

 force between electrified bodies by 

 Coulomb, in France, and indepen- 

 dently by Cavendish in England. 



At the very beginning of the 

 19th century came the extremely 

 important researches of Volta at 

 Pavia, leading to the development 

 of electrokinetics. His prime dis- 

 covery was that two plates of 



