ELECTRICITY FROM T HALES TO FARADAY. 245 



conductors. The only source of electricity which was at the disposal 

 of experimenters up to this time was the electrical machine, improved, 

 as described, by Newton, which furnished intermittent currents (dis- 

 charges) of infinitesimal quantity, as we should say now, but of ex- 

 tremely high pressure. This fact of the enormous pressure resulted 

 in the electricity's forcing its way through very imperfect conductors, 

 so as to cause our investigators to rank many of these latter with the 

 metals. Thus Gray concluded that pack thread was a good con- 

 ductor because it did not oppose sufficient resistance to prevent the 

 flow of his high pressure (or, as we should now say, high voltage or 

 tension) electricity. He tried wire as well, but did not realize it was 

 a better conductor than the thread, although its conductivity was 

 actually in the millions of times as great. In collaboration with his 

 friend Wheeler he conveyed electrical discharges a distance of eight 

 hundred and eighty-six feet, through presumably air-dry pack 

 thread — an achievement which would almost be notable at the 

 present time. He insulated the line by hanging it from loops of 

 silk thread. 



Gray hoped " that there may be found out a way to collect a 

 greater quantity of electric fire, and consequently to increase the force 

 of that power, which, si licet magnis componere parva, seems to be 

 of the same nature with thunder and lightning." 



About this time Desaguliers discovered that those materials 

 which, upon being rubbed, develop electrical charges, are all noncon- 

 ductors, and that, conversely, nonelectrics are conductors. The terms 

 electrics and nonelectrics were applied to bodies respectively capable 

 and incapable of excitation; the words idioelectrics and anelectrics 

 were also used in respectively equivalent senses. 



In France, Dufay discovered that the conductivity of pack thread 

 was greatly improved by the presence of moisture, and he succeeded 

 in conveying a discharge a distance of almost thirteen hundred feet. 

 He suspended himself by silken cords and had himself electrified, and 

 then observed that he could give a shock accompanied by a spark to 

 any person standing on the ground. 



He also established the fact of the two opposite kinds of electri- 

 fication, and gave them the names of vitreous and resinous, from the 

 fact that the former was developed by the excitation of glass and 

 vitreous substances generally, and the latter from that of amber 

 and resins. He observed that the distinguishing characteristic of 

 the two was the fact that opposite charges attracted each other, while 

 similar ones exerted mutual repulsion. Dufay and Gray died within 

 three years of each other, both at the age of forty. Gray having 

 added to the results already mentioned the discovery of the conduct- 

 ing powers of certain liquids and of the human body. 



