

ELECTRICAL CONDUCTION AND INSULATION. 475 



virtue seemingly has no more trouble in traversing eighty 

 feet than as many inches; and then the line is carried 

 backward and forward to increase its length, until it meas- 

 ures over three hundred feet, when the silk threads break 

 under its weight. However, that is easily repaired, thinks 

 Gray, substituting metal wire for the silk; but now, to his 

 dismay, the attraction of the ball disappears. No matter 

 how vigorously they rub the tube, apparently no virtue 

 from it goes upon the line, for the bits of brass foil under 

 the ball at the far end remain motionless. Wheler's 

 happy suggestion of the silk thread supports, now results 

 in a great discovery. Why silk? 



"We are now convinced," says Gray, u that the success 

 we had before, depended upon the lines that supported the 

 line of communication being silk, and not upon their being 

 small." 



More than a century before, Gilbert had cut off electric 

 attraction by interposing silk or water between the electric 

 and the attracted body; and this had been done by Hauks- 

 bee, and, in fact, all the later experimenters. So also 

 the latter believed that substances were divided into elec- 

 trics and non-electrics, although the list of the former was 

 constantly increasing. But no one before had recognized 

 the fact that the electric virtue would apparently refuse to 

 pass over certain substances while freely traversing others, 

 and this even when the first were short bodies and the sec- 

 ond very long. In other words, Gray had discovered the 

 difference between the electric conductivity of bodies de- 

 pending on the substances composing them, and had 

 found in silk threads this conductivity so small as to be 

 inconsiderable. Some bodies evidently conveyed elec- 

 tricity and some did not, and those which did not could 

 be used to prevent the electrical virtue escaping from 

 those which did. Here began the world's practical and 

 useful knowledge of electrical conduction and insulation. 



Wheler's ingenuity rose to the occasion, and, by multi- 

 plying the silk threads, he managed to make the line 



