S IX ELECTRICITY. 



29,1 



longer, but f-*,ill found a power of attrac- 

 tion at thmi' ends. lie then passed on 

 to pack-thread and wire. Hanging ;i 

 thread s, fig. G, from the top window of 

 a house, so that the lower end nearly 

 touched the ground, and twisting the up- 

 per end of the thread round his glass 

 tube R, on briskly rubbing the tube, light 

 bodies were attracted by the lower end 

 B of the thread. 



But Gray's most remarkable experi- 

 ment was this : IIo suspended a long 

 hempen line horizontally by loops of pack- 

 thread, but failed to transmit through 

 it tlm electric power. He then suspend- 

 ed it by ioops of silk and succeeded in 



FIG. 6. 



sending the " attractive virtue" through 

 765 feet of thread. He at first thought 

 the silk was effectual because it was thin ; 

 but on replacing a broken silk loop by a 

 t-till thinner wire, he obtained no action. 

 Finally, he came to the conclusion that 

 his loops were effectual, not because they 

 wore thin but because they were silk. 

 This was the starting-point of our knowl- 

 edge of Insulation. 



It is interesting to notice the devotion 

 of some men of science to their \\<uk. 

 Dr. Wells, who wrote a beautiful CHK.-.V, 

 wherein he explained the oriuiu of dew, 



finished it when he was on the brink of 

 the grave. Stephen Gray was so near 

 dying when his last experiments were 

 made, that he was unable lo write out an 

 account of them. On his death-bed, 

 and, indeed, the very day before his 

 death, his description of them was taken 

 from his lips by Dr. Mortimer, Secretary 

 of the Royal Society, and afterward 

 printed in the " Philosophical Transac- 

 tions." 



One word of definition will be useful 

 here. Some substances, as proved by 

 Stephen Gray, possess in a very high de- 

 gree the power of permitting electricity 

 to pass through them ; other substances 

 ptop the passage of the electricity. 

 Bodies of the first class are called con- 

 ductors ; bodies of the second class arc 

 called insulators. 



You cannot do better than repeat here 

 the experiments of Gray. Push a cork 

 into the open end of your glass tube ; 

 rub the tube, carrving the friction up 

 to the end holding the cork. The cork 

 will attract the balanced lath, shown in 

 fig. 4, with which you have already 

 \vorked so much. 



I5ut the excited glass is here so near 

 the end of the cork that you may not 

 feel certain that the observed attraction is 

 that of the cork. You can, however, 

 prove that the cork attracts by its action 

 upon light bodies which cling to it. 

 Stick a pen-holder into the cork and rub 

 the glass tube as before. The free end 

 of the holder will attract the lath. Stick 

 a deal rod three or four feet long into 

 the cork ; its free end will attract the 

 lath when the glass tube is excited. In 

 this way you prove to demonstration 

 that the electric power is conveyed along 

 the rod. 



7. The Electroscope. Further Inquir- 

 ies on Conduction and Insulation. 



A little addition to our apparatus will 

 now be desirable. You can buy a book 

 of " Dutch metal " for fourpence ; an 1 

 a globular flask like thst shown in fi.j. 

 ?, for sixpence, or at the most a shilling. 

 Find a cork, <:, which tin the fl ik ; pn-a 

 a wire, w, through the cork an 1 bend it 

 near one end at, a right ati-jfle. Attach 

 by means of w:ix to the bent arm, which 

 ought to be about three (ju.-irters of an. 

 inch Jong, two strips, i, of the Dutch 



