Page:Popular Science Monthly Volume 8.djvu/632

614 when his last experiments were made, that he was unable to 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.

One word of definition will be useful here. Some substances, as proved by Stephen Gray, possess in a very high degree the power of permitting electricity to pass through them; other substances stop the passage of the electricity. Bodies of the first class are called conductors; bodies of the second class are called insulators.

We cannot do better than repeat here the experiments of Gray. Push a cork into the open end of your glass tube; rub the tube, carrying 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 worked so much.

But 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, even 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. Further Inquiries on Conduction and Insulation.—A little addition to our apparatus will now be desirable. You can buy a book of "Dutch metal" for fourpence, and a globular flask like that shown in Fig. 5 for sixpence, or at the most a shilling. Find a cork, C, which fits the flask; pass a wire, W, through the cork, and bend it near one end at a right angle. Stick by sealing-wax upon the other end of the wire a little plate of tin or sheet-zinc, T, about two inches