Page:Popular Science Monthly Volume 8.djvu/634

616 a pivot formed by a sewing-needle, and supported on a bent strip of metal, as shown in the figure. By weighting the straw with a little wire near N, you so balance it that the plate N shall be just lifted away from M. The wire w which may be 100 feet long, proceeds from M to your glass tube, round which it is coiled. A single vigorous stroke of the tube by the rubber sends electricity along w to M;



N is attracted downward, the other end of the long straw being lifted through a considerable distance. In subsequent figures you will see the complete straw-index, and its modes of application.

A few experiments with either of these instruments will enable you to classify bodies as conductors, semi-conductors, and insulators. Here is a list of a few of each, which, however, differ much among themselves:

This is the place to demonstrate, in a manner never to be forgotten, the influence of moisture. Assure yourself that your dry silk string insulates. Wet it throughout, and squeeze it a little, so that the water from it may not trickle over your glass tube. Coil it round the tube as before, and excite the tube. The leaves of the electroscope immediately diverge. The water is here the conductor. The influence of moisture was first demonstrated by Du Fay (1733 to