LESSONS IN ELECTRICVfV. 



295 



needle, and supported on a bent strip of 

 mdal, as shown in the figure. By weight- 

 ing the straw with a little wire near j', 

 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 cither of these 

 instruments will enable you to classify 

 bodies as conductors, se'mi-conductors, 

 and insulators. Here is a list of a few 

 of each, which, however, differ much 

 among themselves. 



Conductors. 

 The common metals. 

 Well- burned charcoal. 

 Concentrated acids. 

 {Solutions of tails. 

 Rain water. 

 Li IK n. 

 Living vegetables and animals. 



Semi-conductor*. 

 Alcohol anJ ether. Paper. 

 Dry wood. 8liuw. 



Mmble. 



Insulators. 



Fattv oils. Silk. 



Chalk. Gh;s. 



India-rubber. Wax. 



Dry paper. Sulphur. 



Haii 1 . Shellac. 



A little reflection will enable you to 

 vary these experiments indefinitely. Rul> 

 your excited sealing-wax or glass ajjf.inst 

 the tin plate of your electroscope, and 

 cause the leaves to diverge. Touch the 

 plate with any one of the conductors 

 mentioned in the list ; the electroscope is 

 immediately discharged. Touch it with 

 a semi-conductor ; the leaves fail as le 

 fore, but less promptly. Touch the 

 plate finally with an insulator, the elt-c- 

 tricity cannot pass, and the leaves remain 

 unchanged. 



8. Electrics and Non- Electrics. 



For a long period, bodies were divided 

 into tlcctrics and non-electrics, the former 

 deemed capable of beinnr olectriti >d. the 

 latter not. Thus the amber of the an- 

 cients, and t\ic spars, gems, fossils, stones, 

 glasses, and resins, operated on by Dr. 

 Gilbert, were called electrics, while all the 

 metals were called non-electrics. We 

 must now determine the true meaning of 

 this distinction. 



Take hi succession a piece of brass, of 

 wood coated with tin-foil, a lead bullet, 

 apples, pears, turnips, carrots, cucum- 

 bers uncoatcd wood not very dry will 

 also answer in the hand, and strike them 

 briskly with flannel, or the fox's brush ; 

 none of them will attract the balanced 

 lath, fig. 4, or show any other symptom 

 of electric excitement. All of them 

 therefore would have been onoe called 

 non-electrics. 



But suspend them in succession by a 

 string of silk held in the hand, and strike 

 them again ; every one of them will now 

 attract the lath. 



Reflect upon the meaning of this ex- 

 periment. We have introduced an insu- 

 lator the silk string between the hand 

 and the body struck, and we find that by 

 its introd.iction the non-electric has been 

 converted into an electric. 



The meaning is obvious. When held 

 in the hand, though electricity was devel- 

 oped in each case by the friction, it pass- 

 ed immediately through the hand and 

 body to the caith. This transfer being 

 prevented by tho silk, the electricity, 



