3 c THE POPULAR SCIENCE MONTHLY. 



LESSONS m ELECTEICITY. 1 



HOLIDAY LECTURES AT THE EOYAL INSTITUTION. 

 Br Prof. TYNDALL, F. K. S. 



II. 



SECTION 8. Electrics and Non- Electrics. For a long period, 

 bodies were divided into electrics and non-electrics, the former 

 deemed capable of being electrified, the latter not. Thus the amber 

 of the ancients, and the spars, gems, fossils, stones, glasses, and resins, 

 operated on by Dr. Gilbert, were electrics, while all the metals were 

 non-electrics. We must now determine the true meaning of this dis- 

 tinction. 



Take in succession a ball of brass, of wood coated with tin-foil, a 

 lead bullet, and an apple, in the hand, and strike them briskly with 

 silk, flannel, or the fox's brush ; none of them will attract the balanced 

 lath (Fior. 4), or show any other symptom of electric excitement. All 

 of them, therefore, would have been once 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 experiment. We have introduced 

 an insulator the silk string between the hand and the body struck, 

 and we find that by its introduction the non-electric has been con- 

 verted into an electric. 



The meaning is obvious. When held in the hand, though elec- 

 tricity was developed in each case by the friction, it passed imme- 

 diately through the hand and body to the earth. This transfer being 

 prevented by the silk, the electricity, once excited, is retained, and 

 the attraction of the lath is the consequence. 



In like manner, a brass tube, held in the hand and struck with a 

 fox's brush, shows no attractive power; but when a stick of sealing- 

 wax, ebonite, or gutta-percha, is thrust into the tube as a handle, the 

 striking of the tube at once develops the power of attraction. 



And now you see, more clearly than you did at first, the meaning 

 of the experiment with the heated foolscap and India-rubber. Paper 

 and wood always imbibe a certain amount of moisture from the air. 

 When the rubber was passed over the cold paper, electricity was 

 excited, but the paper, being rendered a conductor by its moisture, 

 allowed the electricity to pass away. 



Prove all things. Lay your cold foolscap on a cold board, sup- 

 ported by warm dry tumblers ; pass your India-rubber over the pa- 

 per ; lift it by a loop of silk, for if you touch it it will discharge itself. 



1 A course of six lectures, with simple experiments in frictional electricity, before 

 juvenile audiences during the Christmas holidays. 



