248 



Lightning Rods, — JVo. 1. 



Vol. IX. 



11. In order to charge a body with the 

 electric fluid, it must first be suspended or 

 supported by some nonconducting materia] 

 to prevent it from escaping. The body is 

 then said to be insulated. 



12. Though both the glass and cushion 

 (9) show electrical phenomena, yet there is 

 a striking difference in their character. 

 Sorne believe that the electricity of the 

 cushion is transferred to and accumulated 

 on the glass; the latter having more and the 

 former less than its natural quantity; hence 

 that of the glass has been called plus, or 

 positive, and that of the cushion minus, or 

 negative electricity. 



13. Others suppose that there are two 

 electric fluids which are ordinarily com- 

 bined, but that they are separated by the 

 friction — one accumulating on the cushion 

 and the otlier on the glass. Having ob- 

 served that glass affords one and resin the| 

 other, when friction is employed, they have 

 called the one vitreous, and the other resin- 

 ous electricity. 



14. If we insulate the brass ball, fig. 1, 



Fig. 1. 



ly the intensity of the charge which a ball 

 is capable of receiving, is in proportion to 

 its density or tlie quantity of matter con- 

 tained in a given space. A ball of lead 

 will hold a more intense charge than a ball 

 of cork. 



17. Let a, b, c, and d, e, f, fig. 2, repre- 



... xVv^'i 

 ^\^ , > \\ ,^ 





^ - ^ y </•/,,', v. /I 





by means of a nonconducting silk thread, 

 and then charge it with electricity — the 

 particles of the fluid will be dispersed by 

 their own repulsive power, wliile the attrac- 

 tion of the ball will solicit them towards its 

 surface, until an equilibrium is obtained be- 

 tween the two forces. The electricity will 

 then have arranged itself around the ball as 

 seen in the figure. This is called an elec- 

 trical atmosphere. 



15. The attraction of the ball decreases 

 as the squares of the distances from its cen- 

 tre increase — consequently the condensation 

 of the fluid will be greatest at the surface, 

 and will decrease in the same ratio with the 

 attracting force. This is called the inten- 

 sity of the charge. 



16. The attracting force, and consequent- 



Fig. 





■■'y-'/a,y? 







sent two balls, the half diameters of which 

 are as the numbers 1 and 3, but composed 

 of different materials, so that they shall con- 

 tain like quantities of matter — then from the 

 known laws of attraction, the attracting force 

 on the surface of a, b, c, will be nine times 

 greater than on the surface of d, e, f,; or in 

 other words, the same force will be diffused 

 over nine times the space on the latter, that 

 it was on the former as is seen at g. Hence 

 if we form an electrical atmosphere around 

 the lesser body, a, b, c, and then describe 

 the circle d, e, f, to represent the larger 

 body, the circle so drawn will cut off just 

 so much of the electricity as the larger ball 

 would be capable of holding. 



18. If we increase the intensity of the 

 charge until the repelling, becomes greater 

 than the attracting force, the fluid will dif- 

 jfuse itself through the surrounding air, or 

 vvill fly off to soriie contiguous body with an 

 explosion and luminous spark. The fonner 

 lis called electrical disjjersion — the latter, 

 the electrical stroke. 



_ 19. The facility with which the disper- 

 sion or stroke takes place, depends, in a 

 great degree, upon the state of the atmos- 

 phere and the form of the electrified body; 

 and the intensity of the charge will be in- 

 fluenced by the same circumstances to the 

 same extent. The space through which 

 the discharge passes, is called the striking 

 distance. 

 20. If a metallic ball be presented to an 



