250 



Lightning Rods, — JVo. 1. 



Vol. IX- 



25. A rod which is insulated, will not 

 effect an electrified body. Let a, fig. 8, be 



Fig. 8. 



an electrified body, and the charsfe kept up 

 by its connexion with the electrical machine. 

 Under a, upon the table, place the instru- 

 ment b, which is a small board holding two 

 short glass tubes at d, and e. The tube d, 

 is open at its lower end, while the one at e, 

 is hermetically closed. Between them, at c, 

 is another glass tube surmounted by a brass 

 ball, and communicating through the tube 

 with the table. Place the pointed wire /, 

 in the open tube d, and it will silently dis- 

 charge «,- remove it to the closed tube e, 

 by which it becomes insulated, and it will 

 no longer do so. But as the intensity of the 

 charge in a, increases, it first begins to pass 

 silently to the point, until it has acquired 

 sufficient explosive force to strike from the 

 side of the wire in a horizontal direction to 

 the ball at c. This I will call the lateral 

 explosion. 



26. The electrical atmosphere which fornis 

 around a sphere, (12) is everywhere equally 

 difl'used, because all parts of its surface are 

 equi-distant from its centre, and its attrac- 

 tion everywhere the same. Bodies with 

 angles or points, have those parts father 

 from the centre, and the electric fluid being 

 less strongly attracted at those parts, is 

 driven by its own repulsion to greater dis- 

 tances—as shown at a, fig. 9. The more 

 acute the angle or point, the greater will 

 be the distance to which the atmosphere 

 will extend, as at b, until the feeble attrac- 

 tion of the small quantity of matter near 

 the point is no longer sufficient to hold the 

 repulsive force in equilibrium, and disper- 

 sion takes place. 



27. If two or more pith-balls be suspended 

 by silk threads and electrified, each one re- 

 spectively will acquire an electrical atmos- 

 phere, and the electricity of the one will 



Fig. 9. 







"^^^^''S^)!^^ 



\ 



repel that of the other, so as to separate 1 

 them to twice the extent of their atmos- W 

 pheres, as is shown fig. 10. But if the • 



Fig. 10. 





/ ' I I '\\> • <■' ;'y I I •! A -^ 



finger be brought near them they will fly to 

 it, — give off their electricity in form of a 

 spark, and then fall back to their original 

 position. 

 28. If we insulate the ball b, fig. 11, and ■ 

 Fig. 11. 



,vA^\\'*"^i'l'i '/' ''/ 





