354 



WRITINGS OF JOSEPH HENRY. 



[1855- 



FiG. 16. 



of the bent metallic lever g h, is depressed, and 

 the fork i brought into contact with the stem 

 of the globe, and thus a perfect metallic con- 

 nection is formed between the latter and the 

 ground. The wire k is then released, the lever 

 falls back, the ball is insulated from the earth, 

 brought down, and applied to an electroscope, 

 and in all cases, when the sky is clear, is found 

 to be negatively electrified. If the wire k be 

 insulated through its entire length, and termi- 

 nated in a bulb at a little distance from the 

 earth, and a pull be given to it by means of a 

 rod of glass, at the instant of contact of the 

 point i with the stem d, the lower bulb will ex- 

 hibit a positive charge of electricity. The ar- 

 rangement will, in fact, be precisely the same 

 as that exhibited in the previous figure, (Fig. 

 15), namely, a vertical conductor, the upper 

 end of which is rendered minus and the 

 lower end plus by the induction of the earth. This eff'ect 

 is entirely due to induction, and is independent of any free 

 electricity which may exist in the air. The results are ex- 

 hibited with the greatest intensity during perfectly clear and 

 dry weather ; and are not observed when the conductor is 

 placed horizontally, but the indications increase as its upper 

 end is gradually brought nearer the perpendicular. 



That these effects are not due to the free electricity of the 

 atmosphere is satisfactorily shown by the original experi- 

 ments of Peltier. For measuring the intensity of the in- 

 ductive influence of the earth he made use of an electrometer 

 represented in Fig. 17 ; in which a 6, is a glass cylinder fur- 

 nished with a wooden foot and a glass cover : through the cen- 

 tre of this is cemented a brass tube carrying a ball c at the 

 top, and an arched straddling wire at the bottom. At the 

 level of the foot of the arched wire is suspended a fine 

 magnetized needle g, the height of which is adjusted by 

 the screw Ti. The intensity of the electricity is measured by 

 the divisions pointed out by the deflected needle on the slip 



