-1859] 



"WRITINGS OF JOSEPH HENRY. 



333 



duced electricity to escape into the ground. If when in this 



condition the knuckle was 

 held near the lower disc 

 and the upper one suddenly- 

 discharged by a spark re- 

 ceived on a ball attached to 

 the end of a wire connected 

 with the earth, a spark was 

 seen to pass between the 

 knuckle and the lower disk. 

 A similar effect was pro- 

 duced when the upper plate 

 was suddenly charged by 

 powerful sparks from the 

 machine, though the inten- 

 sity in this case was some- 

 wdiat less. 



In this experiment, the 

 upper disk may represent a 

 charged thunder-cloud, and 

 the lower one the ground, or 

 any conducting body with- 

 in a house. While the 

 charged cloud is passing over the building, all conducting 

 bodies in it, by this inductive action at a distance, have 

 their natural electrical equilibrium disturbed; the upper 

 part of each body becoming negatively electrified, and the 

 lower part positively ; and if the cloud continue in this 

 position for a few minutes, the free electricity of the lower 

 part of the conductor will be gradually driven into the 

 earth, through the imperfect insulation of the floor. If in 

 this case the lower part of the cloud is suddenly discharged, 

 sparks of electricity may be perceived, and perhaps shocks 

 experienced, by the inmates of the dwelling, produced by 

 the sudden restoration of the equilibrium, due to the removal 

 of the repulsive force of the cloud on the natural electricity 

 of the bodies below. 



The inductive action of the electrical discharge at a dis- 



