332 WRITINGS OF JOSEPH HENRY. [1855- 



material, and while it remains at the top it repels the free 

 electricity of the globe as much as the matter of the globe 

 attracts it. For a similar reason, if a small brass ball be 

 placed on the top of a rod of glass and presented to the 

 globe, but a feeble spark will be elicited ; the inductive in- 

 fluence will act in this case under unfavorable conditions, a 

 portion of the natural electricity, it is true, will be driven 

 down into the lower surface of the ball, and an equal amount 

 of un-saturated matter will exist at the upper surface ; but 

 the attractions and repulsions will be so nearly at the same 

 distance that but a comparatively feeble effect will be pro- 

 duced. An attentive consideration of these facts is essential 

 to a knowledge of atmospheric electricity, and necessary to 

 understand and guard against the effects of the destructive 

 discharges from the thunder-cloud. 



The inductive action we have described takes place at a 

 distance through an intervening stratum of air, but the same 

 effect is produced, and with nearly the same intensity, when 

 the intervening space is occupied with glass or any other 

 non-conducting substance. If a disk of wood, which is a 

 partial conductor, is interposed, the effect will be slightly 

 modified, because an inductive action will take place in the 

 substance of this which will tend to increase the effect in the 

 conductor 0, below. 



As an illustration of the inductive influence of free elec- 

 tricity at a distance on the natural electricity of a conduc- 

 tor, we shall direct the attention of the reader to an arrange- 

 ment exhibited in Figure 9, which is that of an experiment 

 made by the author in Princeton, in 1842*. Two circular 

 disks of wood, a and b, each about 4 feet in diameter, 

 were entirely covered with tin foil ; one was in connec- 

 tion with a large insulated conductor of an electrical ma- 

 chine in the upper story of a building, the other was sup- 

 ported on a glass foot in the lowest story, at the distance 

 of about 25 feet below, with two floors and ceilings interven- 

 ing. The upper disk being charged by the machine, the 

 lower one was touched with the finger, so as to suffer the in- 



* [Proceedings Am. Phil. Society, June 17, 1842. See ante, vol. i, p. 203.] 



