1850] WRITINGS OF JOSEPH HENRY. 291 



ON THE FORMS OF LIGHTNING-RODS.* 



(Proceedings American Association, Adv. of Science, vol. iv, pp. 39-42.) 



August 20, 1850. 



Professor Henry said the question of balls and points had 

 not been fully settled. If electricity acts inversely as the 

 square of the distance, then on the principle of central 

 forces, the induction on a sphere at a distance from the 

 cloud would be the same as if all the matter of the sphere 

 were concentrated in its centre, and consequently the attrac- 

 tion of the ball or sphere on the electricity of the distant 

 cloud would be the same as that of a point. When how- 

 ever the inducing body, or the discharge itself, came near 

 the rod, it would be much more strongly deflected by the 

 point than by the ball, because the former would be electri- 

 fied by induction to a much greater degree of intensity, for 

 the same amount of electricity which would be diffused over 

 the surface of the ball would be condensed in the point, and 

 hence it would tend to rupture the air, and thus give a more 

 easy passage to the discharge. 



His attention had been directed to the action on a ball, by 

 the fixture on the dome of the Capitol at Washington, of a 

 lantern, terminated by a ball. This apparatus had been 

 erected at a great expense, for the purpose of lighting the 

 public grounds. It consisted of a mast reaching to the 

 height of ninety feet above the apex of the dome of the 

 Capitol, terminated by a lantern about five feet in diameter 

 and six or seven feet high. In this were jet gas burners, 

 equal in illuminating power, according to the statement of 

 the projector of the arrangement, to six thousand wax candles. 



After the whole apparatus had been prepared, the speaker 

 was requested to give an opinion as to the effect which 

 the lightning might have upon it. His answer was, that 



* [Remarks on a communication by Professor Elias Loomis, to the Asso- 

 ciation, "On the proper height of Lightning-rods;" in which a reference 

 was made to the question of single or multiple points to the rod.] 



