386 WRITINGS OF JOSEPH HENRY. [1855- 



The question has been agitated whether one point or a 

 number on the same stem is to be preferred? But this 

 question may be readily settled, provided the reason for pre- 

 ferring a point to a ball or a globe is legitimate, since the 

 surface of a ball itself may be considered as made up of an 

 infinite number of points, and therefore a number of points 

 close together must re-act upon each other, and thus ap- 

 proximate in result the effect of a continuous spherical sur- 

 face. In the case of three points on the same stem, the 

 whole amount of inductive effect produced in the rod is 

 practically divided into three parts, and is therefore less con- 

 centrated than in the case of one point; and although at a 

 distance the effect of the three may be equally energetic, yet 

 the one point tends more effectually to rupture the air, and 

 open (so to speak) a passage for the discharge from the cloud. 



In reference to the subject of the termination of rods by 

 balls or points, much discussion took place on the early in- 

 troduction of the invention of Franklin, and the subject was 

 elucidated by a very ingenious experiment made by Beccaria, 

 in 1763, which is quoted by Arago. On the roof of a church at 

 Turin this eminent electrician erected a rod of iron insulated 

 on one of the flying buttresses. The upper part of this rod, 

 which was terminated by a single metallic point, was hinged 

 a few inches below the top, so that by merely pulling a string 

 the point could be directed horizontally, upward, or down- 

 ward. When the point was pulled downward during the 

 presence of a thunder-cloud in the zenith, the lower end of the 

 rod gave no sparks ; but when the point was suddenly directed 

 upward, in a few moments sparks appeared. When the point 

 w^as downward, the rod presented a blunt termination toward 

 the sky ; when upward a sharp point. It might be well to 

 repeat this experiment with some slight variation in the ap- 

 paratus, in order to establish or dis-prove, by direct observa- 

 tion, the inference from theory that a single point acts more 

 energetically than three or four points, terminating the same 

 rod. The substance which terminates the conductor should 

 be such as to preserve its form when subjected to the action 

 of the weather, and be infusible by a stroke of lightning. 



