246 WRITINGS OF JOSEPH HENRY. [1846 



ductor, the more readily must the repulsive action of the 

 free electricit}^ of the cloud drive the natural electricity of 

 the conductor to the farther end of the line, thus rendering 

 more intense the negative condition of the nearer part of the 

 wire, and consequently increasing the attraction of the metal 

 for the free electricity of the cloud. It is not however prob- 

 able that the attraction, whatever may be its intensity, of so 

 small a quantity of matter as that of the wire of the telegraph, 

 can of itself produce an electrical discharge from the heavens, 

 although, if the discharge were started by some other cause, 

 such as the attraction of a large mass of conducting matter 

 in the vicinity, the attraction of the wire might be sufficient 

 to change the direction of the descending bolt, and draw it, 

 in part or in whole, to itself. It should also be recollected, 

 that on account of the perfect conduction, a discharge on any 

 part of the wire must affect every other part- of the connected 

 line, although it may be hundreds of miles in length. 



That the wire should give off a discharge to a number of 

 poles in succession is a fact I should have expected from my 

 previous researches on the lateral discharge of a conductor 

 transmitting a current of free electricity. In a paper on 

 this subject, presented to the British Association in 1837,* I 

 showed that when electricity strikes a conductor explosively 

 it tends to give off sparks to all bodies in the vicinity, how- 

 ever intimately the conductor may be connected with the 

 earth. In an experiment in which sparks from a small 

 machine were thrown on the upper part of a lightning rod, 

 erected in accordance with the formula given by the French 

 Institute, corresponding sparks could be drawn from every 

 part of the rod, even from that near the ground. In a commu- 

 nication since made to this Society, I have succeeded in refer- 

 ringthis phenomenon to thefact,that during the transmission 

 of a quantity of electricity along a rod, the surface of the 

 conductor is charged in succession, as it were, by a wave of 

 the fluid, which, when it arrives opposite a given point, tends 

 to give off a spark to a neighboring body for the same reason 



* [Report of British Association, 1837. See ante, page 101.] 



