-1859] WRITINGS OF JOSEPH HENRY. 399 



surface is increased by flattening a round bar, the conduc- 

 tion is diminished, and a greater intensity is given to the 

 electricity at the edges, tending to increase the lateral es- 

 cape of the fluid. The only proper way of diminishing 

 the resistance to conduction in a rod of metal of a given 

 capacity is to mold it into the form of a hollow cylinder ; a 

 gas-pipe for example will off'er less resistance to conduction 

 than the same weight of metal in the form of a solid cylinder ; 

 but we must not infer from this that a gas-pipe an inch in 

 diameter will conduct better than a solid rod of iron of the 

 same diameter. There is no known law of electricity which 

 would lead us to suppose that by removing the metal from 

 the interior of a rod, we increase its conducting capacity. 

 On the contrary when the discharge is very great in propor- 

 tion to the size of the conductor, it is probable that the dis- 

 charge penetrates through the entire mass. The rod should 

 be of sufiicient size to transmit freely the largest discharge 

 that experience has shown as likely to fall on a building. 

 A rod of three-fourths of an inch of round iron is generally 

 considered sufiicient for this purpose, since a conductor of 

 this capacity has in no case been found to have been fused 

 by a discharge from the clouds. There is no objection on 

 the score of electrical action to using a larger bar, or to the 

 same weight of metal in the form of a hollow cylinder ; in- 

 deed every increase of diameter lessens the resistance to con- 

 duction, and the tendency to give off lateral sparks. 



Lightning-conductors are frequently constructed in this 

 country with points projecting at intervals of two or three 

 feet through their whole length ; this plan has been adopted 

 from some erroneous idea in regard to the action of the con- 

 ductor, and of the proper application of points. The essen- 

 tial ofiice of the conductor is to receive the discharge from 

 the cloud, and to transmit it with the least resistance possi- 

 ble, silently and innoxiously to the great bod}^ of the earth 

 below, and anything which militates against these requisites 

 must be prejudicial. Now in the passage of the electricity 

 through a conductor, it retains its repulsive energy, and 

 hence each point along the rod in succession becomes highly 

 charged, and tends to give off a spark to bodies in the neigh- 



