262 MEMORIAL OF JOSEPH HENRY. 



In this paper he incidentally meets the much debated question 

 whether a lightning-rod is efficient as a conductor by its solidity, or 

 by its surface only. While he had been able to magnetize small 

 needles placed transversely to the edges of broad strips of copper, 

 through which electrical discharges were passed, he could obtain no 

 signs of magnetism in needles when placed transversely near the 

 sides of such strips about mid-way from the edges. In like man- 

 ner he failed to discover any action in a small magnetizing helix 

 placed within a section of gas-pipe and connected with it at either 

 end, when transmitting through the system an electrical spark; 

 while he easily obtained magnetic effects with a galvanic current 

 passed through the same arrangement.* From these and other 

 experiments he was led to believe that mechanical electricity tends 

 to pass mainly along the exterior surface of a conductor, and accord- 

 ingly that Ohm's law of conduction is not applicable to lightning 

 or mechanical electricity, f 



Some popular uneasiness having been excited in 1846, in conse- 

 quence of telegraph poles being occasionally struck by lightning, 

 and of the supposed danger to travellers along highways likely to 

 result therefrom, a communication on the subject addressed to Dr. 

 Patterson, one of the Vice- Presidents of the American Philosophical 

 Society, was read before the Society, and referred to Professor 

 Henry for report. This was in the very infancy of the electro- 

 magnetic telegraph ; as it had not then been in existence more than 

 a couple of years. Henry responded in a communication read 

 June 19th, 1846, to the effect that while telegraph wires as long 

 conductors were eminently liable to receive discharges of atmos- 

 pheric electricity both from charged clouds and from the varying 

 electrical condition of the air at distant points along the line (as for 



* In passing a galvanic current through an iron tube, he obtained the evidence 

 of an induction from both the inside and the outside of the tube, but in opposite 

 directions. 



t This very important question cannot be regarded as even yet decisively 

 settled: eminent authorities maintaining that electricity in flow of whatever 

 origin observes equally the ratio of proportionality to area of cross section in 

 the conductor. Probably the law of conductivity varies with circumstances. 

 RITCHIE remarks that "if a metallic rod be raised to a red heat, its power of 

 conducting common electricity is increased, whilst its conducting power for 

 voltaic electricity is considerably diminished." (Journal of the Royal Institution 

 of Great Britain, Oct. 1830, vol. i. (n. s.) p. 37.) 



