PHILOSOPHICAL SOCIETY OF WASHINGTOX. 275 



or b}" 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 manner he failed to discover any action in a small magne- 

 tizing 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 accordingly tliat Ohm's law of con- 

 duction is not applicable to mechanical electricity. f 



Some popular uneasiness having been excited in 184G, in con- 

 sequence of telegraph poles being occasionally struck by light- 

 ning, and of the supposed danger to travellers along highways 

 likely to result therefrom, a communication on the subject ad- 

 dressed 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, 184(i, to the 

 effect that while telegraph wires as long conductors were emi- 

 nently liable to receive discliarges of atmospheric electricity 

 both from charged clouds and from the varying electrical condi- 

 tion of the air at distant points along the line (as for example 

 even by a fog or precipitation of vapor at one station) as also 

 from induction at a distance, the danger to travellers along a 

 telegraph road would be very slight, unless a person should be 

 standing or passing quite close to a pole at the moment of its 

 being struck. He however recommended that for the protection 

 of the poles, they should be provided with conductors. "The 

 effects of powerful discharges from the clouds may be prevented 

 in a great degree by erecting at intervals along the line and 

 beside the supporting poles a metallic wire connected with the 

 earth at the lower end, and terminating above at the distance of 

 about half an inch from the wire of the telegraph. By this 



* In passing a galvanic current tlirongli an iron tnl>e. be obtained the 

 evidence of an induction from both the insi<le and the outside of tiie tube, 

 but in opposite directions. 



t This very important question cannot be regarded as even yet de- 

 cisively settled : — eminent authorities maintaining that electricity in Jl,w 

 — 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. 



49 



