550 



magnetized by a not very powerfully excited helix surrounding it. 

 In the recent experiments the magnetizing force was (we may infer) 

 far greater. 



It is to be remarked that the results now brought forward do not 

 afford ground for a quantitative comparison between the effects of the 

 same degree of magnetism, on the resistance to electric conduction 

 along and across the lines of magnetization, in either one metal or the 

 other, in consequence of the oblong form of the specimens used in 

 the experiment. It is probable that in each metal, but especially in 

 the nickel of which the specific inductive capacity is less than that 

 of iron, the transverse magnetization was more intense than the 

 longitudinal magnetization, since the poles of the electro-magnet 

 were brought closer for the former than for the latter. 



I hope before long to be able to make a strict comparison between 

 the two effects for iron at least, if not for nickel also ; and to find 

 for each metal something of the law of variation of the conductivity 

 with magnetizing forces of different strengths. 



XX. " On the Electric Conductivity of Commercial Copper of 

 various kinds." By Professor W. THOMSON, F.R.S. 

 Received June 17, 1857. 



In measuring the resistances of wires manufactured for subma- 

 rine telegraphs, I was surprised to find differences between different 

 specimens so great as most materially to affect their value in the 

 electrical operations for which they are designed. It seemed at first 

 that the process of twisting into wire-rope and covering with gutta- 

 percha, to which some of the specimens had been subjected, must be 

 looked to to find the explanation of .these differences. After, how- 

 ever, a careful examination of copper-wire strands, some covered, 

 some uncovered, some varnished with india-rubber, and some oxidized 

 by ignition in a hot flame, it was ascertained that none of these circum- 

 stances produced any sensible influence on the whole resistance ; and 

 it was found that the wire-rope prepared for the Atlantic cable (No. 14 

 gauge, composed of seven No. 22 wires, and weighing altogether 

 from 109 to 125 grains per foot) conducted about as well, on the 

 average, as solid wire of the same mass : but, in the larger collection 



