146 Royal Society. 



The author then examines the laws which were supposed to connect 

 the conducting power of a wire for electricity, with its length and 

 diameter, and which, according to Professors Cumming and Barlow, 

 varies directly as the diameter, and inversely as the square root of 

 the length ; but, according to MM, Becquerel and Pouillet, directly 

 as the square of the diameter, and inversely as the length. He points 

 out the false conclusions of M. Becquerel, and that he has, in fact, 

 deduced the value of two unknown quantities from one equation; and 

 that M. Pouillet having arrived at his through the fallacious indica- 

 tions of the common deflecting galvanometer, they are equally erro- 

 neous. The author then shows that the law pointed out by Cumming 

 and Barlow is, in ordinary cases, nearest the truth ; though, under 

 certain circumstances, the limits of even that law may be passed. 

 Hence, and from a series of experiments with the torsion galvanome- 

 ter, he arrives at the unexpected conclusion, that there is no deter- 

 minate law of conduction, either for the length or diameter of the 

 wire, but that it must vary, in every case, with the size of the plates, 

 and the energy of the acid solution used in exciting them. This con- 

 clusion the author shows to be in accordance with the views of con- 

 duction which he had previously published ; namely, that there is no 

 actual transfer of electricity, but that all the phenomena result from 

 the definite arrangement of the electric fluid essentially belonging to 

 the conducting wire. 



The second part of this paper relates to certain properties of elec- 

 tro-magnets. No attempt seems to have been hitherto made to in- 

 vestigate the law which connects the lifting power of electro-magnets 

 with their length. The author found, bv experiments with two soft 

 iron horse-shoe electro-magnets, to each of which the same short 

 horse-shoe lifter was adapted, and the circuit of one four times that 

 of the other, that their lifting powers were nearly inversely as the 

 square root of their lengths. By increasing the strength of the battery 

 with which they were connected, their lifting powers approached more 

 nearly to a ratio of equality 5 by diminishing it, the ratio increased in 

 favour of the shorter magnet. Hence the law in this case seems to 

 be as indefinite as in that of common electric induction, and the re- 

 lation of the powers to vary with the energy of the inducing voltaic 

 influence. By another experiment, the author shows that all that is 

 necessary in preparing a powerful electro-magnet is simply to roll a 

 ribbon of copper about a short bar of soft iron, and to use a short horse- 

 shoe lifter of soft iron. The quality of the iron has great influence on 

 the power of an electro-magnet ; and the author found that the worst 

 part of a bar of the worst iron he could procure made by far the best. 

 A bar electro- magnet, four feet long, which scarcely retained any 

 power when its connexion with the battery was broken, on being re- 

 connected with it, in the same direction as before, was rapidly con- 

 verted into a powerful magnet; but after being removed, and its 

 wires now connected with the opposite poles, it required a long time 

 to convert it into a magnet of much inferior power; as if the atoms 

 of electricity, having been first put in motion in one direction, are 

 afterwards more easily turned in that direction than in the contrary. 



