R. Shida. Determinations 



[Nov. 23. 



tional effect ; whereas, for a magnetising force below that value, the- 

 simple reversal of that force is not so effective as to annnl the per- 

 manent effects of "ons and offs," or even to reverse the magnetic 

 polarities of the wires. But an equally, if not more, remarkable result 

 is found in the fact that the intensity of magnetisation of soft iron 

 wires is greater or less while it is pulled than while it is unpulled, 

 according as the magnetising force is below or above a certain critical 

 value — a result which confirms that given in Sir William Thomson's 

 paper on the " Electrodynamic Qualities of Metals," Part VII. It 

 is quite evident, however, that this critical value is different, not only 

 for different kinds of soft iron wire, but for different amounts of the 

 pull to which the wire is subjected. The singularity of the existence 

 of a critical point in a soft iron wire is only intensified by the fact 

 that, whilst the permanent magnetisational effects of " ons and offs " 

 on a wire of soft iron and of steel (pianoforte wire at least) are 

 similar in kind, there is found no such thing as a critical point in the 

 latter, in which the magnetisation is greater in the case of "off" than 

 in the case of "on " for every degree of magnetising force. 



For high magnetising forces the curves of the intensity of mag- 

 netisation all become asymptotes parallel to the line of abscissae,, 

 proving that there is a limit to the magnetisability of iron and steel, 

 as was first shown by Joule. In the case of " off," the maximum 

 intensity of magnetisation is found to be approximately 1420, both 

 in the soft iron wires and in the steel pianoforte wire, and in the cases 

 of " on," it is more or less below that value, the minimum magnetising 

 force corresponding to that magnetisation being in each case roughly 

 80 units ; while in the glass-hard-tempered steel wire, to which no 

 weight was applied at all, the maximum magnetisation is found to be 

 slightly lower. 



The steepness of the commencement of the curves of magnetic sus- 

 ceptibility in the case of the soft iron wires is striking, owing to 

 retentiveness ; indeed, the magnetic susceptibility of these wires 

 varies through a vast range. Taking, for example, the case of the 

 soft iron wire of No. 22 B.W.G., in which the weight used for " ons 

 and offs " is 8 kilos., the susceptibility for the magnetising force 

 (about *545) of the earth's vertical component ; is roughly 730 for 

 "on" (8 kilos.) and 330 for " off," it is about 65 at the critical point 

 (about 15), while it is only about 17 and 17^ in the cases of "on" 

 and " off " respectively for the minimum magnetising force (about 80) 

 corresponding to the maximum magnetisation. On the other hand, 

 the susceptibility-curves for the steel wires are neither so steep nor 

 so regular as those for the iron wires, but have a few maxima and 

 minima ; it is, however, all but certain that by using a heavier weight 

 than the one actually used for "ons and offs," these irregularities in 

 the curves can be got rid of, and at the same time, the magnetic sus- 



