1890.] on Magnetic Phenomena. 53 



circular electro-magnet and a semicircular armature of soft iron, a 

 weight of more than 200 lbs. per square inch was easily carried, though 

 the current was very far indeed from being infinite ; it was, in fact, 

 about 5 amperes. 



If, as we have seen to be the case, there is no limit to the number 

 of magnetic lines which can be induced in an iron bar, then, theoreti- 

 cally, there can be no limit to the lifting-power which an electro- 

 magnet can be made to exhibit. Practically, however, a limit is 

 imposed by the fact that we cannot command an unlimited current 

 of electricity, nor would wires of any known material convey it even 

 if we could. With sufficient current a little 3-inch electro-magnet 

 might no doubt be made to lift a weight of a ton, but any attemj)t to 

 pass such a current would result in the immediate fusion, or even 

 vaporisation, of the wire-coils by the intense heat that would be 

 generated. 



The lifting power of an electro-magnet with an iron armature is 

 proportional to the square of the total number of magnetic lines 

 which run through the iron, inclusive of those due simply to the 

 current in the coil. Ewing's experiments enable us to determine the 

 greatest weight that a magnetised iron bar could support by itself, 

 without any assistance from the surrounding coil. In the case of his 

 specimen of iron it would be about 260 lbs. per square inch of 

 section. 



The permeability of an iron rod depends not only upon the inten- 

 sity of the field in which the rod is placed, but also to some extent 

 upon the physical condition of the iron, and is affected by such causes 

 as mechanical stress or changes of temperature. If, for instance, we 

 hang an iron wire vertically in a not very strong field, and stretch it 

 a little by attaching a weight to its lower end, we shall find that the 

 stretching causes a temporary increase in the longitudinal permea- 

 bility of the wire. But if the experiment be repeated in a strong 

 field, the effect will be reversed ; the same load which before 

 increased the permeability of the wire will be found to diminish 

 it. In a field of a certain medium strength which can be determined 

 by trial, the stretching will have no effect at all upon the permeability. 

 This value of the field is called, after the first discoverer of the 

 phenomenon, the " Villari Critical Point" for a certain load. 



The permeability of a nickel wire appears to be always diminished 

 by stretching, whatever the strength of the field or the magnitude of 

 the load. 



As the magnetic qualities of a rod of iron or other magnetisable 

 metal are affected by a temporary strain or slight alteration of its 

 form, so it has been found that the form of such a rod may be slightly 

 altered by magnetising it. By the aid of very delicate apparatus it 

 is possible to show that in a continually increasing field the length of 

 an iron bar is at first increased, and afterwards diminished ; that of a 

 cobalt bar is at first diminished, and afterwards increased ; while that 

 of a nickel bar is always diminished. The following table shows the 



