62 Mr. Shelford Bidwell [Feb. 21, 



possibly exceed a certain definite limit, and that, in fact, no magnetic 

 force, however great, could induce in iron more than about 18,000 

 lines per centimetre. This conclusion, which seemed to be in agreement 

 with Weber's theory of magnetism, was generally accepted as correct. 

 Unfortunately, however, Rowland's experiment did not go quite far 

 enough. If he had been able to carry his magnetising force a little 

 beyond 65 units, he would have seen that there was no such limit as 

 he supposed. More recently, an induction of 18,000 has been 

 actually obtained with a field of only 200, the permeability being 90. 

 With the stronger field of 585, the induction was found to be 20,000 ; 

 and quite lately, Professor Ewing, employing a field of 24,500, 

 has obtained an induction of 45,300, the permeability being 1-9. 

 Ewing concludes that there is no limit whatever to the degree to 

 which magnetic induction may be raised ; and there can be no doubt 

 that he is right. 



But while Ewing's experiments tend to show that the number of 

 magnetic lines which can conceivably be made to run through a piece 

 of iron is indefinitely great, they at the same time clearly indicate that 

 the number of additional lines in excess of those contained in the 

 field before the iron was placed there, has a very definite limit. This 

 limit, for the piece of wrought iron which he used, appears to have 

 been about 21,000, and it w^as practically reached with an external 

 field of about 2000. For this sample of iron we may, therefore, say 

 that in fields of 2000 and upwards, 



B = H + 21,000. 



Closely connected with the questions which have just been 

 discussed, are the further questions : — What are the conditions 

 affecting the lifting-power of an electro-magnet ? and. What is the 

 greatest lifting-power attainable ? 



One point of fundamental importance was settled experimentally 

 by Joule many years ago. He found that the power of a uniform 

 electro-magnet varies directly as the sectional area of the iron core, 

 so that, for example, a magnet with a section of two square inches 

 would, other things being equal, carry twice the weight that could 

 be supported by one with a section of only one square inch. Joule 

 also studied the effect of varying the strength of the current passing 

 through the surrounding coil, and ascertained that while up to a certain 

 point increase of current was accompanied by marked increase of 

 lifting-power, yet when the current exceeded a more or less definite 

 limit, further increase of it produced comparatively little effect. 

 Reasoning upon his experiments, he formed the opinion — in which 

 long afterwards Rowland concurred — that no current, however great, 

 "could give an attractive power equal to 200 lbs. per square 

 inch." 



It has, however, since been shown that this statement is not quite 

 true. In the course of some experiments made in 1886, with a semi- 



