Mr. R. H. M. Bosanquet on Electromagnets. 299 



specimens, cannot be accepted as having more demonstrative 

 force than belongs to preliminary investigations. So far as 

 work of this description goes, my own paper in Phil. Mag. 

 June 1884 (xvii.) p. 531, showed in detail the properties of 

 bars ; and specially pointed out that the course of the values 

 did not at all indicate any definite limit to the magnetism, 

 and that the existing idea of saturation was a complete mis- 

 take (p. 535). This is now being published as a new discovery. 



In subsequent papers I have shown that rings do not ex- 

 hibit the same behaviour ; and though some ring specimens 

 admit of the magnetism being forced up very high, yet others 

 do not. 



Further, where the magnetism is forced up very high in 

 rings, the whole magnetization function appears to change 

 under the influence ; and the values thus obtained are not 

 generally capable of being satisfied by the same expression, 

 which represents the behaviour of the specimen under induc- 

 tions less than 23 = 18,000. 



The difference in this respect between rings and bars is 

 already indicated in my paper, Phil. Mag. 1885 (xix.) p. 79 ; 

 and it is there pointed out that the cause is probably the dif- 

 ference in distribution, i. e. that in bars the lines of force are 

 crowded closely only at the equatorial section. 



The law of similar solids is easily deduced from the fact 

 that magnetic resistance (quotient of potential by induction) 

 is of linear dimensions. Hence, in similar electromagnets, 

 where the inductions are the same, the number of current 

 turns required is proportional to the linear dimension*. 



The present two series of experiments were made on cylin- 

 drical bars having the following dimensions: — The pole-pieces 

 of the second set were circular, had a diameter five times that 

 of the bar, and thickness equal to the diameter of the bar. 

 The object of the experiments on the bars with pole-pieces 

 was mainly to obtain numbers which might assist to serve as 

 bases for a knowledge of the behaviour of the field -magnets of 

 dynamos. 



* I have long been under the impression that this law was enunciated 

 in substance by Sir W. Thomson. But after carefully going through the 

 reprinted papers, the nearest I could find to it is at p. 435 (' Electrostatics 

 and Magnetism '). But this statement does not include the proposition 

 that the number of current-turns is proportional to the linear dimensions. 

 And I am therefore doubtful whether the law is really due to Sir W. 

 Thomson. 



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