252 Prof. Thomson on the Forces experienced by inductively 



used, especially if the upper pole, over which the watch-glass 

 is suspended, be flat. An electro-magnet with, for core, a hol- 

 low cylinder of soft iron open at the ends, would even repel a 

 small ferromagnetic body capable of moving along the axis, 

 in some positions, and attract it a little further off, since there 

 would be variations of force in this case precisely similar to 

 those explained with reference to points in the line of motion 

 of the ball in experiment 2. 



22. The most striking experiments adduced by M. Pliicker 

 to support his hypothesis, that u diamagnetism increases more 

 rapidly than magnetism" when the magnetizing force is in- 

 creased, are those in which the force experienced by a small 

 inductively magnetized body in a constant position is tested 

 for different strengths of the same electro-magnet, produced by 

 using a greater or less number of cells in the exciting battery. 

 At the recent Meeting of the British Association in Edin- 

 burgh, I ventured to suggest that a change in the distribution 

 of magnetic force in the neighbourhood of the magnet, accom- 

 panying an increase or dimi?iutio?i in the strength of the gal- 

 vanic current, might have contributed to- produce some of the 

 singidar phenomena which had been observed; and that there 

 is some considerable change in the distribution of force in the 

 neighbourhood of an electro-magnet with a soft iron core in a 

 state of intense magnetization when, for i?ista?ice, the strength 

 of the current is doubled, seems extremely probable when we con- 

 sider that a piece of soft iron in a state of intense magnetiza- 

 tion cannot be expected to be as open to fresh magnetization as 

 it would be if not magnetized in the first instance* On the 

 same occasion I remarked, that some experiments made by 

 Mr. Joule in connexion with his researches on changes of 

 dimensions produced in iron bars by magnetic influence, ap- 

 peared to indicate diminished inductive capacities in states of 

 intense inductive magnetization*. At that time I was not 

 aware of the recent experimental researches of Gartenhauser 

 and Muller on the magnetization of soft iron ; but 1 have 

 since met with a Number of Poggendorff 's Annalen ( 1 850, 

 No. 3, published last April) containing an account of these 

 researches t ? which completely confirms the second part of 

 the conjecture I had thrown out. Whether or not, how- 

 ever, the change in the distribution of force is of such a kind 

 as to account for the phenomena by which M. Pliicker sup- 

 ports the conclusion which has been quoted, it is impossible 

 to pronounce without a complete knowledge of the circum- 



* Phil. Mag. 1847j vol. xxx. pp. 76, 225. Also Sturgeon's Annals, Aug. 

 1840. 



t " Ueber die Magnetisirung von Eisenstaben diirch den Galvanischen 

 Strom : von J. Muller." 



