ELECTRO-MAGNETISM AS A MECHANICAL POWER. 537 



butes the greatness of this effect jJi'incipally to the weight of the arma- 

 ture, and also to the rounded form of this and also of the poles of the 

 magnet ; but he seems to think the great thickness of the wire, namely, 

 8'4 mill., of no moment. With this important particular the reader is 

 not acquainted till the end of the paper, where a table of the diameter 

 of the wires is added. He infers from his experiments, " that a tempo- 

 rary magnet (as he calls the electro-magnets) can only acquire a mag- 

 netic power proportional to its mass;" and says, " experience will show 

 what is the smallest electro-motive surface required to give the maxi- 

 mum of power];" and adds, " these experiments will become the more 

 necessary when electro-magnetic power has been applied to some useful 

 purpose." 



The following remarks of Dal Negro on the property of some pieces 

 of iron either not to take any magnetism at all, or only to take it under 

 certain circumstances from the inverted electric current, were to me 

 very mysterious and enigmatical. He says : 



I. (1.) "I had several cylindrical soft-iron horseshoes made, of different 

 weight, and experimented with them according to Sturgeon's method ; 

 for the most part none of them were at all magnetic. Indeed, in a small 

 bar of iron Avhich was cut into four pieces, and the single pieces made 

 into magnets of the above-mentioned size, only one of them became a 

 powerful magnet; the others were little or not at all magnetic." 



(2.) " In the same way curved square bars gave no appreciable re- 

 sults : it appears from this that the cylindrical form is essentially ne- 

 cessary to the development of this temporary magnetism. I also endea- 

 voured, without success, to magnetize hollow cylinders." 



(7.) " During the first experiments it often happened that when the 

 weight which the magnet could sujiport had reached its maximum, all 

 on a sudden the horseshoe would become incapable of re-acquiring 

 magnetism, not even so much as to be able to support the keeper again. 

 Van Moll also appears to have observed this phaenomenon." 



" Fortunately it appeared that by continually weakening [^abstmn- 

 pfen'\ (?) the same magnet, one is enabled to repeat the experiments, 

 and each time make it support a considei'able weight." 



III. (5.) " It is remarkable that I did not observe with these two mag- 

 nets (namely, the two strongest,) the phsenomena mentioned in the first 

 part of this treatise. No. (7). I am much inclined to believe that this 

 depends upon the magnet being made to support the greatest possible 

 weight for a longer or shorter time. But here I must not omit to 

 mention, that often, when I removed from the magnets the helices 

 which I had been using, either for the purpose of altering the number 

 of coils or the thickness of the wire composing them, the magnets for 

 several dags would not t(dic np the least magnetism. On continuing 

 these exi)erinuiils I ol)laineil the same pha-nonicnon with tiic magnet C 



