ELECTRO-MAGNETISM AS A MECHANICAL POWER. 537 
butes the greatness of this effect principally 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.) “Thad 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 which 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 support 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 phenomenon.” 
“ Fortunately it appeared that by continually weakening [abstwm- 
pfen](?) the same magnet, one is enabled to repeat the experiments, 
and each time make it support a considerable weight.” 
Ill. (5.) “It is remarkable that I did not observe with these two mag- 
nets (namely, the two strongest, ) the phenomena mentioned in the first 
part of this treatise, No. (7). Iam 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 days would not take up the least magnetism. On continuing 
these experiments I obtained the same phenomenon with the magnet C 
