butes the greatness of this effect principally to the weight of the armature, 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 temporary magnet (as he calls the electro-magnets) can only acquire a magnetic power proportional to its mass;" and says, "experience will show what is the smallest electro-motive surface required to give the maximum 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 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 results: it appears from this that the cylindrical form is essentially necessary to the development of this temporary magnetism. I also endeavoured, 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 phænomenon."
"Fortunately it appeared that by continually weakening [abstumpfen](?) the same magnet, one is enabled to repeat the experiments, and each time make it support a considerable weight."
III. (5.) "It is remarkable that I did not observe with these two magnets (namely, the two strongest,) the phænomena 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 take up the least magnetism. On continuing these experiments I obtained the same phænomenon with the magnet C
