106 PROCEEDINGS OF THE AMERICAN ACADEMY 



cate, however, with sufficient exactness for my present purpose, that 

 the strength of a magnet, as compared with its weight, is very much 

 less for large magnets than for small ones. It is not difficult to explain 

 this general fact. When one bar of iron is magnetized by another, 

 according to the laws of ordinary magnetic induction, the action is su- 

 perficial. The most we can do is to bring the surfaces of the two bars 

 into contact on one side. If the mass of iron is thick, the interior por- 

 tions are so far removed from the inducing magnet, that they receive 

 only a part of the magnetic development of which they are susceptible. 

 This difficulty is removed, if we begin by dividing the thick bar into a 

 number of thin pieces. These may be magnetized separately, and to 

 saturation. When we come to reunite them, we encounter another 

 difficulty. Each piece tends to induce a magnetic state, opposite to its 

 own, in its neighbours ; a state, accordingly, which is opposite to that 

 which its neighbours have already acquired. As soon as the pieces 

 are brought together, a part of the magnetism, originally developed, 

 becomes latent again ; and the united strength of all is not so great as 

 the sum of the pow^ers possessed by the parts when tried separately. 



" In 1820, it was discovered that the conducting-wire of a galvanic 

 battery possessed magnetic properties, and was capable of inducing 

 magnetism in a bar of soft iron placed at right angles to its own length. 

 This elementary force, first announced by Arago, was soon multiplied 

 in a wonderful degree by the application of Schweigger's principle to 

 it. At length large masses of iron, which defied the ordinary methods 

 of touch, were magnetized to saturation. Magnets which acquire their 

 magnetism from the induction of electricity (electro-magnets) have been 

 made of such a size and power as to lift ten thousand pounds. Professor 

 Henry gave an account, in Silliman's Journal^ of an electro-magnet con- 

 structed by him, as early as 1831, which, weighing twenty-one pounds, 

 was able to lift seven hundred and fifty pounds ; that is, more than 

 thirty-five times its own weight. I frequently experiment with electro- 

 magnets of a half-pound weight, which lift one hundred and seventy- 

 five times their own weight. With a good current, magnets no heavier 

 than this may be made to lift five hundred times their own weight. 



" At the present day, the old statical theory of magnetism has been 

 supplanted by the electro-dynamical theory of Ampere. According 

 to the views of this eminent physicist, all magnets are in one sense 

 electro-magnets. In the electro-magnet properly so called, the in- 

 ducing currents are obtained from a galvanic battery, and are made to 



