36 HEXRY A. ROWLAND 



The relations of these quantities are given by the following equa- 

 tions : 



, _ 

 - 



3k A 



The first determination of the value of any of these quantities was 

 made by Thalen. But more important experiments have been made 

 by Weber, Von Quintus Icilius, and more recently by M. Eeicke and 

 Dr. A. Stoletow. 2 The first three of these in their experiments used 

 long cylindrical rods, or ellipsoids of great length; the last, who has 

 made by far the most important experiments on this subject, has used 

 an iron ring. The method of the ring was first used by Dr. Stoletow 

 in September, 1871; but more than eight months before that, in Jan- 

 uary, 1871, I had used the same method, but with different apparatus, 

 to measure the magnetism. He plots a curve showing the variation of 

 K ; but he plots it with reference to E as abscissa instead of R * , and 

 thus fails to determine the law. His method of experiment is much 

 more complicated than mine, so that he could only obtain results for 

 one ring; while by my method I have experimented on about a dozen 

 rings and on numerous bars, so that I believe I have been enabled to 

 find the true form of the function according to which /* varies with the 

 magnetism of the bar or the magnetizing-force. 



Many experiments have been made on the magnetism of iron without 

 giving the results in absolute measure. Among these are the experi- 

 ments of Muller, Joule, Lenz and Jacobi, Dub, and others. The ex- 

 periments have been made by the attraction of electromagnets, by the 

 deflection of a compass-needle, or, in one case, by measuring the in- 

 duced current in a helix extending the whole length of the bar. By 

 the last two methods the change in the distribution of magnetism over 

 the bar when the magnetism of the bar varies is disregarded, if indeed 

 it was thought of at all : even in a recent memoir of M. Cazin * we have 

 the statement made that the position of the poles is independent of the 

 strength of the current. He does not give the experiment from which 

 he deduces this result. Now it is very easy to show, from the formula 



'Phil. Mag., January, 1873. 



3 Annales de Chimie et de Physique, Feb., 1873, p. 171. 



