264 Mr. H. A. Rowland on Magnetic Distribution. 



in. 



Among the various methods of measuring linear magnetic dis- 

 tribution, we find few up to the present time that are satisfactory. 

 Coulomb used the method of counting the number of vibrations 

 made by a magnetic needle when near various points of the 

 magnet. Thus, in the curve of distribution most often repro- 

 duced from his work, he used a magnetized steel bar 27 French 

 inches long and 2 lines in diameter placed vertically; opposite 

 to it, and at a distance of 8 lines, he hung a magnetic needle 3 

 lines in diameter and 6 lines long, tempered very hard ; and the 

 number of oscillations made by it was determined. The square 

 of this number is proportional to the magnetic field at that point, 

 supposing the magnetism of the needle to be unchanged ; and 

 this, corrected for the magnetism of the earth, gives the mag- 

 netic field due to the magnet alone. This for points near the 

 magnet and distant from the ends is nearly proportional to the 

 so-called magnetic surface-density opposite the point. At the 

 end Coulomb doubled the quantity thus found, seeing that the 

 ,bar extended only on one side of the needle. 



It will be seen that this method is only approximate, and 

 almost incapable of giving results in absolute measure. The 

 effect on the needle depends not only on that part of the bar 

 opposite the needle, but on portions to either side, and gives, as 

 it were, the average value for some distance; in the next place, 

 the correction at the end, by multiplying by 2, seems to be in- 

 adequate, and gives too small a result compared with other 

 parts. For at points distant from the end the average surface- 

 density at any point will be nearly equal to the average for a 

 short distance on both sides, while at the end it will be greater 

 than the average of a short distance measured back from the 

 end. To these errors must be added those due to the mutual 

 induction of the two magnets. 



The next method we come to is that which has been recently 

 used by M. Jamin, and consists in measuring the attraction of 

 a piece of soft iron applied at different points of the magnet. 

 In this case it does not seem to have been considered that the 

 attraction depends not only on the magnetic density at the 

 given point, but also on that around it, and that a piece of 

 soft iron applied to a magnet changes the distribution imme- 

 diately at all points, but especially at that where the iron is 

 applied. The change is of course less when the magnet is of 

 very hard 'steel and the piece of soft iron small. Where, how- 

 ever, we wish to get the distribution on soft iron, it becomes a 

 quite serious difficulty. Another source of error arises from the 

 fact that the coefficient of magnetization of soft iron is a func- 



