55 PROCEEDINGS OF THE AMERICAN ACADEMY. 



homogeneous throughout, and although the larger values oi B are given 

 in the tables rather more exactly than the observations warrant, the 

 slow-moving ballistic galvanometers employed permitted of very accu- 

 rate measurements of the flux changes in the testing coil. 



After a good HB diagram, accurately drawn on a large scale, has 

 been obtained for a given kind of iron or steel, it is possible to find out 

 how nearly the mean value of the magnetic induction in a given ring 



TABLE I. 



Annealed Norway Iron Rod 1.25 cms. in Diameter, magnetized in a 



Uniform Solenoid about five Meters long. Results 



obtained by the Method of Reversals. 



made of this material would differ from the real induction correspond- 

 ing to the mean value of the field in the metal, for any given excitation. 

 Suppose, for example, that the ring is to be a toroid and that the radius 

 of the circular cross section is to be a, while the centre of the section 

 is distant c cms. from the axis, OY, of revolution of the ring. Suppose 

 that the excitation is to be such as to make the value of //, at points 

 distant c from OY, He, then the value of // at a point P (Figure 1) is 

 He • c/(OP). Let the numerical value of this quantity be computed for 

 say n -f 1 points evenly dividing the space WV, and let the numerical 

 values of B corresponding to these values of // be read with the help 

 of a lens from the HB diagram. Let P represent one of the points of 



