PEIRCE. — MAGNETIC TESTS UPON IRON AND STEEL RINGS. 89 



division, let y represent the product of the value of B corresponding 

 to the value of // at P, and the width, ST, of the ring at P, and let 

 a curve be drawn with the v/'s as ordinates and the OP's as abscissas. 

 The ratio of the area under this curve, — obtained by the help of a 

 good Amsler's planimeter, — to 7ra^, gives the mean value (//) of the 

 magnetic induction in the ring. The average value of the field {H) is 



2 (}{ 



— ^(c — Vc' — a^)and the value {B") of the induction correspond- 

 ing to this value of H can be found from the HB diagram. 



TABLE II. 



Bod of Bessemer Steel 1.25 cm. in Diameter, magnetized in a Uniform 



Solenoid about five Meters long. Besults obtained 



by the Method of Reversals. 



An illustration may help to make the details of the process more in- 

 telligible. Consider a toroid of the Norway iron, the circular cross 

 section of which has a radius of one centimeter and the mean radius (c) 

 of which is 7 centimeters. If the excitation is to be such that the 

 value of the magnetic field at C is unity, the values of // at points dis- 

 tant 6, 6.1, 6.2, 6.4, 6.6, 6.8, 7.0, 7.2, 7.4, 7.6, 7.8, 8 cms. respectively 

 from the axis of revolution of the ring, are 1.167, 1.147, 1.129, 1.094, 

 1.061, 1.030, 1.000, 0.972, 0.946, 0.921, 0.897, 0.875, and the values 



