﻿468 Mr. A. Campbell on the Use of 



(10) For the same frequency, initial flux-density, and 

 material, increase of diameter of the specimen increases the 

 critical distance, has little or no effect on the critical retar- 

 dation (hence making dOjdx near the origin less), and dimin- 

 nishes the leakage coefficients : in fact, we find that the critical 

 distance is approximately proportional to the diameter, and the 

 critical value of the leakage coefficient inversely proportional 

 to the diameter. (See § 14 and Tables XXL, XXI1L, XXIV.) 



(11) For the same specimen and initial flux, increase of 

 frequency increases the initial value of d6/dx, increases the 

 critical values of the retardation and the leakage coefficient, 

 and slightly diminishes the critical distance. (See figs. 1, 2, 

 and the Tables.) 



(12) For the same specimen and frequency, increase of 

 initial flux increases the critical distance (see fig. 4), increases 

 the critical value of the retardation (fig. 4), and reduces the 

 critical value of the leakage coefficient (see fig. 5) : also for 

 low values of the initial flux F , ddjdx near the origin increases 

 with F 1? but seems to approach an upper limit for high values 

 of F . (See fig. 4.) 



L. On the Use of Chilled Cast Iron for Permanent Magnets, 

 By Albert Campbell, B.A. (From the National 

 Physical Laboratory.)* 



EARLY in the past year an interesting paper f was 

 published by Mr. B. 0. Peirce of Harvard University, 

 drawing attention to the fact that chilled cast-iron is in many 

 instances a suitable material for permanent magnets. As the 

 subject is of interest to scientific experimenters and of con- 

 siderable importance to instrument makers, I undertook some 

 time ago a short research upon it, with a view, firstly of 

 obtaining some measurements by standard methods, and 

 secondly of finding, if possible, an easy method of chilling 

 the material so as to give good results. 



Form of Test Pieces. — The cast iron tested was of ordinary 

 commercial quality and was obtained in the form of rods and 

 rings. The rods, which were of rectangular section, were 

 shaped to dimensions usual in such tests, viz., 10 cm. x 

 1 cm. x 1 cm. 



* Communicated by the Physical Society : read January 26, 1906. 



t Amer. Acad. Pro'c. xl. 22. pp. 701-715, April 1905. Dr. Watson 

 has hindly drawn my attention to Mr. J. R. Ash worth's experiments on 

 chilled cast-iron rods (Proc. Roy. Soc. vol. lxii. p. 210, Dec. 9, 1897) ; he 

 found that the maguetic quality of these was comparable with that of 

 tungsten steel. 



