View related articles 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 retardation (hence making d~/dx near the origin less), and diminnishes the leakage c~efficients : in t~tct, we find that the critical distance is approximately proportional to the diameter, and the critical value of the leakage coefficient inversely proportional to

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... diameter. (See w 14 and Tables XXI., XXI]I., XXIV.) (11) For the same specimen and initial flnx, increase of frequency increases the initial value of dO/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 ~he retardation (fig. 4), and reduces the critical value of the leakage coefficient (see fig. 5): also for low values of the initial flux Fo, dO/dx near ~he origin increases with F1, but seems to approach an upper limit for high values ofF 0. (See fig. 4.) L. On the Use of Chilled Cast lron :or Permanent Magnets. Bff ALBERT CAMPBELL, B.A. (From the National Physical Laboratory.) a ARLY in the past year an interesting papert 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 ibr permanent magnets. As the subject is of interest to scientific experimenters and of considerable 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, ~n easy method of chilling the material so as to give good results.