﻿Chilled Cast Iron for Permanent Magnets. 469 



By using rods of these dimensions it is easy to compare 

 the results with those for various kinds of steel which we 

 have already tested or with those published by Madame Curie * 

 and other observers. 



Two rings were tested ; they were of rectangular section, 

 their mean diameters being nearly equal (12'5 and 13*0 cm.). 

 Their cross-sections, however, were very different, being 

 l'OO sq. cm. and 6'0 sq. cm. respectively. The object of 

 testing a thick and a thin ring was to find if our method of 

 chilling was effective for the hardening of thick castings. 



Heat Treatment. — All the test pieces were heated to 

 1000° C. in a gas muffle furnace, the temperature being 

 measured by a thermo-jtinction in the usual way. Each 

 piece was removed from the furnace and quickly chilled in 

 water at the temperature of the room. As Mr. Peirce has 

 pointed out, great care is necessary in handling the cast iron 

 at tbis high temperature (so near its melting-point) for it 

 becomes very brittle. For this reason the thick ring was 

 placed in the furnace on a |J -shaped piece of wrought iron, 

 it was lifted out by means of this support, and the two were 

 plunged together into cold water. By this method the brittle 

 material could be handled without risk of breakage. 



Tests on Rods. — After the chilling, the rods were mag- 

 netized to saturation by means of a very strong magnetic field. 

 This was produced by a solenoid consisting of 70 turns with a 

 length of 16 cm. and having a resistance of about 0"1 ohm. 

 A large current was sent through this coil by connecting it 

 for a very brief interval to a 50-volt circuit (by the process 

 commonly known as ''flashing'''). Each rod was then 

 tested for 



(1) The maximum remanent flux density B (at the medial 



section of the bar). 



(2) The coercivity H , i. e. the value of the demagnetizing 



magnetic force required to annul this remanent 

 magnetism. 



(1) In order to measure the medial B, a small square 

 search-coil of 20 turns of very fine wire was used ; it was 

 just large enough to slip along the rod. The search-coil was 

 connected to a calibrated ballistic galvanometer, and, when 

 the coil was slipped off the rod, the resulting deflexion gave 

 the required B in the usual manner. 



(2) The coercivity was found by Madame Curie's method, 

 as follows : — The magnetized bar was fixed at the centre of a 

 long solenoid, a search-coil being so arranged that it could be 



* Bulletin, de la Societe d 'Encouragement pour V Industrie. Nationale, 

 Jan. 1898. 



