188 PROCEEDINGS OF THE AMERICAN ACADEMY. 



constant motion by a set of four i^ropellers run by a small motor, was 

 heated to a constant definitely determined temperature by means of a 

 Simplex Electric Heater attached to a 110 volt circuit and dominated 

 through a relay by a delicate thermostat. The annealing effects of 

 very hot water upon hard cast iron had to be avoided, but the water in 

 the second tank was usually made uncomfortably warm for the hand. 



In making cast iron magnets, it is very necessary that the iron j ust 

 before it is chilled shall be much hotter than it is safe to heat ordinary 

 tool steel in making it hard. Dr. Campbell, of the National Physical 

 Laboratory, Teddington, Middlesex, England, finds that a temperature 

 of 1000° C. has been sufficient for the iron he has used, but some 

 specimens of American iron seem to work best at a slightly higher 

 temperature, just below the melting point. If a massive piece of cast 

 iron weighing, say, fifty pounds be heated thus hot and then chilled in 

 a proper bath, the material, as magnetic tests can be made to show, 

 becomes hard throughout, whereas it is practically impossible to make 

 a similar piece of tool steel glass-hard inside. The experiments of 

 Chernoff" upon a certain kind of steel, made more than forty years ago, 

 showed that if the temperature from which the steel was chilled was 

 made higher and higher, from, say, 400° C, the hardening effect was 

 almost inappreciable until a cherry red was reached, when suddenly the 

 chilled specimen was found to be glass-hard. It is not very surprising, 

 therefore, that cast iron shows very little temper when chilled from a 

 temperature of 800° C. or 900° C, but may easily be made glass-hard 

 if its temperature just before the chilling is high enough, say 1050° C. 

 for some kinds. 



The rods were heated for the hardening, under a compressor 

 blast, in a special gas furnace made for the purpose by Messrs. J. 

 Connors and J. Coulson, and most of them were placed inside an iron 

 tube to protect them from direct exposure to the flames. In annealing 

 the rods they were packed in iron filings inside an iron tube closed at 

 the ends by screw caps and heated thoroughly to a white heat for 

 possibly 30 minutes before the tube was packed in ashes for many 

 hours. Although the work was done with the greatest care, it soon 

 appeared that it is usually impossible, at least by this particular 

 annealing process, to bring a piece of cast iron once made glass-hard 

 back to as low a resistivity as it originally had, and if the piece be 

 repeatedly hardened and annealed, its resistivity in the relaxed state 

 increases every time the cycle is passed through. The diameter of the 

 piece also increases perceptibly much as the cast iron bars of a fire box 

 grate grow longer with hard use. Two or three examples will show the 

 complicated nature of the phenomena involved. 



