MAGNETIC PROPERTIES OF PERMINVAR 23 



the alloy. The alloys also contained small amounts of carbon (less 

 than .03 per cent), silicon (less than .1 per cent) and traces of sulphur 

 and phosphorus. The alloys were cast into bars 18 in. long and 3/4 in. 

 in diameter. The bars were rolled or swedged into 1/4 in. rods and 

 drawn from that size to .062 in. diameter wire. This wire was flattened 

 and trimmed into tape 1/8 in. X .006 in. The material was annealed 

 several times in the reduction process, for the cold working hardened 

 the alloys rapidly and made them difficult to work. 



To prepare the tape for heat treatment and subsequent magnetic 

 measurements, about 30 ft. of it was wound spirally into a ring of 

 3 in. inside diameter, the ends being spot welded to the adjacent 

 turns. Care was taken to wind the rings loosely to prevent the 

 turns of tape from sticking during annealing. 



A number of such rings were packed in a nichrome pot. Some iron 

 dust was usually placed in the pot to take up the oxygen and thus 

 prevent the oxidation of the rings. Further protection was secured 

 by luting the joint between the pot and its cover. The pot was 

 placed in an electrical resistance furnace, the temperature of the 

 furnace raised to 1000° C. and held at that temperature for one hour. 

 The current was then turned off and the pot cooled with the furnace. 

 Ten hours were required for the furnace to cool to the temperature 

 of the room. Between 700° C. and 400° C. the rate of cooling was 

 approximately 1.5° per minute. 



Three rings of each composition were always annealed together. 

 One of these rings received no further heat treatment. The second 

 ring was placed for 15 minutes in a furnace held at 600° C, then 

 removed and cooled rapidly on a copper plate. In some cases, the 

 third ring was heated 24 hours at 425° C. 



In the discussions and in the figures and tables, the rings which 

 received the first heat treatment only are referred to as "annealed," 

 those reheated to 600° C. and rapidly cooled as "air quenched," and 

 those held for a long time at 425° C. as "baked." 



Magnetic Measurements 



Permeabilities at low magnetizing forces were measured on un- 

 wound rings with an inductance bridge, and an a.c. permeameter.^ 

 From these measurements initial permeabilities were computed. For 

 elevated temperature, measurements were made with a similar perme- 

 ameter provided with a furnace compartment.^ The bridge was also 

 used for measuring permeabilities to small a.c. magnetizing forces 

 when d.c. forces are superposed on the magnetic circuit. For these 



3 G. A. Kelsall, /. 0. S. A. and R. S. I., 8, pp. 329-338, 1924. 

 ^ G. A. Kelsall, J. O. S. A. and R. S. I.,S, pp. 669-674, 1924. 



