456 



BELL SYSTEM TECHNICAL JOURNAL 



which the magnetism disappeared and the curve for decreasing temper- 

 ature is the same as for increasing temperature over a short range. 

 At 500° C. the curve for decreasing temperature does not follow its 

 original path but continues to drop until the temperature reaches 

 about 425° C. From that point on until room temperature is reached 

 the return curve is nearly horizontal. 



At room temperature the permeability is only 2,000, a drop of 5,000 

 from its value at the beginning of the run. With the alloy in this 

 condition, if a second cycle is run, the permeability both for increasing 



140000, 



120000 



100000 



10000 



80000 



z 



^ 



60 000 1 



40000 



20000 



20 



40 60 80 100 120 140 



COOLING RATE — DEG. CENT. PER SEC. 



180 



Fig. 17 — Initial (mo) and maximum _(/xmax) permeabilities for 78.5 permalloy for 

 different rates of air quenching from 600° C. 



and decreasing temperature will be substantially the same as the one 

 in the figure for decreasing temperature. By heating the sample to 

 600° C. and air quenching, the magnetic properties at the beginning 

 of the test are restored. 



The connection between the permeability of this composition and 

 the heat treatment in the temperature range below 600° C. is also 

 illustrated by a series of tests in which annealed rings were air quenched 

 from temperatures below 600° C. The rings were placed in a furnace 

 and heated at 600° for 15 minutes. The temperature was then 

 decreased slowly to 550° C. and held until the alloys had reached a 

 constant condition. One of the samples was then taken out of the 

 furnace and air quenched. The temperature of the furnace was then 



