Magnetic Permeahilitij of various Alloi/s of Iron. 



83 



Group 5. — Aluminium Steels. 



It will be seen, upon referring to Plate II., that the curve is a very smooth 

 one: the samples being fairly uniform in tlieir composition, except as regards 

 the percentage of aluminium. The curve shows us that the reduction of con- 

 ductivity by adding comparatively small percentages of aluminium to iron is 

 most startling, being greater than that of any other metal so far examined. 

 Between 0'75 and 5'5 per cent, of added aluminium, the specific resistance 

 increases 48*0 microhms, equivalent to an increase of 10-1 microhms for every 

 1 per cent, of aluminium added to the iron ; between 2*25 and 5 '5 per cent, of 

 aluminium, nearly the same increase occurs, viz. 9'5 microhms. (See also p. 94.) 



The alloy No. 11671 with 5A per cent, of aluminium was soft and ductile: a 

 sample was rolled into strip, and drawn into wire No. 20 B.W.G., and the specific 

 resistance and temperature coefficient of both carefully determined. The mean 

 specific resistance of the annealed wire was found to be 74*69 microhms per c.c. 

 at 15° C. ; the resistance of the strip was slightly lower. The specific resistance 

 of the annealed rod of the same alloy, calculated from the conductivity given in 

 Group 5, was somewhat lower. The percentage variation of resistance for increase 

 of temperature was found to be 0-063 per 1°C. between 0° and 150° C. This 

 is remarkably good, being exactly ten times less than iron, and nearly as low 

 as German silver, which has a temperature coefficient of 0"044. As the resistance 

 of this aluminium steel is three and a half times greater than German silver, 

 it promises to be a valuable and cheap alloy for use in resistance coils, though we 

 cannot yet speak of its behaviour after repeated heating and cooling. 

 We now come to the alloys of silicon and iron : — 



Group 6. — Silicon Steels. 



IKANS. EOT. BUB. SOC, N.S. VOL TIL, PAKT IV. 



