462 
DR. J. HOPKIRSON ON MAGNETIC AND OTHER PHYSICAL 
Table 36. 
Re.sistance of 
secondary and leads. 
Temperature. 
Total induction. 
Permeability. 
0-77 
°C. 
9'0 (room) 
67-7 
1-08 
2-20 
476-0 
93-1 
1-95 
3-00 
757-0 
101-7 
2-19 
3-23 
816-0 
71-7 
1-45 
3-30 
841-0 
72-0 
1-42 
3T4 
787-0 
72-0 
1-.38 
2-80 
674-0 
92-3 
1-99 
0-79 
8-8 (room) 
94-5 
1-99 
As the changes in the temperature were in this case made somewhat rapidly, the 
temperature of the ring lags behind the temperature of the cojDper. 
These show: first, that at no temperature does this steel become at all strongl}^ 
magnetic; second, that at a temperature of a little over 750° C. there is a substantial 
reduction of permeability ; third, that above this temperature the substance remains 
slightly magnetic ; fourth, that annealing somewhat increases the permeability of the 
material. 
Resistance of Iron at High Temperatures. 
These exjDeriments were made in a perfectly simple way. Coils of very soft iron 
wire, pianoforte wire, manganese steel wire, and copper wire were insulated with 
asbestos, were bound together with copper wire so placed as to tend by its .conduc¬ 
tivity for heat to bring them to the same temperature, and were placed in an iron 
cylindrical box for heating in a furnace. They were heated with a slowly rising 
temj^erature, and the resistance of the wires was successively observed, and the 
time of each observation noted. By interpolation the resistance of any sample at any 
time intermediate between the actual observations could be very app7’Oximately 
determined. The points shown in Curves XXXV., XXXVI., XXXVIL, were thus 
determined. In these curves the abscissse represent the temperatures, and the 
ordinates the resistance of a wire having unit resistance at 0° C. Curve XXXVII. 
is manganese steel, whiiih exhibits a fairly constant temperature coefficient of 
O'OOllO; Dr. Fleming gives 0'0012 as the temperature coefficient of this material. 
Curve XXXV. is soft iron; at 0° C. the coefficient is 0'0056 ; tire coefficient gradually 
increases with rise of temperature to 0'019, a little below 855° C.; at 855° C. the 
coefficient suddenly, or at all events very rapidly, changes to 0'007. Curve XXXVI. 
is jiianoforte wire; at 0° C. the coefficient is 0’0035 ; the coefficient increases with 
rise of temperature to 0'016, a little below 812° C.; at 812° C. the coefficient 
suddenly changes to 0‘005. The actual values of the coefficients above the points of 
change must be regarded as somewhat uncertain, because the range of temperature 
