PEOPERTIES OP IRON AT A HIGH TEMPERATURE. 
445 
the maximum temperature it exceeded 10,000 ohms, and generally lay between 
10,000 and 20,000 ohms. The ring’ to be examined, with its coils of copper wire, 
was placed in a cylindrical cast-iron box, and this in a Fletcher gas furnace, the 
temperature of which was regulated by the supply of gas. The temperatures were 
estimated by the resistance of the secondary coil. It was observed that the 
resistance of this coil at the ordinary temperature increased slightly after being 
raised to h. high temperature : this I attribute to oxidation of the wire where it 
leaves the cast-iron box. However, it introduced an element of uncertainty into the 
determination of the actual temperatures, amounting, perhaps, to 20 ° 0 . at the 
highest temperature. This error will not affect the differences between neighbouring 
temperatures^ with which we are more particularly concerned. 
The resistance of the ballistic galvanometer is 0’43 ohm; to this additional 
resistances were added to give the necessary degree of sensibility. The ratio of 
two successive elongations of the galvanometer is (1 -|- >’)/! = 1T2/1. The time of 
oscillation T and the sensibility varied a little during the experiments, but so little, 
that the correction woidd fall within the limits of errors of observation in these 
experiments. 
The total induction = 
7 —R.A.IO®, where C is the current which 
In 
gives the dedection a, n is the number of turns in the secondary coil, R the resistance 
of the secondary circuit, A the mean of the first and second elongations on reversal of 
the current in the primary. 
The magnetising force = 47 rmc//, where m is the number of turns in the primary, 
I the mean length of lines of force in the ring, c the current in absolute measure in the 
primary. 
With my galvanometer as adjusted, a Grove’s cell, the E.M.F. of which was at the 
time determined to be I’SOO volt, gave a deflection of 158’5 divisions through a 
resistance of .00,170 ohms, whence 
Hence 
d 
1-800 
158-5x50,170 
0-0000002264, 
T =13-3. 
C 
a. Itt 
5-09 X 10"h 
The ring method of experiment is open to the objection that the magnetising force 
is less in the outer than in the inner portions of the ring. The results, in fact, give 
the average r^esults of forces which vary between limits. 
Wrought Iron .—The sample of wrought iron was supplied to me by Messrs. 
