1890-91.] Prof. C. G. Knott on Electric Resistance of Cobalt 307 
all experiments, and were measured with great care. If again 
the resistances of the contacts had changed to any great extent, 
this would declare itself in the measured resistances at 7° C. made 
before and after the first severe heating. In Table III. these 
measured resistances, corrected for connections, are given. They 
were all made at 7° C., except the first pair (taken immediately 
before the first heating), for which the temperature was 7° ’5 C. 
Table III. 
Resistance in Legal Ohms of 
When measured. 
Platinum. 
Cobalt. 
•8525 
•09724 
Before 1st heating. 
•85028 
•09135 
After 1st ,, 
•85028 
*09354 
„ 2nd ,, 
•85013 
•09674 
„ 3rd „ 
•85232 
•09978 
, , 4th , , 
The fall in resistance after the first heating is probably due to 
some change in the contact resistances — decrease evidently. But 
even if this were large enough to sensibly affect the second sig- 
nificant figure in the calculated value of the temperature coefficient, 
its effect would be to diminish this coefficient. Consequently, 
we must accept the conclusion that the first excessive heating has 
profoundly influenced the qualities of cobalt as regards its change 
of resistance with temperature. 
Table III. shows us also that whereas the platinum resistance 
at 7° C. has not been changed at all by the second heating, and 
only slightly by the third, the cobalt resistance goes on steadily 
increasing. After the experiments were completed, the cobalt was 
indeed found to be much altered by oxidation. It had become 
exceedingly brittle, and broke into small pieces when it was being 
detached from the copper rods. This steady deterioration in condi- 
tion of the cobalt explains the inferiority in point of regularity of 
the third and especially the fourth experiment, as compared with 
the first and second. 
It is matter of surprise that, in spite of the great alteration in 
structure taking place in the cobalt strip, the general behaviour of 
