LABORATORY XÔTEy. 



291 



If we p.s:>mne thîit the change« in the platinum re>«i«tance follow 

 the same law n^i in the earlier experiment with the oil, the rise of tem- 

 perature which will just (l(juble the resistance is about 680° C; and the 

 interval from 1 to 1.2 may be taken as c<jrresponding approximately to 

 a rise of tem})erature (jf 136°C. According to the ex])eriment in oil, 

 the resistance of the cobalt would ha^'e Ijeen increased in the ratitj 

 1.42-48 to unity by this rise of temperature. It is p.pparent then that 

 under the influence of the first excessive heating tlie cobalt has been 

 considerably altered in its pro])erties, so tliat the average temperature 

 cœfficient for resistance up to 150°C. has been increased by a quarter. 



That the successive heatings caused a marked change in the 

 structure of the wire <jr strip is shown l)y the variations in the measur- 

 ed resistance at 7°C. These are given in Taljle III. 



Table III. 



The fall in resistance alter the first heating is no doubt due to 

 some change in the contact resistances. It characterises both the 

 platinum and cobalt. !Subse(pient heatings howe\er do not change 

 the platinum to any great extent until the very last experiment ; but 

 their effect on the cobalt is very marked. After the experiments were 

 completed the col);dt was found to be nuich ahered l)v oxidation. It 

 was exceedingly brittle and broke into small pieces when it w^as beinij- 



