456 Variations in the Resistance of certain Amalgams. 



if dissociation were taking place during the whole period of 

 cooling, the change from a decreasing to an increasing resist- 

 ance would be more gradual. 



Guillaume has observed bars of nickel-steel over many 

 months at temperatures near that of the room, and in some 

 cases he found a gradual elongation requiring a year for its 

 completion. This he attributes to a gradual dissociation. 

 The analogy to this for the zinc amalgams is the gradual 

 change, extending over several weeks, which manifests itself 

 by a gradual fall in the resistance. The point that seems 

 most difficult to explain is that the final is greater than the 

 initial resistance immediately after the completion of a tem- 

 perature cycle. If nothing further than dissociation of the 

 compound that exists from 100° down to 25° occurs, which 

 would be the case to be in strict analogy with M. Gruillaume's 

 theory, it is difficult to see why the partial dissociation 

 occurring when the amalgam is cooled from 25° down to 15° 

 should cause an increase in resistance, while when it is kept 

 near this latter temperature for some time, a further disso- 

 ciation causes a fall in resistance. Hence, as suggested 

 above, it seems probable that an intermediate compound is 

 first formed having a high specific resistance, and that on 

 standing it is a modification of this which causes the fall in 

 resistance. 



Further, Guillaume considers that when equilibrium is 

 attained at a given temperature the proportion of dissociated 

 elements present in the same reversible alloy is a function of 

 the temperature, and is always the same no matter how often 

 this temperature is reproduced. If something similar occurs 

 with the amalgams it would explain why, when a spiral was 

 cooled down to 65° and kept at that temperature for several 

 hours, the resistance showed no tendency to increase to the 

 value it had when the temperature was being raised ; but, for 

 reasons already given, a more likely explanation seems to be 

 the persistence unaltered through a wide range of tempera- 

 ture of the compound which is present at 100° C. 



In conclusion, I must thank Prof. Thomson for the assist- 

 ance I have derived from his suggestions during the course 

 of the experiments. 



Cavendish Laboratory, 

 Cambridge. 



