viii THERMO-ELECTRIC CRYOSCOPY 157 



Such junctions are usually formed at the binding-screws 

 between brass and copper or between two different samples 

 of copper. Strained places even in the copper leads may 

 also act as thermo- junctions. Another source of trouble is 

 strains in the galvanometer suspension, which lead to con- 

 tinual changes in the position of the zero on the scale. 

 The slowness of the galvanometer needle to take up its 

 final position may also be mentioned as introducing uncer- 

 tainty in deciding on the true magnitude of the deflection. 



In view of these sources of error, it is evidently of great 

 importance to have as few connections in the circuit 

 as possible, and, where the latter are unavoidable, to 

 secure that they are balanced by similar connections kept 

 at the same temperature. 



Construction of the thermocouples. In order 

 to eliminate one usual set of connections from the 

 circuit, i.e., that between the thermocouple and the leads, 

 it was arranged to utilise the ends of the copper leads 

 themselves as one pair of elements in the junctions. 

 These leads, which had a diameter of 0*1 7 mm., extended 

 right from the junctions to the reversing key (to be 

 described later). The other pair of elements of the 

 couple were formed of the ends of a continuous iron, 

 nickel, german silver, " constantin," or " eureka " wire. 



For the work in hand, the eureka-copper junctions were 

 found most suitable. The eureka alloy has a high thermo- 

 electric value when forming a junction with copper, and 

 so is capable of giving a large deflection for a small tem- 

 perature difference. Its comparatively great resistance 

 enables one to adjust very conveniently the sensitiveness 

 by increasing or diminishing the length of the eureka in 

 the couple. Its low coefficient of variation of resistance 

 with temperature secures that this convenient resistance 

 introduces practically no error ; and when, as in the 

 apparatus to be described, it is enclosed in the freezing- 

 chamber, the error is so small that it may be disregarded. 



