vim THERMO-ELECTRIC CRYOSCOPY 163 



was made in the following way : The bare ends of the 

 galvanometer leads were fixed on the inner surfaces of 

 the jaws of a spring wooden clip. When the clip was 

 closed upon the vertical support of the thermocouple 

 leads, connection was established between the two pairs 

 of leads, and the circuit was complete. By releasing the 

 clip and rotating it round a vertical axis through 180, 

 and clamping it again on the support, the current from 

 the couple may be reversed through the galvanometer. 



In this form of reversing key, the junctions being of 

 the same metal and if desired made of the same piece 

 of metal, thermo-electric effects set up by temperature- 

 difference at the junctions are reduced to a minimum. 

 Notwithstanding this, it was found that these differences 

 of temperature were a source of error. To maintain the 

 junctions on the opposite sides of the support at the same 

 temperature and so eliminate the error, these connections 

 were made underneath liquid petroleum, contained in a 

 beaker, on the bottom of which rested the support of the 

 thermocouple leads. The petroleum was kept stirred 

 during observations. 



Arrangements for galvanometer. It is found 

 convenient to have the galvanometer leads a con- 

 siderable length, so as to allow a suitable distribution 

 of the parts of the apparatus ; consequently, it is 

 essential that they should have a sufficiently large cross- 

 section, so as to offer but a small resistance ; otherwise 

 changes in temperature, from which it is impossible to 

 shield them, will alter the sensitiveness of the apparatus. 

 With the key described, there is no objection to having the 

 galvanometer leads of different copper wire and heavier 

 than those coming from the junctions. 



Some special precaution is also needed to secure that 

 the junctions at the binding-screws and those in the 

 galvanometer are at the same temperature. In the case 

 of these connections it is all the more necessary, because 



m 2 



