140 



A RESPIRATION CALORIMETER. 



whole. The thermal junction system in the air current is likewise 

 capable of being placed in series with the galvanometer. There are, 

 then, eleven different thermal junction connections to be made with the 

 galvanometer. 



Any system of switches for such a number of connections must obvi- 

 ously be somewhat complex, and, furthermore, the wear and tear on 

 them would be such as to render their use extremely unsatisfactory. 

 The sliding contacts would frequently become covered with dirt or grit, 



IfiQ. 39. Unit Key of Mercury Switch. By depressing the key the two ends of a copper link are 

 caused to dip into mercury in holes in a wooden block. The mercury insures connection of the 

 screw and nut with the proper electrical devices. 



and the working parts would soon give way. An ingenious method of 

 using mercury contacts for the thermal junction systems, thereby avoid- 

 ing poor contacts and excessive wear and tear, was devised by our 

 mechanician, Mr. S. C. Dinsmore. Later, this mercury contact device 

 was incorporated in an instrument which combined a mercury switch 

 and a bridge system. 



The connection between each thermal junction circuit and the galva- 

 nometer is made by dipping two copper links into four mercury cups, 

 two of which are connected with the wires leading from the thermal 



