THE 73° CALOKIE. 



381 



flask protecting the coupling. Finally it flows out through the fourth 

 thermometer tube, to a pump which returns it to the constant-head 

 reservoir above A. Distilled water is used, and renewed from time to 

 time. 



It was intended to choose the temperature so that the ratio of the 

 75° calorie to the 20° calorie might be obtained. The actual tem- 

 peratures chosen happened to be somewhat more easily handled with 

 the thermostats. 



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THeRnOiTATi 



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CALORiriCTCR 



The resistance thermometers contain about 33 ohms of No. 42 

 platinum wire, coiled in helixes of inside diameter .02", silver-soldered 

 to copper leads, which are passed in the shape of ribbon \" wide be- 

 tween copper leaves of somewhat greater width, and 2\" long, from 

 which they are insulated by mica. . They are held between these leaves 

 by a tight winding of copper ribbon. The purpose of this arrangement 

 is to make certain that the leads come to the temperature of the ther- 

 mometer case, and to this end the copper leaves referred to are silver- 



