THE CALORIMETER. 19 



what cooler than is desired. The effect of this cooling, therefore, is then 

 counterbalanced by passing an electric current of varying strength through 

 the heating wire. By this manipulation it is unnecessary that the observer 

 manipulate more than one instrument, namely, the rheostat, while formerly 

 he had to manipulate valves, compressed-air cocks, and rheostat. The 

 arrangement for providing for the amount of compressed air and water is 

 shown in fig. 13, in which it is seen that a small drop-sight feed-water valve 

 is attached to the pipe C leading into the dead air-space surrounding the 

 calorimeter chamber. Compressed air enters at B and the amount entering 

 can be regulated by the pet-cock. The amount of water admitted is readily 

 observed by the sight feed-valve. When once adjusted this form of apparatus 

 produces a relatively constant cooling effect and facilitates greatly the 

 manipulation of the calorimetric apparatus as a whole. 



THE THERMO-ELECTRIC ELEMENTS. 



In order to detect differences in temperature between the copper and 

 zinc walls, some system for measuring temperature differences between these 

 walls is essential. For this purpose we have found nothing that is as prac- 

 tical as the system of iron-German-silver thermo-electric elements origi- 

 nally introduced in this type of calorimeter by E. B. Eosa, of the National 

 Bureau of Standards, formerly professor of physics at Wesleyan University. 

 In these calorimeters the same principle, therefore, has been applied, and it 

 is necessary here only to give the details of such changes in the construc- 

 tion of the elements, their mounting, and their insulation as have been made 

 as a result of experience in constructing these calorimeters. An element 

 consisting of four pairs of junctions is shown in place as T-J in fig. 25. 



One ever-present difficulty with the older form of element was the ten- 

 dency for the German-silver wires to slip out of the slots in which they had 

 been vigorously crowded in the hard maple spool. In thus slipping out of 

 the slots they came in contact with the metal thimble in the zinc wall and 

 thus produced a ground. In constructing the new elements four pairs of 

 iron-German-silver thermal junctions were made on essentially the same 

 plan as that previously described,* the only modification being made in the 

 spool. While the ends of the junctions nearest the copper are exposed to 

 the air so as to take up most rapidly the temperature of the copper, it is 

 somewhat difficult to expose the ends of the junctions nearest the zinc and 

 at the same time avoid short-circuiting. The best procedure is to extend 

 the rock maple spool which passes clear through the ferule in the zinc 

 wall and cut a wide slot in the spool so as to expose the junctions to the 

 air nearest the ferule. By so doing the danger to the unprotected ends of 



* W. O. Atwater and F. G. Benedict: A respiration calorimeter with appliances 

 for the direct determination of oxygen. Carnegie Institution of Washington Pub- 

 lication No. 42, p. 114. (1905.) 



