30 



CALORIMETERS FOR STUDYING RESPIRATORY EXCHANGE, ETC. 



sequent experimenting the resistance thermometer could not be used with 

 satisfaction. More recently, plans were made to incorporate some of the 

 results of the rapidly accumulating experience in the use of resistance ther- 

 mometers and consequently an electric-resistance thermometer was devised 

 to meet the conditions of experimentation with the respiration calorimeter 

 by Dr. E. F. Northrup, of the Leeds & Northrup Company, of Philadelphia. 

 The conditions to be met were that the thermometers should take rapidly 

 the temperature of the ingoing and outcoming water and that the fluctua- 

 tions in temperature difference as measured by the resistance thermometers 

 should be controlled for calibration purposes by the differences in tempera- 

 ture of the mercurial thermometers. 



For the resistance thermometer, Dr. Northrup has used, instead of copper, 

 pure nickel wire, which has a much higher resistance and thus enables a 



J" A 



Fig. 16. — Details of resistance thermometers for water-circuit. Upper part of 

 figure shows a sketch of the outside of the hard-rubber case. In lower part 

 is a section showing interior construction. Flattened lead tube wound about 

 central brass tube contains the resistance wire. A is enlarged part of the case 

 forming a chamber for the mercury bulb. Arrows indicate direction of flow 

 on resistance thermometer for ingoing water. 



much greater total resistance to be inclosed in a given space. The insu- 

 lated nickel wire is wound in a flattened spiral and then passed through a 

 thin lead tube flattened somewhat. This lead tube is then wound around 

 a central core and the flattened portions attached at such an angle that the 

 water passing through the tubes has a tendency to be directed away from 

 the center and against the outer wall, thus insuring a mixing of the water. 

 Space is left for the insertion of the mercurial thermometer. With the 

 thermometer for the ingoing water, it was found necessary to extend the 

 bulb somewhat beyond the resistance coil, so that the water might be 

 thoroughly mixed before reaching the bulb and thus insure a steady tem- 

 perature. Thus it was found necessary to enlarge the chamber A (fig. 16) 

 somewhat and the tube leading out of the thermometer, so that the bulb of 

 the thermometer itself could be placed almost directly at the opening of the 

 exit tube. Under these conditions perfect mixing of water and constancy 

 of temperature were obtained. 



