Nov. 22. I9I5 Improved Respiration Calorimeter 329 



As the slider moves back and forth on the bar which supports the shde 

 wire, it carries a pen which draws a curve on ruled paper by which the 

 movement of the contact point on the slide wire is expressed in tempera- 

 ture. The total width of the paper scale, 25 cm., represents a difference 

 of 2 degrees between the temperature of the water entering and that of 

 the water leaving the heat absorbers, and corresponds exactly to the 

 length of the slide wire by which the bridge is balanced for the inequality 

 of resistance in the thermomicter coils resulting from such a temperature 

 difference. The temperature difference indicated by the position of the 

 pen on the paper scale coincides with that to which the position of the 

 contact point on the slide wire is equivalent. The paper scale is ruled 

 with 100 lines, each representing 0.02 degree, and as the distance between 

 the lines is 2.5 mm., the curve may easily be interpreted to o.oi degree. 

 The paper is moved forward at a very regular rate, approximately 7.5 cm. 

 per hour, by the motor which moves the slider, the speed of the motor 

 being regulated by a governor so that it is uniform, even with wide 

 fluctuations in voltage of the current by which the motor is driven. 

 Since the necessary changes in the position of the slider are made every 

 7 seconds, the cur\-e gives a practically continuous record of the tem- 

 perature difference. • 



The difference between the temperature of the water as it enters and 

 that as it leaves the heat absorber may thus be easily read at any 

 instant to o.oi degree. The accuracy of the measurement of tempera- 

 ture difference by the apparatus may be tested at any time, even during 

 the course of an experiment, without interfering with the record, and 

 such tests are made at frequent intervals. In the water channel in the 

 center of each resistance-thermometer bulb is the end of a differential 

 thermoelement of 0.125 "^ni- copper and constantan wires, having 11 

 junctions in each end, inclosed in 4-mm. glass tubing, with thin wall. 

 The element remains permanently in position, though it may be easily 

 removed if necessary. The terminals of the element are joined by insu- 

 lated I -mm. copper wire to binding posts on the observer's table, from 

 which connection can be made with a potentiometer whenever a test is 

 to be made. With this differential thermoelement, which has been cali- 

 brated over a wide range of temperature at the United States Bureau of 

 Standards, an electromotive force of over 4.5 microvolts results from a 

 difference of o.oi degree in the temperatures of the two ends. By means 

 of the potentiometer and galvanometer with which it is employed, an 

 electromotive force of half that magnitude is easily measured; conse- 

 quently temperature differences may be measured by it to an accuracy 

 at least as good as o.oi degree. Measurements made with this apparatus 

 therefore serve to indicate the accuracy of those with the recorder. 

 The agreement of results obtained by the tvv'o methods of measuring the 

 increase in the temperature of the water flowing through the heat ab- 



