THE RESPIRATION APPARATUS. 41 



Calibration of the pans. In order to know accurately the actual vol- 

 ume of air in the system as a whole, it is necessary to take into con- 

 sideration the fluctuating volume of air in the pans, and consequently 

 a calibration showing the volume of air inclosed by the rubber dia- 

 phragms at different positions is essential. By calculations based upon 

 measurements of dimension it is possible to determine the volume of 

 air in the pans when both diaphragms are down. 



By means of the valve in the main air-pipe leading to the chamber 

 (see fig. 17) and the mercury valves at the exit end of the absorber 

 system, the section of the circuit to which the pans are attached may 

 be sealed off. Furthermore, by means of a lead weight attached to a 

 hook from which the rubber disk is hung, one of the pans may be kept 

 empty. It is thus seen that if air is admitted at any point in this 

 portion of the ventilating air-pipe under these conditions the rubber 

 diaphragm on the other pan will become inflated. 



There is no condition in which both pans need to be read when only 

 partly filled, and in practice one can be kept either full or empty. It 

 is necessary, therefore, to calibrate completely but one of the pans, and 

 this has been done only with that shown in the foreground of figure 17. 

 It is sufficient for the other pan to determine the actual amount of air 

 contained when full, and in order to facilitate reading and insure accu- 

 racy it is customary either to place a weight on the disk and so expel 

 all air from the pan or to attach the weight to the outer end of one of 

 the bicycle spokes on the opposite side of the wheel to insure filling. 

 That there shall always be a rise and fall through exactly the same 

 distance, two screws are inserted in the rim of the wheel at such a point 

 that when the pan is weighed empty a screw strikes against the fork, 

 and thus relieves the extra weight. Similarly, a screw placed in the 

 bicycle rim on the other c ide of the fork prevents the weight from 

 raising the rubber diaphragm beyond a definite point. Between these 

 two points, therefore, it is necessary to know the volume of air required 

 to fill the diaphragm. This was found by forcing room air through 

 the Elster meter, then through sulphuric acid to remove all moisture, 

 and finally into the system until the rubber diaphragm had reached its 

 highest point. This was easily detected, for at the moment the screw 

 in the rim of the bicycle wheel touches the fork during the upward 

 movement of the diaphragm an electrical contact is made, causing a 

 bell to ring. From the readings of the meter, including its temperature 

 and the pressure on the manometer, and the readings of the barometer 

 and thermometer, the volume of dry air thus added to the system was 

 readily computed. This amount, plus the constant volume of air con- 

 tained in the pan below the bottom of the diaphragm when in its lowest 



