[MURRAY] HELIUM CONTENT 29 



and G a galvanometer. In one arm of bridge there is a comparison 

 protected spiral corresponding to the exposed spiral previously men- 

 tioned. In the case of detection of hydrogen in air the exposed and 

 protected spirals are symmetrical and of equal resistance; in the case 

 of detection of air in hydrogen the two spirals are unsymmetrical, 

 and the exposed spiral about one-quarter the resistance of the pro- 

 tected spiral. 



When a reading is to be made, switch S is closed and switch E 

 turned to A — the "calibration" position. By means of the resistance 

 at C, the deflection of the indicator is adjusted to a calibration de- 

 flection. In this way the current is standardized. Switch E is then 

 turned to B, the "on" position, and the indicator read. 



Instruments Calibrated 



The instruments calibrated were: 



1. Shakespear Hydrogen Purity Meter No. 35944. 



2. Detail Explorer No. 39273 with Indicator No 37868. (100- 

 70% scale). 



3. Detail Explorer No. 39272 with Indicator No. 37868. (0- 

 20% scale). 



4. Permeameter Ej^plorer No. 39275 with Tinsley Galvanometer 

 No. 5753. 



With the exception of the mirror galvonometer, made by H. 

 Tinsley & Co., the instruments were manufactured by the Cambridge 

 Scientific Instrument Co., Ltd. 



Experimental Apparatus 



The general arrangement used in the calibration work is shown 

 diagrammatically in Fig. 2. 



Gold-leaf filter-tubes were placed on each side of the sensitive 

 gas cell to act as a protection against mercury vapour. The gas 

 mixture on leaving the mercury aspirator set passed through a phos- 

 phorus pentoxide drying-tube and gold-leaf filter to the gas chamber. 

 On leaving the chamber the gas passed through a gold-leaf filter, 

 over a mercury-trap and mercury manometer to the mercury-pump 

 which was used to create a flow of gas when required. If desired, a 

 sample of the gas mixture could be pumped off and collected over 

 mercury as indicated in the figure. 



^ Shakespear. Advisory Committee for Aeronautics. A new Permeability 

 Tester for Balloon Fabrics. Feb., 1917. 



