Chap. 12] 



MISCELLANEOUS GEOPHYSICAL METHODS 



893 



the heated platinum filament and are burned with the oxygen of the air, 

 thereby raising the temperature of the platinum filament which in turn 

 increases its resistance. To attain sufficient sensitivity, the measurements 

 must be made with the gas mixture at rest, and the current must be turned 

 on after the gas has been introduced into the chamber Hence, a ballistic 

 galvanometer is best suited for the purpose. The instrument is calibrated 

 by a mixture of air and methane and is said to be sensitive to one part in 

 10 million. However, at the smaller dilutions the sensitivity drops rapidly. 

 For instance, in one of the original Laubmeyer instruments, the deflection 

 was 2.8 scale divisions for a dilution of 1 : 10^ and 2.0 scale divisions for a 



American Askania Corp. 



Fig. 12-10. Graf-Askania double-chamber combustible gas detector. 



dilution of 1:10 . The measurements are made by comparing the reaction 

 of the galvanometer in an empty combustion chamber with that in one 

 filled with the gas-air mixture. Difficulties arise from the fact that the 

 voltage of the battery supplying the filament current cannot be kept suffi- 

 ciently constant. 



This is avoided in the double-chamber instrument^^ shown in Fig. 12-10 

 in which the two combustion chambers are arranged in opposite arms of a 

 bridge circuit, much in the same manner as in the regular double-filament 

 combustible gas detector (see Fig. 11-35). The zero instrument is a Zeiss 

 loop galvanometer which may be used also for photographic recording. 

 The bridge is first balanced for air in both chambers, and the bridge 



« A. Graf, Oel und Kohle, 11(36), 644-648 (Sept. 22, 1935). 



