510 MR. J. B. PBTAVEL ON THE HEAT DISSIPATED BY 



practical difficulties in the construction of so thin a film are considerable, and in 

 designing a new instrument it would be advisable either to reduce the size of the 

 filuTor to increase its thickness. The very thin platinum, obtained in the way we 

 have just described, though capable of withstanding a large amount of vibration, 

 cannot resist the slightest mechanical strain, a touch with a single hair being sufficient 

 to break it. Great care is needed in the manipulation of the film from the moment 

 the silver is dissolved off' it until it is screwed up in position in the instrument. 

 During this time it has to receive an electrolytically deposited coating of platinum 

 black, be washed several times, and carefully dried. When once in position in the 

 instrument there is, however, comparatively little chance of an accident. 



Fig. 8. 



Section a.b. 



The desiderata of a good bolometer may be enumerated as follows : 



1. Great sensitiveness. 



2. Constancy of zero. 



3. Rapidity of action. 



The first and third conditions are fulfilled, thanks to the extreme thinness of the 

 platinum and to its high temperature coefficient. The sensitiveness was found to be 

 increased by coating the films on one side only with platinum black, and by placing 

 behind them a nickel mirror. With regard to rapidity of action it was found in the 

 course of some subsequent experiments that the platinum rises in half a second to 

 over 90 per cent, of its maximum temperature. 



The second condition, namely the constancy of the zero point, is the most difficult 

 to fulfil. It has already been pointed out that the four arms of the bridge must be 

 in pairs as nearly as possible identical, not only with regard to resistance, but also 

 with regard to shape, size, position, and temperature coefficient. But these pre- 



