566 PROCEEDINGS OF THE AMERICAN ACADEMY. 



bridge current both direct and reversed to eliminate any local disturb- 

 ances in the bridge and also with the heating current both direct and 

 reversed. A Siemens & Halske direct-reading precision milliampere- 

 meter was used to measure this current. From repeated observations 

 the current which gave the same heating effect as the light beam was 

 i = 0.865 amp. The resistance between the binding posts A and B was 

 measured with the lamp on and gave r = 0.278 ohm. The intensity of 

 the beam in erg-seconds was thus: r i" 1 X 10 7 , or 0.278 X 0.75 X 10 7 . 



Using 0.92 as the reflection coefficient of silver, the pressure computed 

 from the energy measurements was p = 1.34 X 10 - * dynes. The 

 observed pressure was only 78 per cent of this value. No correction for 

 the diffuse reflection of the blackened bolometer face nor for the differ- 

 ence in reflecting power between the two faces of the silver coating, 

 discovered later, was made. The two corrections, however, nearly bal- 

 ance, so no considerable change in the result would have been effected 

 by using them. 



It was later discovered that, in dissolving the silver from the platinum 

 when the bolometer was made, the acid had eaten away the silver from 

 the strips A and B for a distance of nearly a millimeter under the 

 asphalt. The resistance 0.278 ohm given for the disc was thus too high. 

 It was impossible to redetermine the resistance by the direct method 

 because of an accident to the bolometer by which the disc was nearly 

 severed from the strip B. The disc was therefore carefully torn away 

 from its supports, mounted on a glass plate and cut on a dividing engine 

 into strips, 1 and 2 mms. wide, parallel to A B. The resistance along 

 these strips was measured by the fall of potential method. The resist- 

 ance was found to vary slightly in different parts of the disc due to lack 

 of uniformity in the thickness of the metal. After many measurements, 

 an average value was reached and the resistance of the disc computed 

 theoretically as follows : — 



The resistance of a conducting sheet of infinite extent, when the 

 current enters and leaves the sheet by electrodes * of relatively great 



conductivity, is - — ~ , where o- is the resistance of any square of the 



4 7T O 



sheet, and C is the electrostatic capacity of the two electrodes. If the 

 electrodes are cylinders, the lines of flow are circles orthogonal to them. 

 When the sheet, in place of being infinite, is bounded by one of these 



circular lines of flow, the resistance is - — -^. In particular, if the elec- 



2 tv C 



* J. J. Thomson, Electricity and Magnetism, 2d Edition, p. 314. Cambridge, 1897. 



