NICHOLS AND HULL. — PRESSURE DUE TO RADIATION. 567 



trodes of radii r are on a diameter of this circular sheet of radius R, 

 theu the resistance can be shown to be 





loge 2r^ 



Assuming for the moment that the leading-in strips of the bolometer 

 (Fig. 1) were of great conductivity compared to that of the thin platinum 

 sheet and that they terminated in circular arcs orthogonal to this circular 

 sheet, the resistance would be 0.922 X cr, giving to r the value of 2.79 

 mms. and to R the value 11.25 mms. But the leading-in strips ter- 

 minated on the boundary of the large circle. The resistance was there- 

 fore altered by two facts, — the lines of flow were changed and the 

 distance between the electrodes was increased. The latter is the impor- 

 tant item. It is necessary therefore to find approximately the resistance 

 of these gibbous portions of the large disc previously considered as 

 electrodes. This may be done by estimating the area of these parts and 

 by considering the average equipotential line as midway between the 

 chord and arc of the cylindrical electrode. It results that the amount to 

 be added on account of this calculation is 0.471 X cr. Hence the resist- 

 ance between the electrodes is now (0.922 + 0.471) cr = 1.393 X a. 

 The value of o- as found by the fall of potential method was 0.148 at 

 19° C. When corrected for the temperature of the disc exposed to the 

 lamp, cr becomes 0.1 GO. Hence the resistance of the disc when hot was 

 1.393 X 0.160 = 0.221 ohm.* Substituting this computed value of the 

 resistance in place of the one used, the energy of the standard beam 

 becomes 0.221 X 0.75 X 10'' ergs-seconds and 



1.92 X 0.221 X 0.75 x 10' , ^^ ,^. , 



» = -^^ — 1.05 x 10"^ dynes. 



^ 3 X lO^o ^ 



This result is in accidental agreement with the observed pressure. If 

 necessary corrections, determined by later experiment, had been applied, 

 the difference between the observed pressure and the pressure computed 

 from the energy measurements would have been about three per cent. 

 Moreover the probable error of the final result was roughly double this 

 amount. 



In the November number of the Annalen der Physik for 1901 Pro- 

 fessor Lebcdew f pul)lished the results of a more varied series of 



* Tlie resistance of a triiil disc was measured experimentally with tlie result 

 that the experimental value differed from the theoretical by ahout one per cent, 

 t P. Lebedcw, Ann. I'hys., VI. 4i;;J (IWl). 



