THE PRESSURE DUE TO RADIATION. 119 



4. Radiation pressure, from its nature, must reach its maxinuim 

 value instantly, while observation has shown that gas action begins at 

 zero and increases with length of exposure, rising rapidly at first, 

 then more slowl}" to its maxinuun effect, which, in many of the cases 

 observed, was not reached until the exposure had lasted from two and 

 a half to three minutes. For large gas pressures an even longer expos- 

 ure was necessary to reach stationary conditions. The gas action ma}" 

 be thus still further reduced l)v a ballistic or semiballistic method of 

 measurement. 



In the number of the Annalen der Physik for Noveml)er, 1901, Pro- 

 fessor Lebedew'^' published the results of a more varied series of 

 measurements of radiation pressure than the early measurements of 

 the present writers. 



Professor Lebedew's'^ estimate of the accuracy of his work is such 

 as to admit of possible errors of 20 per cent in his final results. An 

 analysis of Professor Lebedew's paper and comparison with our pre- 

 liminar\' experiments seem to show that his accidental errors were 

 larger than ours, but through an undiscovered false resistance in the 

 bolometer our final results were somewhat further from the theory 

 than his. Either of the above researches would have been sufficient 

 to establish the existence of a pressure due to radiation, but neither 

 research ofi:'ered, in our judgment, a satisfactory (luantitative confirma- 

 tion of the Maxwell-Bartoli theorv. 



latp:k pressure measurements. 



Deseriptiooi of apparatus; the torsion halance. — The form of suspen- 

 sion of the torsion balance, used to measure radiation pressure in the 

 present study, is seen in fig. 1. The rotation axis (/ h was a fine rod 

 of drawn glass. A drawn-glass cross arm c, bent down at either end 

 into a small hook, was attached to the axis. The surfaces C and D, 

 which received the light ])eani, were circular microscope cover-glasses, 

 12.8 nnn. in diameter and 0.17 nun. thick, weighing- approximately 

 51 mg. each. To distinguish the two vanes from each other, in case 

 individual differences should appear in the measure.! ents, and also to 

 mark the two faces of each vane for subsequent recognition, a letter 

 C was marked on one and D on the other by diamond scratches. 

 Through each glass a hole 0.5 mm. or less in diameter was drilUnl near 

 the edge, by means of which the glasses could be hung on the hooks 

 on the cross arm c. On opposite sides of the rotation axis at <l two 

 other drawn-glass cross arms were attached. The cover-glass(>s slipped 

 easily between these, and were thus held securely in one plane. 



('V. Lebedew, Ann. der Phys., 6, 433, 1901. 

 &P. Lebedew, Ann. der Phys., 6, 4o7, 1901. 



SM 1903 9 



