118 THE PRESSURE DUE TO RADIATION. 



The writers'* presented the results they had obtained by measure- 

 ments of radiation pressure at eight different gas pressures, in a pre- 

 liminary communication to the American Physical Society, meeting 

 with Section B of the American Association at Denver, August 29, 

 1901. The main arguments underlying the method of measurement 

 of the radiation pressure ma}^ here be given. 



In the experiments of earlier investigators every approach to the 

 experimental solution of the problem of radiation pressure had been 

 balked by the disturbing action of the gases which it is impossible to 

 remove entirely from the space surrounding the body upon which the 

 radiation falls. The forces of attraction or repulsion, due to the 

 action of gas molecules, are functions, first, of the temperature differ- 

 ence between the body and its surroundings, caused by the absorption 

 by the body of a portion of the rays which fall upon it; and, second, 

 of the pressure of the gas surrounding the illuminated body. In the 

 particular form of apparatus used in the present study, the latter 

 function appears very complicated, and certain peculiarities of the gas 

 action remain inexplicable upon the basis of any simple group of 

 assumptions which the writers have so far been able to make. 



Since we can neither do away entirely with the gas nor calculate its 

 effect under varying conditions, the only hopeful approach which 

 remains is to devise apparatus and methods of observ^ation Avliich will 

 reduce the errors due to gas action to a minimum. The following 

 considerations led to a method by which the elimination of the gas 

 action was practically accomplished in the present experiments: 



1. The surfaces which receive the radiation, the pressure of which 

 is to be measured, should be as perfect reflectors as possible. This will 

 reduce the gas action by making the rise of temperature due to absorp- 

 tion small, while the radiation pressure will l)e increased; the theory 

 recjuiring that a beam totally reflected shall exert twice the pressure 

 of an equal beam completel}^ absorbed. 



2. By studying the action of a beam of constant intensity upon the 

 same surface surrounded by air at different pressures certain pressures 

 may be found where the gas action is less than at others. 



3. The apparatus — some sort of torsion balance — should carry two 

 surfaces symmetrically placed with reference to the rotation axis, and 

 the surfaces of the two arms should be as nearly equal as possible in 

 ever}^ respect. The surfaces or vanes should be so constructed that if 

 the forces due to gas action (whether suction or pressure on the warmer 

 surface) and radiation pressure have the same sign in one case, a reversal 

 of the suspension should reverse the gas action and bring the two forces 

 into opposition. In this way a mean of the forces on the two faces of 

 the suspension should be, in part at least, free from gas action. 



«E. F. Nichols and G. F. Hull, Science, 14, 588 (October 18, 1901); Phys. Rev., 13, 

 293 (November, 1901); Astrophysical Journal, 15, p. 62 (January, 1902). 



