THE PRESSURE DUE TO RADIATION. 123 



The bolometer at K. (tig-. 2) was of sheet platinum, 0.001 mm. thick, 

 rolled in silver. The strip was cut out in the form shown in fig. 3 

 and mounted on a thin sheet of slate 8. Two windows had been cut 

 in the slate behind the strips at ABCD where the silver had lieen 

 removed, lea^ ing the thin platinum. The platinum surfaces were 

 blackened b}' Kurlbaum's process. The image from L5 (tig. 2) fell 

 at D. The silver ends between A and C were connected with E and 

 F, respectivel}'. On the heavy wire EF a sliding contact c served to 

 balance the bridge, all four arms of which- are shown in the figure. 



METHODS OF OBSERVATION. 



The observations leading to the results given later were of three 

 different kinds: (1) the calibration of the torsion balance; (2) the 

 measurement of the pressure of radiation in terms of the constant of 

 the balance, and (3) the measurement of the energy of the same l)eam 

 in erg-seconds h}^ the rate of temperature rise of a blackened silver 

 disk of known mass and specific heat. 



1. The determination of the constant of the torsion l)alance was 

 made by removing the vanes C and D and accurately measuring the 

 period of vibration. Its moment of inertia was easily computed 

 from the masses and distribution of the various parts aljout the axis 

 of rotation. The moment of torsion for 1 mm. defiection on a scale 

 105 cm. distant was 0.363X10 '' dyne X cm. This value divided by 

 one-half the distance between the centers of the light spots on the two 

 vanes gave the force in dynes per scale division deflection. As the 

 light spots were circles 11.25 nun. in diameter, the area of the image 

 was ver}' nearly 1 cm.'; hence the above procedure gave roughly the 

 pressure in dynes per square centimeter. 



2. In the measurements of radiation pressure it was easier to refer 

 the intensity of the beam at each exposure to sonie arbitrary standard 

 which could be kept constant than to try to hold the lamp as stead}^ 

 as would otherwise have been necessary. For this purpose, the bolom- 

 eter at R (fig. 2) was introduced, and sinudtaneous observations 

 were made of the relative intensity of the reflected beam by the deflec- 

 tion of the galvanometer Go and the pressure due to th(> transmitted 

 beam b}" the defiection of the torsion balance. The actual defi(>ction 

 of the balance was then reduced to a deflection corresponding to a gal- 

 vanometer deflection of loo scale divisions. The galvanometer sensi- 

 tiveness was carefully tested at the l)eginning and end of each even- 

 ing's work. All observations of pressure were thus reduced to the 

 pressure due to a beam of fixed intensity. 



At each series of radiation pressui-e measurements two sets of 

 observations were made. In one of these sets static conditions were 

 observed, and in the other the deflections of the balance due to short 



