NICHOLS AND HULL. — PRESSURE DUE TO RADIATION. 573 



ment of the pressure of radiation in terms of the constant of the balance; 

 and (3) the measurement of the energy of the same beam in erg-seconds 

 by the rate of temperature rise of a blackened silver disc, of known mass 

 and specific heat. 



1. The determination of the constant of the torsion balance 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 about the axis of rotation. The 

 moment of torsion for 1 mm. deflection on a scale 105 cm. distant was 

 0.363 X 10 -6 dyne X cm. This value divided by one-half the distance 

 between the centres 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 mm. in diameter the area of the image was very nearly 1 (cm.) 2 , 

 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 some arbitrary standard 

 which could be kept constant than to try to hold the lamp as steady as 

 would otherwise have been necessary. For this purpose, the bolometer 

 at R (Fig. 3) was introduced, and simultaneous observations were made 

 of the relative intensity of the reflected beam by the deflection of the 

 galvanometer G 2 , and the pressure due to the transmitted beam by the 

 deflection of the torsion balance. The actual deflection of the balance 

 was then reduced to a deflection corresponding to a galvanometer deflec- 

 tion of 100 scale divisions. The galvanometer sensitiveness was carefully 

 tested at the beginning and end of each evening's work. All observa- 

 tions of pressure were thus reduced to the pressure due to a beam of 

 fixed intensity. 



At each series of radiation pressure measurements, two sets of observa- 

 tions were made. In one of these sets, static conditions were observed, 

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

 were measured. In the static observations, each vane of the balance 

 was exposed in turn to the beam from the lamp, the exposures lasting 

 until the turning points of the swings showed that stationary conditions 

 had been reached. The moment of pressure of radiation and gas action 

 combined would thus be equal to the product of the static deflection 

 and the constant of the balance. The torsion system was then turned 

 through 180° by rotating the outside magnet, and similar observations 

 were made on the reverse side of the vanes. All turning points of the 

 swinging balance in these observations were recorded. From the data 



