NICHOLS AND HULL. — PRESSURE DUE TO RADIATION. 565 



through the slate. Copper washers were soldered to the silver strips 

 and binding posts were attached. 



The torsion balance was removed from under the bell-jar, the bolom- 

 eter was put in the place of one of the vanes and was covered by the 

 bell-jar. Connections to the bolometer were 

 made as schematically shown in Fig. 1. The 

 disc P was the exact size of the light image 

 thrown on the vanes in the pressure measure- 

 ments. The intention was to heat the disc by 

 allowing the image to fall on it, and then, 

 with the light turned off, to heat it to the 

 same temperature by sending a current through 

 it from A to B. If r be the resistance from 

 A to B in ohms, when exposed to the lamp, 

 and i be the current in amperes which gives 

 the same temperature in P as that given by 

 the absorbed radiation, then i 2 r X 10 7 will 

 be the activity of the beam in erg-seconds. 

 The temperature of the disc, whether exposed 

 to the radiation or heated by the current, was 

 shown by the resistance, to D-E, which 

 was made one arm of a Wheatstone Bridge. 

 The relation of the heating current to the 

 bridge was adjusted as follows : With the key 



iTopen, so that no current flowed through the bridge, the heating current 

 from six storage cells B 2 was turned on, and the sliding contact at F so 

 set that the bridge galvanometer zero was not changed by reversing the 

 heating current. The point F, equipotential to c, was found very near the 

 middle of the wire a b, which showed the current distribution of P to be 

 symmetrical with respect to a diameter at right angles to A B. The key 

 K was then closed making the bridge current, and the bridge was bal- 

 anced. The bolometer was next exposed to the radiation, and simultaneous 

 observations of the intensity of the beam were made on galvanometer G l 

 (Fig. 1), and the lamp galvanometer G 2 . The deflection of galvanom- 

 eter G x was reduced to standard lamp (a deflection of 100 divisions), 

 as was done in the pressure observations. After shutting off the light 

 the heating current was turned on. It was regulated by means of the 

 variable resistance R x (Fig. 1), so that nearly the same throw was ob- 

 tained from the galvanometer G l as when the bolometer was exposed to 

 the lamp. All deflections of the galvanometer G 1 were taken with the 



Figure 1. 



