496 PROCEEDINGS OF THE AMERICAN ACADEMY 



One Daniell cell gave a deflection with the electrometer of 1.3 centi- 

 meters. The resistance of the platinum strip when cold was .2 of an 

 ohm. It will be seen from the above results that the current varied 

 approximately from 8 to 6 webers, with an electromotive force of from 

 3.8 to 2.G volts, while the resistance varied from .47 to .44 of an ohm, 

 the resistance when cold being .2 of an ohm. The range of the in- 

 dications of the electrical instruments was comparatively small, while 

 the light varied enormously. It is evident that the chief dilliculty of 

 this method is in measuring a strong current with accuracy : for an 

 increase in the current represented by a fraction of a degree of the 

 tangent galvanometer will result in a very large increase in the light 

 from the incandescent strip. 



I next endeavored to ascertain if a thermal junction enclosed in an 

 Edison incandescent lamp, at the centre of the carbon loop, would be 

 sensitive to changes in the heat radiation of the lamp. It is evident 

 that, if this were the case, the carbon loop might be raised to the same 

 point of incandescence in successive times, assuming that the thermal 

 junction at this point of incandescence receives the same amount of 

 radiant energy. Mr. Edison kindly provided me with a lamp in which 

 one thermal junction of an alloy of iridium platinum and platinum 

 was inserted at the centre of carbon loops. The other junction was 

 placed in ice and water. The thermo-electric force of this combination, 

 however, was extremely feeble. The difiiculty of inserting wires of 

 other metals into glass prevented me from carrying this idea further. 

 Instead of the thermal junction, a small loop of extremely fine platinum 

 wire was placed at the centre of a carbon loop in an Edison lamp. 

 This fine wire constituted a bolometer strip and made one branch of a 

 "Wheatstone's bridge, it being my intention to place a similar strip in 

 another branch of the bridge, thus making a bolometer. The lamp 

 was placed in a photometer box, and its light was compared with 

 that of a candle as it was raised from a red sjlow to a li'dit of fifteen- 

 candle power. At the same time the resistance of the fine platinum 

 wire was measured by a Wheatstone's bridge. The following table 

 gives the results. 



Resistance of the Distance of Carbon Lamp Distance of Candle from 



Strip in Olims. from Piiotometcr Disk. Photometer Disk. 



14.42 70 cm. 40 cm. 



14.45 85 " 40 " 



14.55 98 " 40 " 



14.62 108 " 40 " 



