130 J. Trowbridge — A Standard of Light. 



l-3 cm . 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 elec- 

 tromotive force of from 3'8 to 2'6 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 indications of the elec- 

 trical instruments was comparatively small, while the light 

 varied enormously. It is evident that the chief difficulty 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 center 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 succes- 

 sive 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 center of carbon loops. The other junction 

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

 combination, however, was extremely feeble. The difficulty 

 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 

 center of a carbon loop in an Edison lamp. This fine wire 

 constituted a bolometer strip and made one branch of a Wheat- 

 stone'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 com- 

 pared with that of a candle as it was raised from a red glow to 

 a light of fifteen-candle power. At the same time the resist- 

 ance of the fine platinum wire was measured by a Wheatstone's 

 bridge. The following table gives the results : 



Resistance of the 

 Strip in Ohms. 



14-42 



14-45 



14-55 



14-62 



This method seems to be quite sensitive. The change in 

 resistance is large when estimated by the number of ohms nec- 

 essary to restore a balance to the bridge. It was noticed that 

 at a certain point a comparatively small increase in heat radia- 

 tions was accompanied by a large change in the amount of 



Distance of Carbon Lamp 

 from Photometer Disk. 



Distance of Candle from 

 Photometer Disk. 



70 cm 



4 cm 



85 



40 



98 



40 



108 



40 



