NICHOLS. — THE VISIBLE RADIATION FROM CARBON. 



91 



had been constructed for the purpose of securing uniform temperature 

 for the standard clock of the physical laboratory, and other similar 

 apparatus. The range of temperature in this room fluctuated through- 

 out the entire investigation be- 

 tween 18°C. and 19°C. The 

 range was so small and the 

 variations occurred so gradu- 

 ally that no changes of electro- 

 motive force of a size which 

 it was necessary to consider in 

 these measurements could have 

 arisen other than those included 

 in the usual correction for 

 temperature. 



The two cells were compared 

 with each other from time to 

 time by setting them in opposi- 

 tion to one another in circuit 

 with a sensitive galvanometer 

 and noting the deflection pro- 

 duced. It was found that 

 although one of them was sup- 

 plying current to the 100,000 

 ohm circuit of the potentiome- 

 ter, during the times when it 

 was necessary to close the key 

 of that circuit, the difference 

 of electromotive force between 

 the used and unused cell was 

 always very small, never more 

 than a few hundred thousandths 

 of a volt. At the end of the 

 entire set of measurements, the 

 difference was 0.00006 volts. 

 The absolute electromotive 



force of these cells was checked by comparison with Clark cells of the 

 II form and of the test-tube form, constructed in this department in 1898. 

 As a result of these comparisons it was found that the electromotive 

 force of the cell used in the potentiometer might be taken at 1.430 volts 

 at 18.° 



6 mm 



4 mm 



Figure 11. 



