JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. 9 NOVEMBER 4, 19 19 No. 18 



PHYSICS. — The spectral photoelectric sensitivity of molybdenite 

 as a function of the applied voltage. W. W. CoblENTZ 

 and H. KahlER, Bureau of Standards. 



In a previous investigation it was observed that the spectral 

 photoelectric sensitivity of molybdenite is confined practically 

 within three spectral bands, the maxima of which are separated 

 by equal intervals, when plotted in terms of frequency instead 

 of wave lengths. 



The present investigation was undertaken in order to test the 

 validity of this frequency relation, using for the purpose a quartz 

 prism which gave twice the dispersion formerly obtained with a 

 fluorite prism. The sample of molybdenite was soldered to 

 copper wire terminals, and operated in an evacuated chamber 

 as in previous work. The photoelectric substance, the dry 

 battery and the d'Arsonval galvanometer were connected in 

 series. The deflection caused by the small dark current was 

 overcome by rotating th^ suspension head of the galvanometer. 



The results obtained with this apparatus appear to verify 

 the previous observations, indicating that the frequency max- 

 ima of photoelectrical sensitivity are separated by equal intervals, 

 which decrease with temperature; the arbitrary wave-number 

 being w = 40 at 25 °C. and w = 30 at — i75°C. 



A new voltage phenomenon. — A photo-negative action (re- 

 sistance increase) has been observed in certain samples of sele- 

 nium,^ when exposed to the total radiation from an incandescent 



1 RiES, Phys. Zeits. 9: 569. 1908. Brown, Phys. Rev. 33: i. 191 1. 



537 



