806 



SCIENCE 



[N. S. Vol. XLII. No. 1092 



writer found that copper-oxide (Cu„0, presum- 

 ably) shows the effect quite unmistakably. 



Without going into details here as to the 

 mode of production of copper-oxide cells or 

 bridges, it may be stated that copper-oxide has 

 a much lower speciiic resistance than either 

 selenium or stibnite and is much the more 

 transparent toward red light (layers having a 

 thickness of more than 1 mm. are stiU slightly 

 translucent). The fundamental facts which 

 have been established for this new light-sensi- 

 tive substance are : 



1. The conduction is electronic and not 

 electrolytic. 



2. The increase in conductivity, occasioned 

 by light, is distinctly different from that pro- 

 duced by a heating effect. 



3. The conductivity increases with the ap- 

 plied voltage, i. e.. Ohm's law is not obeyed 

 (voltage effect). 



4. The region of increased conductivity 

 spreads slightly to portions of the material not 

 illuminated (transmitted effect). 



5. The region of highest sensibility lies in 

 the ultra-violet near A =■ 2800 A. F. 



6. Cooling in liquid air increases the per- 

 centage change in conductivity and displaces 

 the sensibility maximum in the red toward 

 shorter wave-lengths. 



Y. The relation between the radiant energy 

 absorbed {E) and the resultant change in con- 

 ductivity (C) is very approximately of the 

 form G^KE§ where Z is a constant and /? 

 lies near 0.5. 



While the percentage change in conductivity 

 upon illiunination is much less than that of 

 selenium and stibnite, the comparatively high 

 conductivity of copper-oxide makes the abso- 

 lute increase quite large. The best cell which 

 the writer has thus far constructed has a re- 

 sistance of 15,200 ohms at 17° C. for 1 volt. 

 The change in conductivity occasioned by the 

 light from a 40-watt tungsten lamp at 20 cm. 

 is about 15 per cent. The area exposed to 

 radiation is about 12 mm.^ If this cell be con- 

 nected to a 2-volt cell and a galvanometer 

 (forming part of a simple potentiometer) a 

 sensitive device for detecting radiant energy is 

 produced. Exposing the cell to daylight in a 



moderately lighted room throws the galva- 

 nometer spot of light violently off the scale. 

 Monochromatic radiations which are quite too 

 feeble to affect a sensitive radio-micrometer, 

 bring about large deflections when allowed to 

 fall on the copper-oxide cell. If the cell be 

 connected to a telephone receiver and battery 

 and if an intermittent light beam of definite 

 frequency be allowed to fall on the cell, a clear, 

 musical note is heard. 



The preceding discussion is to be looked 

 upon as being of a preliminary nature. A sys- 

 tematic search for light-sensibUity is being 

 undertaken and a complete account of the work 

 will appear later. 



A. H. Pfund 



Johns Hopkins TJniversitt, 

 November, 1915 



EADIOACTmTY OP UNDERGROUND WATERS IN 

 PROVIDENCE AND THE VICINITY 



Some idea of the distribution of radium 

 salts near the surface of the earth may be ob- 

 tained from a study of the relative amounts of 

 radimn emanation dissolved in underground 

 waters. Within the last ten years a number of 

 the better known springs and wells in Amer- 

 ica, Europe and Japan have been examined for 

 emanation content. Some of the activities 

 obtained have been tabulated by Schlunt and 

 Moore^ also by Dole.^ 



Fig. 1. 



1 U. S. Geological Survey Bulletin 395, 1909. 

 ^ U. S. Geological Survey, "Mineral Eesources 

 of the U. S., 1913," Part II., pp. 393-440. 



