ROSA AND VINAL: THE SILVER VOLTAMETER 451 



comparison with white lead, what is gained in higher emissivity 

 in the infra-red in the zinc oxide paint, is lost by its higher absorp- 

 tion (lower reflecting power) in the visible spectrum, so that 

 radiometrically it is no more efficient than white lead. 



The white lead paint was removed from the sheets, A-II and 

 B-II, which were then painted with a matte layer of the lamp- 

 black used in the experiments already described. When exposed 

 directly to the sun the temperature of the lampblack sheet A-II 

 was about 16° higher than the zinc oxide paint, A-I; the actual 

 temperature of the lampblack being about 52? 2 for solar radi- 

 ation of intensity, Q = 1.16. Similarly the temperature of the 

 lampblack sheet, B-II was 17?5 higher than the zinc oxide plate 

 B-I, the actual temperature of the aluminum sheet covered 

 with lampblack paint being about 53? 3 C. and the room tem- 

 perature being about 23? 1 C. The infra-red reflecting at 8.8/* 

 differs but little for these two substances, -hence, there is no great 

 difference in their emissivities for low temperatures. In the vis- 

 ible spectrum the absorptivity of the zinc oxide is only about 

 30 per cent and for lampblack it is 97 per cent. Hence, the 

 lampblack must become the hotter; for it absorbs energy at 

 three times the rate, and it emits energy (low temperature radi- 

 ation) ?l\ practically the same rate, as does the zinc oxide paint. 



ELECTROCHEMISTRY.— The silver voltameter. I. E. B. Rosa 

 and G. W. Vinal, Bureau of Standards. To appear in the 

 Bulletin of the Bureau of Standards. 



The earliest use of electrochemical decomposition as a means 

 for the measurement of electricity appears to have been by Gay 

 Lussac and Thenard 1 about 1811, but it remained for Faraday 

 to enunciate the conditions on which it may be used for the exact 

 measurement of current. He declared his gas voltameter to 

 be the "only actual measurer of voltaic electricity which we at 

 present (1833) possess." Because of this, he named it a "volta- 

 electrometer." The earliest use of the silver voltameter of which 

 we are aware was by Poggendorff in 1847. Since then, about 



1 These and other references will be given more in detail in the full paper 

 and also the places where they may be found. 



