On Normal Elements for Electrometric Measurements, 173 



lime-light extends much further. But it might be possible 

 to establish a sort of spectroscopic scale of temperatures if 

 the lines which are successively developed as the temperature 

 rises were carefully noted. Thus the appearance of the iron 

 line T seems to synchronize with temperature of about 3000°. 

 The lithium blue line is invisible in the flame of an ordinary 

 Bunsen burner, but is just visible at the temperature of the 

 inner green cone formed by reducing the proportion of gas 

 to air in such a burner, while in the exploding gas the green 

 line too is seen. It seems to need a temperature above 3000° 

 to get the aluminium lines at H. Probably no line is ever 

 abruptly brought out at a particular temperature — it will 

 always be gradually developed as the temperature rises; yet 

 the development may be rapid enough to give an indication 

 which may be useful in default of means of more exact 

 measurement. In former papers treating of spectroscopic 

 problems (Proc. Roy. Soc. vol. xxxiv. p. 130, and xxix. p. 489) 

 we have more than once adverted to the necessity of the study 

 of the spectra both of flames and of the electric discharge 

 under modified conditions of pressure. The projected expe- 

 riments on the arc in lime-crucibles have not yet been carried 

 out; but the present is a first instalment of a study of flame- 

 spectra under such conditions. 



XXIII. On Normal Elements for Electrometric Measurements. 

 By H. W. Beetz*. 



IN order to measure a difference of potential in absolute 

 units a normal element is required, having a perfectly 

 definite electromotive force, which can either be put together 

 once for all, or can always be set up in exactly the same 

 way and without too much trouble. According to Kittler's 

 experiments'!", a Daniell element, having a porous cell, does 

 not at all fulfil these conditions, since its electromotive 

 force is subject to continuous change. On the other hand, 

 an element composed of chemically pure zinc and pure 

 copper, in dilute sulphuric acid and copper-sulphate solution 

 of definite strength, the solutions being placed in connexion 

 by means of a siphon-tube terminating in small openings and 

 filled with the same sulphuric acid, has always the same elec- 

 tromotive force, and since it can easily be put together, forms 



* Translated from Wiedemann's Annalen, xxii. p. 462. 

 t Sitzungsber. d. k. layer. Akad. d. Wiss. p. 467 (1882) ; Wied. Ann, 

 xvii. p. 865 (1882). 



