690 



Theory of the apparatus. 



Of late jears mmiberless new electrometers liave been constructed 

 which, for the greater part, possess great sensibility and are to be 

 considered as modifications of two of the principles, known until 

 now; viz: that of W. Thomson's "quadranteleetrometer" and the 

 principle realised in the "Hankel-Bohnenberger" electrometer. To the 

 first belong among others the measuring instruments of: Dolezat.ek^), 

 MüLLY "), Hoffmann '), Parson *), to the second principle those of 

 LuTZ and Edelmann '), Elster and Geitel '), Wulf "). 



Besides the part of the measuring system which is charged to 

 the tension to be determined, there are also in all these electro- 

 meters two conductors, which are kept at constant potential during 

 the measurement. The electrometer described here possesses, it is 

 true, this latter quality, but yet cannot be reduced to any of the 

 princi[)les mentioned; in shape it somewhat resembles the antique 

 measui'ing instrument of Kohlrausch-Deli,man '), which also has a 

 cylindrical measuring space with two metal needles. As these needles, 

 however, are charged together to the tension to be determined and 

 in consequence repel each other, so the similarity spoken of here 

 is not mentioned with regard to the principle of. measuring, but 

 only with regard to the exterior of both instruments and the system 

 Kohlrausch-Dellmann has to be looked upon more as a realisation 

 of the simple gold-leaf principle, while torsion has been made use 

 of at the same tinie. The electrometer which concerns us here, 

 however, strives after the combination of the following conditions: 



1. Simplicity in the arrangement of the system (Cf. I, pp. 22 and 26). 



2. Great sensibility by making use of the small toi'sion of thin wires. 



3. Utilizing as much as possible the lines of force which arise 

 through addition of charge to the system for the motion on the 

 movable conductor. 



As to the third condition, in communication 1 the motives were 

 already indicated why 1 thought better to abandon entirely the 

 principle of the quadranteleetrometer") (cf. 1 p. 26); at the same 



1) F. DoLEZALEK, Ann. d. Pbys. 26, p. 312, 1908. 



s) C. MüLLY, Phys. Z. 14, p. 237, 1913. 



») G. Hoffmann, Ann. d. Phys. 52, afl. 7, p. 665, 1917. 



*) A. L. Parson. Phys. Rev. N. S. Vol VI. p. 390, 1915. 



5) G. W. LuTZ, Phys. Z. 9, p. 100, 1908. 



6) J Elster and H. Geitel, Phys. Z. 10, p. 664, 1909. 



7) Theod. Wulf, Phys. Z. 15, p. 250, 1914. 



8) Pogg. Ann. Bd. 72. 



^) The drawback of the horizontal wing-surface holds for the measurement 

 of a definite quantity of charge, of course not of fixed potentials. 



