THE SMITHSONIAN INSTITUTION. 339 



tro-motive force. The correctness of this assumption lias been tacitly 

 received by most physicists, although a direct experimental confirma- 

 tion had not been attempted on account of the imperfection of the appa- 

 ratus. Kohlrausch has at length supplied this omission. He con- 

 verted the exceedingly sensitive electrometer of Dellman into a meas- 

 uring instrument of great accuracy. By combining this instrument 

 with a condenser (Pog. Ann. LXXV, 88) he succeeded in determining 

 the electroscopic tension at the poles of an open, simple battery, with, 

 such, exactness that there can be no longer any doubt of the correct- 

 ness of the above-mentioned principle. 



Kohlrausch has, at the same time, proved by this investigation 

 that Dellman's electroscope, as it comes from his hands,, is adapted 

 to the most delicate electrical researches. For a more detailed de- 

 scription of the instrument and its use, we refer the reader to 

 the excellent treatise already cited. The comparison of the electro- 

 motive force with the tension of an open battery may be found in a 

 third memoir in volume LXXV of Poggendorff's Annalen, page 

 220. To render the results of this investigation comprehensible, we 

 must first give the modus operandi more fully by which the values of 

 the electroscopic tension can be derived from, the measurements ma^di^ 

 by the instrument. 



Kohlrausch's electrometer can be used as a measurinip^ instrument 

 in two ways, namely: 



1. By placing the upper divided circle, which we shall term the tor- 

 sion circle, at 90°, the movable needle will form an angle of 90° with 

 the fixed metal strip. The needle and strip are now brought into 

 communication, the electricity to be measured communicated to them, 

 and then the connexion between needle and strip broken. The tor- 

 sion circle being now turned back to 0, the needle will form an arc 

 with the strip as much greater as the electrical charge is stronger. 



The electrical charge which produces a deflection of 10° being 

 denoted by 1, the strength of the electrical charge belonging to each 

 angle of deflection can be determined. For the details of this compu- 

 t<ation I refer the reader to Kohlrausch's memoir in volume LXXII 

 of Pogg's Ann. On page 385 he gives a table, indicating the corres- 

 ponding electrical tension hr each angle of deflection, which holds 

 good, of course, only for his own instrument. For clearer compre- 

 hension of the matter we will present an extract from this table : 



