376 RECENT PROGRESS IN PHYSICS. 



the number of degrees the torsion was diminished after the second 

 measurement ; also by t and t' the two intervals of time, then the den- 

 sity of the electricity on ball II, at the moment the torsion a was mea- 

 sured, is equal to 



. Suppose, for example — 



Z*=369° i' = 2'.5 



c=20°; 



then, at the instant in which the density a was measured, the electri- 

 cal density on the other proof ball was 



369 H- 20 X 1^ X ^ = 369 -f- 31°.3 = 400.3; 



and the required ratio of the two densities 



67 



400 



,, = 0.167. 



Biess computed the results by this method of observing "with pairs 

 of proof bodies," not according to formula (1), which is only an ap- 

 proximation, but according to another approximate formula, the deri- 

 vation of which, however, cannot be termed elementary. The results, 

 however, of the computation, according to formula (1), correspond 

 so closely with those obtained by Reiss, that there need be no hesita- 

 tion in using it. 



The results of the first of the above two examples corresponded per- 

 fectly when computed according to both formulas ; in the second 

 example, the value found, according to Biess, was 400°. 8, while we 

 have made it 400°. 3, a difference which has hardly any effect upon 

 the required quotient. 



§ 14. Electroscopes to which the principle of the torsion balance 

 IS applied. In the 53d volume of Poggendorf s Annalen is a descrip- 

 tion of two electrometers, or rather electroscopes, which may be con- 

 sidered as small torsion balances ; the first by Belbnan, (page 606,) 

 the second by Oersted, (page 612.) 



Fig. 12. Delhnan's instrument is represented in fig. 12. The 



mouth of a white preserve jar, 8 or 10 inches high^ is 

 closed with a piece of cork, B. Through this cork is 

 passed a tolerably stout wire, C, with a hook at its lower 

 end, in which is hung a thread of untwisted silk. The 

 thread carries a small rod of shellac, D, with a little 

 ball of elder pith, a, fastened at one end. (Pressure 

 with clean fingers readily removes all angles from the 

 pith.) 



The glass is pierced at a, and a pin, ^ y^ is fastened in the hole by 

 shellac, with the head, j, outside; on the inside a pith ball, /?, is stuck 

 uj^on the pin, the point of whick must not go through the ball. The 

 wire, C, is drawn up until a and /? are on a level ; the wire is then 



