OF ELECTRO-DYNAMIC FORCES. 507 



The two instruments, i. e. the galvanometer and the dynamo- 

 meter, are complementary to each other, so that when the same 

 momentary current is transmitted through both, and the deflec- 

 tion of both instruments thus produced is observed, both the 

 duration and the intensity of the momentary current can be de- 

 termined from these two observations. This reciprocity is based 

 upon the circumstance that the observed deflection of both in- 

 struments depends in the same manner upon the duration of the 

 momentary current, i. e. it is proportional to it, whilst it is not 

 dependent in the same manner upon the intensity of the current, 

 because the deflection of the galvanometer is in proportion to 

 the intensity of the current. 



Let s and j indicate the duration of the oscillations of the gal- 

 vanometer and dynamometer ; 



e' and e' the deflection at which both instruments remain when 

 the same constant current of the intensity i'is transmitted through 

 them ; 



Whilst e and e indicate the extent of the deflection which both 

 instruments attain in consequence of a momentary current of the 

 duration and of the intensity i ; the following equation then 

 gives the duration © : — 



„_1 ss s' e e 

 TT f e' e s 



and the following that of the intensity of the current i : — 



■ _ 5_ ^ i' J. 

 s ' s' ' ' e' 



s, c, e', e', i', e and s in these formulae are magnitudes which 

 can be determined by observation. 



This combination of the dynamometer with the galvanometer 

 is of special importance in physiology, to investigate accurately 

 the excitation of the nerves by galvanic currents. For it is 

 found that nerves of sensation especially are quickly deadened 

 by continued currents, and hence that for such experiments 

 momentary currents are frequently required to be used. But 

 the observed impressions of sense depend less upon the dura- 

 tion of the current than upon its intensity ; and it is essential 

 to be acquainted with both. 



2 M 2 



