( 26(^ ) 



physiology, the investigation of the action-currents of a nerve. Here 

 the galvanometer has to fiilfill conditions which in many respects 

 differ from (hose described above. Choosing as onr object the nerve 

 of a frog, from wliich the current must be led to the galvanometer, 

 we shall have to count with a great external resistance, e.g. ot 

 10^ ohms. 



Compared with this tiie resistance of the galvanometer, even that 

 of thread n^ 14 may l)e called small. The potential difference caused 

 by the action of the ner\e, and available for the current to be 

 measured, is considerably greater than tiiat which is met with in 

 the investigation of tlie human electrocardiogram, but tlie duration 

 of a nerve action current is shorter and is measured by only a few 

 thousandths of a second. 



These data show us the way in choosing a quartz thread. 



In the first place we easily perceive tiiat the differences in the 

 ol^mic resistance of the cpiartz threads can only have an insignificant 

 influence on the intensity of the action current, since the resistance 

 of the nerve itself in the circuit is preponderant. Further, the deflec- 

 tion of the quartz thread must be very quick, hence the tension 

 great ; and since an oscillating deflection must be avoided, it will be 

 desirable to adjust the tension so that the motion of the string is 

 brought to the limit of aperiodicity. But even under these circum- 

 stances the deflection is not quick enough fen* accurately i-eproducing 

 the action current of the ner\T. We must therefore apply means 

 that enable us to increase the velocity of deflection without the 

 motion becoming oscillatory. We shall have to try to increase the 

 damping, and can for this purpose apply with good result the 

 "condenser method" formerly described by us. ^) 



So we come to requirements here which are opposed to those 

 which we had repeatedly to put in the above described methods. 

 Whereas applying a vacuum had then to be considered an important 

 advantage, now increasing the damping becomes an urgent necessity. 



Under these conditions the conception of a normal sensitiveness 

 comes out to its full advantage and it may be briefly stated that of 

 a number of threads of equal ohmic resistance that with the greatest 

 normal sensitiveness is to be preferred. If the external resistance in 

 the circuit is great compared with the resistance of the galvanometer, 

 then of a number of threads with equal normal sensitiveness tliat 

 Avith the greatest ohmic resistance will have to be preferred. 



For the investigation of the action current of the nerve of a frog, 



1) See these "Proceedings" 7, p. 315. 1904. 



