Currents on Transmission of Excitation. 



495 



5. Phenomenon of Reversal under Moderate Intensity of Current. 



In studying the effect of increasing intensity of polarising current, the 

 concomitant variations of conductivity appeared at first sight to be very 

 puzzling. The results obtained at different stages were, however, very 

 definite. In the first stage with very feeble current there was no per- 

 ceptible change of conductivity. At the second stage with increasing 

 current the conductivity variation increased at a rapid rate, and soon 

 attained a maximum. After this, at the third stage, the conductivity 

 change underwent a decline, and then abolition. The effect outwardly 

 resembled that induced at the first stage. There was, however, a difference, 

 for a critical point had now been reached beyond which there was a 

 complete reversal of normal conductivity variation. These different effects 

 will be clearly understood from the following tabular statement : — 



Table III. — Conductivity Variations at Different Stages. 





Heterodromous current. 



Homodromous current. 



I stage 



II „ 



ni „ :. 



IV „ 



Enhanced conductivity 



Conductivity change reduced to 

 zero at critical point 



Reversal : diminution of conduc- 

 tion culminating in a block 



No perceptible change. 



Depression of conductivity culmi- 

 nating in a block. 



Conductivity change reduced to 

 zero at critical point. 



Enhancement of conduction. 



I shall now describe a typical experiment which will clearly demonstrate 

 the phenomenon of reversal. 



Experiment 4. — In this I was desirous of obtaining with an identical 

 specimen alternate records showing (1) normal effect under feeble current, 

 (2) reversed effect under moderate current, and (3) normal effect once more 

 under the original feeble current. It took two hours to obtain these six 

 records (fig. 5) at intervals of 20 minutes. The specimen was vigorous, 

 and therefore was little affected by fatigue. The normal enhancement of 

 conductivity under heterodromous current was observed at as low a current- 

 intensity as 1 microampere. In the first of the pair of records (1) we 

 find the interval between stimulus and response under heterodromous 

 current to be 18 tenths of a second ; this was prolonged to 21 tenths under 

 homodromous current ; the current was next raised to 3 microamperes, 

 and we observe in the pair of records (2) the reversal of normal effect by a 

 block of conduction under heterodromous, and transmission under homo- 

 dromous current; the time-interval in the latter case is 20 tenths of a 



