That is to say, the passage of a feeble current modifies conductivity for excitation in a selective manner. Conductivity is enhanced against, and diminished with, the direction of the current.
The minimum current which induces a perceptible change of conductivity varies somewhat in different specimens. The average value of this minimal current in autumn is 1.4 microampères. The effect of even a feebler current may be detected by employing a test stimulus which is barely effective.
TABLE VI.—SHOWING EFFECTS OF UP-HILL AND DOWN-HILL CURRENTS OF FEEBLE INTENSITY ON PERIOD OF TRANSMISSION THROUGH 15 MM.
| Number. | Intensity of current in microampères. | Period for up-hill transmission. | Period for down-hill transmission. |
| 1 | 1.4 | 14 tenths of a second | 16 tenths of a second |
| 2 | 1.4 | 13 tenths„ of a second„ | 15 tenths„ of a second„ |
| 3 | 1.6 | 19 tenths„ of a second„ | Arrest. |
| 4 | 1.7 | 12 tenths„ of a second„ | 14 tenths of a second. |
Having demonstrated the effect of direction of current on the velocity of transmission, I shall next describe other methods by which induced variations of conductivity may be exhibited.
DETERMINATION OF VARIATION OF CONDUCTIVITY BY METHOD OF MINIMAL STIMULUS AND RESPONSE.
In this method we employ a minimal stimulus, the transmitted effect of which under normal conditions gives rise to a feeble response. If the passage of a current in a given direction enhances conductivity, then the
