TABLE VI.--SHOWING EFFECTS OF UP-HILL AND DOWN-HILL CURRENTS OF FEEBLE INTENSITY ON PERIOD OF TRANSMISSION THROUGH 15 MM. +-------+-------------+---------------------+---------------------+ |Number.|Intensity of | Period for | Period for | | | current in | up-hill | down-hill | | |microampères.| transmission. | transmission. | +-------+-------------+---------------------+---------------------+ | 1 | 1.4 |14 tenths of a second|16 tenths of a second| | 2 | 1.4 |13 " " |15 " " | | 3 | 1.6 |19 " " |Arrest. | | 4 | 1.7 |12 " " |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 intensity of transmitted excitation will also be enhanced; the minimal response will tend to become maximal. Or excitation which had hitherto been ineffectively transmitted will now become effectively transmitted. Conversely, depression of conductivity will result in a diminution or abolition of response. We may use a single break-shock of sufficient intensity as the test stimulus. It is, however, better to employ the additive effect of a definite number of feeble make-and-break shocks. We may again employ additive effect of a definite number of induction shocks, the alternating elements of which are exactly equal and opposite. This is secured by causing rapid reversals of the primary current by means of a rotating commutator. The successive induction shocks of the secondary coil can thus be rendered exactly equal and opposite. _Experiment 42._--Working in this way, it is found that the transmitted excitation against the direction of current becomes effective or enhanced under ‘up-hill’ current. A current, flowing with the direction of transmission, on the other hand, diminishes the intensity of transmitted excitation or blocks it altogether. Henceforth it would be convenient to distinguish currents in the two directions: the current in the direction of transmission will be distinguished as _Homodromous_, and against the direction of transmission as _Heterodromous_. AFTER-EFFECTS OF HOMODROMOUS AND HETERODROMOUS CURRENTS.