1. The flow of current through a small metal electrode, such as is commonly used in stimulation experiments, has been studied. The wave-form resulting from rectangular voltage pulses has been examined with various electrodes, preparation impedances and analogue networks.

2. Variation of pulse repetition rate, when all other parameters are constant, causes wide variation of current flow and lesser variation of wave-form.

3. An attempt has been made to relate these changes to physical changes occurring at the electrode tip during polarization of the electrodes. Calculations indicate that the amount of gas generated by the currents commonly used in stimulation experiments would produce gross distortion of the tissues in the absence of diffusion, and may do so in practice when large currents are used.

4. It is concluded that changes of threshold with changes of pulse repetition rate are as likely to be due to the physical effects described here as to the physiological characteristics of the stimulated nervous system, to which they have sometimes been ascribed.

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