A direct current bridge with vacuum tube detector is described for measuring the resistance of living cells. Methods for evaluating the surface of contact with the protoplasm, and the leakage around the cell wall, allow us to calculate the effective resistance of the protoplasm. In Valonia ventricosa this is usually at least 10,000 ohms per square centimeter and is often much higher. This is in agreement with the very slight ionic interchange observed in normal Valonia.

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