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A capacitance-resistance bridge for the measurement of the dielectric properties of human blood at 100 kc is described. The use of inductively coupled ratio arms in the bridge greatly reduces the effect of stray impedances to ground. This allows the glass measuring cell to be mounted in a transparent plastic thermostatting box, without metallic shielding. Thus, the physical condition of the blood can be observed throughout the period of coagulation. Observed changes in the capacitance and resistance of blood sample during coagulation are briefly outlined, particularly the effects observed when the blood samples are taken from donors who are known to have abnormal blood states.