Noise measurements have been performed on oxide‐coated cathodes at frequencies of 8 Mc and of 30 cycles. At 8 Mc, where thermal noise and shot noise predominate, the measurements show thermal noise at high cathode temperatures (pore conduction region) and at low cathode temperatures (grain conduction region) with a pronounced noise peak caused by shot noise in the temperature range where pore conduction switches over to grain conduction.
Noise measurements at 30 cycles where flicker noise (with a 1∕f noise spectrum) predominates, indicate that the pore noise corresponds to the flicker noise expected for a semiconductor with a voltage‐independent conductivity, whereas the grain noise corresponds to the flicker noise expected for a semiconductor with voltage‐dependent conductivity. The measurements also show that the pores are inherently noisier than the grains at low frequencies. The results agree with a qualitative theoretical analysis.