On the excess noise of dc current-carrying thin metal wires in dilute gases of different kind and pressure

Abstract

The excess noise of dc current-carrying thin metal wires in gases of different kind and pressure has been investigated. The noise depends on the kind of the gas. Therefore its origin must be in the gas or in the boundary between the metal and the gas. By means of a simple theory the power spectrumW _φs of the random heat current ϕ_s flowing into the gas (surface source) can be calculated from the observed noise voltage. At frequencies above 100 HzW _φs is given by a Nyquist formula, at lower frequencies an additional term 2 · 10⁻³ · Φ ²₀ /N_hf is superimposed (Φ₀ = dc heat current,f=frequency). The numberN _h characterizes this Hooge-contribution. A lot ofN _h -values for different metals, gases, pressures and heating currents have been measured.N _h is smaller for light gases and small wire diameters. For heavy gasesN _h is about ten times larger. A possible qualitative explanation is given. At higher pressures and heating currents thef ⁻¹ dependence turns over into af ⁻³ one. This may be due to gas convection.