Investigation of Environmental Noise

Large excess resistance noise has been observed in small conductors surrounded by air. The conductor resistance was found to have a well-defined average power spectral density over the observed frequency range from 10-4 Hz to 200 Hz, and the dependence of the noise was measured as a function of bias current, temperature, temperature coefficient of resistance, spatial correlation, and state of the surrounding air. From this study, three different mechanisms were identified that produce the noise. The room temperature fluctuations were measured and found to have a spectral density nearly proportional to f-2 over the observed six order of magnitude frequency range. The lowest frequency noise around 10-4 Hz could be predicted from the measured temperature fluctuations using the temperature coefficient of resistance. Above 10-3 Hz and below 1 Hz, enclosing the wire in a box greatly reduces the noise, and placing the wire in a vacuum eliminated the predominant noise. This noise was directly related to the temperature of the conductor, somewhat proportional to the temperature coefficient of resistance, independent of bias current, and has a correlation length smaller than the specimen size. The highest frequency noise does not depend on the conductor temperature, temperature coefficient of resistance, or the presence of air. It had a very strong dependence on bias current, and a long spatial correlation. The mechanism that generates this noise is not understood.