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When recording bioelectric signals, electrodes are used as sensors. These sensors normally add noise to the wanted signals and can influence the signals in an unwanted way. In normal EEG practice, for example, surface electrodes are used which are not always in perfect condition and, therefore, can contribute considerable noise. Measurements were performed on different types of electrodes in both the time and the frequency domains. Results show that if there is electrode noise, it is in a low-frequency band and is extremely difficult to distinguish from the biological signal.Different types of electrodes in varying degrees of degradation, and of different ages were also tested. For low-frequency EEG signals, silver electrodes having a well chlorided surface gave the best results for both time signals and frequency spectra. Poorly prepared electrodes can cause considerable noise which can easily be confused with other low-frequency biological sources such as the delta activity of the EEG.