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The production of an e.m.f. is one of the fundamental properties of the living cell and, owing to the nature and magnitude of this e.m.f., special amplifying methods and input circuits are required. The paper reviews the standard methods of amplifying and recording biological potentials, and the application of these methods to recording the electro-encephalogram, or the record of potentials developed in the human cerebral cortex. The magnitude of the potentials produced by the brain varies between 5 and 1000 Â¿V, the wave form being alternating, irregular and of very low frequency. The frequency of the output from an abnormal brain (e.g. epileptics and cerebral tumours) differs appreciably from that of the normal, and from this and other indications it is possible to diagnose mental diseases and locate with considerable accuracy the site of tumours. One of the authors has recently developed apparatus for analysing waves of low frequencies (1Â¿20 c./s.) on the mechanical resonance principle.