A mechanical rectifier for use at commercial frequencies has been constructed. Accidental electromotive forces have been so far eliminated that it can be used with a sensitive direct-current galvanometer as a detecting instrument. Four galvanometers have been tried and the results are described. With the most sensitive galvanometers used, the sensitivities of the system at 50 cycles were 0.05 microvolt (without the critical damping resistance), and 40 micromicroamperes per millimeter at one meter.
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