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e A number of years ago I noticed that a copper sulphate cell with copper electrodes, one of which was considerably larger than the other, showed an apparent rectifying effect for alternating currents of the order of those produced by speaking with the mouth pressed against an ordinary telephone receiver. In the original experiment the rectifying cell, the telephone, and a d'Arsonval galvanometer of moderate sensibility were connected in series. The lowest tone spoken into the telephone produced a deflection of the galvanometer, and a louder one drove it from the scale. Recently 1 have again taken the matter up, desiring, if possible, to find an explanation of the rectifying effect, and also to test the usefulness of the detector in the laboratory. To test the sensibility of the detector for alternating currents, a known alternating current was passed through a bridge wire of about 1 ohm resistance, and from this, by means of sliding contacts, various potential differences were applied to the detector (see fig. 1). A form of key was used which kept the detector circuit short-circuited Fig. 1.—Connections for detector (form 1) .

Detector for very small alternating currents and electrical waves