Depending upon the kind of electrode system employed, a sinusoidal electric current gives rise to at least five phenomena. (1) With a moving electrode on the skin or on the roof of the mouth (fricative effect), the subject receives the strongest sensation of hearing. He may hear either the first harmonic or the first and second harmonics, or the second harmonic alone, depending upon the applied voltage and upon the placement and properties of the electrode. (2) With a large‐area electrode any place on the skin of the head, if the skin is dry the subject hears the second harmonic; if it is wet, he hears nothing. (3) When the electrode is immersed in salt solution in the ear (the most common method), the subject hears a complex tone containing mostly second harmonic, suggesting the action of a square‐law transducer. (4) When the electrode is on the mucous tissue inside the middle ear (with eardrum removed), the subject hears the first harmonic and/or a noise. (5) When the electrode is in contact with the epidermis of the meatus, the subject hears the first harmonic, and at low frequencies he may also hear a noise. The experimental results seem to indicate that the hearing of a tone under any of these five conditions is probably due to vibrations set up outside the cochlea, although there appear to be at least four different transducing mechanisms. Contrary to earlier hypotheses, the tympanic membrane is apparently not involved in the conversion of the electrical energy into mechanical vibration.