The last category includes instruments such as theremins or synthesizers, which he called radioelectric instruments.
Francis William Galpin provided such a group in his own classification system, which is closer to Mahillon than Sachs-Hornbostel. For example, in Galpin’s 1937 book A Textbook of European Musical Instruments, he lists electrophones with three second-level divisions for sound generation [“by oscillation,” “electro-magnetic,” and “electro-static”], as well as third-level and fourth-level categories based on the control method. Sachs himself proposed subcategories 51, 52, and 53, on pages 447-467 of his 1940 book The History of Musical Instruments. However, the original 1914 version of the system did not acknowledge the existence of his 5th category.
Present-day ethnomusicologists, such as Margaret Kartomi [page 173], and Terry Ellingson [PhD dissertation, 1979, p. 544] suggest that, in keeping with the spirit of the original Hornbostel Sachs classification scheme, if one categorizes instruments by what first produces the initial sound in the instrument, that only subcategory 53 should remain in the electrophones category. Thus it has been more recently proposed that, for example, the pipe organ [even if it uses electric key action to control solenoid valves] remain in the aerophones category, and that the electric guitar remain in the chordophones category, and so on.
Thus, in present-day ethnomusicology, an electrophone is considered to be only musical instruments which produce sound primarily by electrical means. It is usually considered to constitute one of five main categories in the Hornbostel-Sachs scheme of musical instrument classification [though it is not actually present in the scheme published in 1914].