Jörg Georg Adam Mager (6 November 1880 - 5 April 1939) was a German musician and inventor, who was a pioneer of early electronic music.

In 1911, after hearing an organ that had become slightly detuned due to temperature variations, Mager began researching quarter tones, and built a harmonium based on quarter-tonal divisions.

[1] In the 1920s, Mager developed the Electrophon (later renamed to the Sphärophon), a keyboard instrument that produced sound using heterodyning.

In the late 1920s Mager received a government grant, and used this money to found a laboratory near Darmstadt; he also began an Electronic Music Society at around the same time.

[4] Although he created a number of unique electronic instruments, including the Kurbelsphärophon and the Kaleidophon, all of his examples were destroyed by Allied bombing raids during World War II.