What is the X in X-mas?

# dynamophone

earliest musical instrument to generate sound electrically. It was invented in the United States by Thaddeus Cahill and introduced in 1906. The electrophonic instrument was of the electromechanical type, and it used rotating electromagnetic generators (and thus was a predecessor of the Hammond organ) to produce electric impulses that were converted into sound by telephone receivers. It was soon superseded by more practical electronic instruments.