Clocker, for clock, galvanic skin response sensor and digital delay system, is a minimalist electronic music piece by Alvin Lucier conceived in 1978, though Lucier felt there did not exist an appropriate digital delay system till 1988: "With this new equipment, the sounds of the delayed clock now matched those of the original, creating clear copies and with them a more convincing illusion of time expanding and contracting. Later I added a bank of fixed delays which, as they splay out from the voltage controlled delay, create multiple reflections that almost convince the listener that the room is changing size."
What was played through the digital delay system, a Digitech RDS 7.6 with a continuously variable voltage control output, was the sounds of a clock, a Westclox Silver Bell Monogram, ticking, with the delay controlled by a galvanic skin response sensor (GSR). The GSR, by sending a small current through the body, reads minute differences in skin resistance which in turn is influenced by the performers thoughts and emotions: "I had wanted to make a work in which a performer could speed up and slow down time, stopping it, if possible, simply by thinking."