Gray telegraph relay patent model

This relay was constructed early in 1867 by noted electrical inventor Elisha Gray to accompany a patent application. He received US Patent 69,424 later that year for an improved telegraph relay. Gray's idea was to use a second set of electromagnet coils rather than a spring to retract the armature after a signal pulse. He wrote that the pulse's strength could vary so much that a relay's spring needed constant adjustment. His design compensated automatically for the variation in pulse strength.