instrument measuring change in wave formations of electricity
• (n.) An instrument for autographically recording the wave forms of varying currents, esp. rapidly varying alternating currents.
On'do·graph noun [ French onde wave, Latin unda + -graph .] (Electricity) An instrument for autographically recording the wave forms of varying currents, esp. rapidly varying alternating currents. 
An ondograph is an instrument for graphically recording oscillatory variations, as in alternating current.