harmonica bug

harmonica bug

[här′män·ə·kə ‚bəg]
(electronics)
A surreptitious interception technique applied to telephone lines; the target instrument is modified so that a tuned relay bypasses the switch hook and ringing circuit when a 500-hertz tone is received; this tone was originally generated by use of a harmonica.
References in periodicals archive ?
Years ago when it was first introduced, it was called a harmonica bug.