A hand held test instrument measures cable lengths by applying a square wave signal to a conductor of a pair and detects the induced current in the other conductor of the pair. A synchronous detector measures the induced current, which is representative of the capacitance of the cable, which is representative...http://www.google.com.au/patents/US6573733?utm_source=gb-gplus-sharePatent US6573733 - Capacitance measuring technique for estimating cable length
