One more: if you're toning a circuit, you'll get the loudest tone if the two leads you're squirting tone onto are completely open. A GFCI receptacle will foul things up--a gfi has an injector coil in it to impress a slight voltage on the neutral, so that a ground fault on the neutral, even with no load on the circuit, will trip the gfi. this coil is a moderate impedance short from hot to neutral. A low-voltage transformer for a doorbell or alarm system also bridges the hot & neutral, and will reduce the tone amplitude. Also in commercial situations, look out for emergency lighting fixtures, which have a transformer/battery charging circuit.
Cliff
