"The greatest boon to divorce lawyers and spies since the Mickey Finn.  A microminiaturized wire recorder.  The wire is spring driven so that it can't be spotted by a snooper circuit.  The insides are transistors and resistors and capacitors and stuff, all packed in plastic - you could drop it out of a cab and not hurt it.  The power is about as much radioactivity as you would find in a watch dial, but shielded, The wire is good for twenty-four hours.  Then you slide out a spool and stick in another one - the spring is part of the spool, already wound."