First metal detector by Alexander Graham Bell, how did it work?
According to this document:


  He knew that a metallic object near an inductor changes the value of
  the inductance. If a listening device, such as a telephone, was
  connected to an audio frequency source, the change in induction would
  also change the audio signal. This change would be heard by the
  listener. With Bell’s recent invention, the telephone, he thought that
  he could hear that change.


...


  Bell successfully detected bullets in sides of beef and shrapnel in
  Civil War veterans. He made several attempts with variations on the
  basic apparatus to locate the bullet in the President’s body. But, he
  was unable to find the location of the bullet in Garfield. The
  physicians in attendance attempted to find the bullet by inserting
  their fingers into the bullet hole. While the idea of sterilization
  had been developed by Joseph Lister in England by this time, most
  doctors in the United States did not take it seriously. Thus, on
  September 19, 1881, eleven weeks after being shot President Garfield
  died.


Why didn't it work?


  The problem was the bed he was in. Coil spring mattresses had just
  been invented. In fact, a national campaign hadn't even been started
  yet at the time of the assassination. The White House was one of the
  few that had the coil spring mattresses at the time. Very few people
  had even heard of them. Thus, Bell's and Newcomb's invention was
  detecting metal -- unfortunately they didn't realize that it was the
  coil springs.


The first document gives details of the interruptor coil and ticking clock used by Bell to generate alternating current in the circuit.