Telephonic bullet probe.
A bulb connected to conducting wire is placed between the patient’s teeth (or inside the rectum/vagina). The operator then holds the receiver to his ear with one hand and passes a metal probe through the wound with the other hand in search of the bullet. Once the probe touches a foreign metal object the circuit is complete, and a grating/clicking sound will transmit through the receiver. All electrical current in this device is derived from the patient’s body; no other battery is used.
From A. D. Rockwell’s The medical and surgical uses of electricity, 1903