In the episode Court Martial, Dr. McCoy uses this device to nullify the amplified heartbeats of all personnel on the Enterprise bridge, leaving only the single heartbeat of supposedly deceased Lt. Cmdr. Finney, actually still alive, but not at all well. It reappears as a background medical instrument in many future TOS episodes, including among others, Space Seed, Journey To Babel, Day Of The Dove, and Spock's Brain. Also pictured in The Making Of Star Trek, where it's labeled 'Heartbeat Reader'.
This is the same model vintage Shure 585 Unisphere microphone as the original prop, with screen accurate modifications including repainted orange-red equatorial band and rotating 'reset' ring at the base.
Prop builder not identified.
