A Lieutenant using a trident scanner.
A trident scanner was a 23rd century piece of Federation technology that was used for repairing power relays. (DS9: "Trials and Tribble-ations")
Ensign Harper attempted to disconnect the M-5 computer with a trident scanner, but was vaporized by the advanced computer for his troubles. (TOS: "The Ultimate Computer")
Spock used a trident scanner while he attempted to repair the Galileo, after the shuttlecraft had been brought down on the Companion's homeworld. The scanner, however, was short-circuited and destroyed by the Companion when Spock attempted to make physical contact with it. (TOS: "Metamorphosis")
In 2267, Lieutenant Commander Montgomery Scott, leading a repair party aboard the USS Constellation, employed a trident scanner to cross-connect the warp drive control circuits to those of the impulse engines. (TOS: "The Doomsday Machine")
A trident scanner also appears briefly in the first bridge scene in Star Trek V.
Background
According to the September 1968 Star Trek reference guide, The Making of Star Trek, this device was described in production photos as a "(Spock built) ray generator and energy neutralizer."
Sonic disruptor
The scanner units appear to be a reuse of the sonic disruptors fabricated for "A Taste of Armageddon".
The term used here, "trident scanner," comes solely from the script for "Trials and Tribble-ations," where the device is apparently named after its general shape, which resembles a trident.