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A lieutenant using a trident scanner

Scotty repairing the Constellation

A trident scanner was a 23rd century piece of Federation technology that was used for repairing power relays. (DS9: "Trials and Tribble-ations")

Spock used a trident scanner while he attempted to examine the circuits aboard the Galileo, after the shuttlecraft had been brought down to Gamma Canaris N. 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")

The same year, Ensign Harper used a trident scanner to connect the M-5 multitronic unit to the USS Enterprise's main power plant. He later attempted to disconnect M-5 with a trident scanner, but was vaporized by the advanced computer for his troubles. (TOS: "The Ultimate Computer")

Another model of the trident scanner was used on the bridge aboard the USS Enterprise-A while it was undergoing repairs in 2287. (Star Trek V: The Final Frontier)

In 2381, Ensign D'Vana Tendi found a trident scanner while cataloguing Kerner Hauze's collection. (LD: "Kayshon, His Eyes Open")

Sonic disruptors, design components

According to the final draft of the script for "Metamorphosis", dated 3 May 1967, this device was called "a ray-generator of some sort." The September 1968
Star Trek reference guide, The Making of Star Trek, described this device in production photos as a "(Spock built) ray generator and energy neutralizer."
The term "trident scanner," was first described almost thirty years later, in the script for "Trials and Tribble-ations," where the device is apparently named after its general shape, which resembles a trident.
The scanner units appear to partially incorporate piece of the sonic disruptors fabricated for "A Taste of Armageddon".