Trident scanner

A trident scanner was a 23rd century piece of Federation technology that was used for repairing power relays. 

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. 

In 2267, Ensign Harper used a trident scanner to connect the M-5 computer to the USS Enterprise (NCC-1701)'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. 

The same year, Lieutenant Commander Montgomery Scott, leading a repair party aboard the USS Constellation (NCC-1017), employed a trident scanner to cross-connect the warp drive control circuits to those of the impulse engines. 

Another model of the trident scanner was still used aboard the USS Enterprise (NCC-1701-A) in 2287. 

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."

The scanner units appear to be a reuse of the sonic disruptors fabricated for.

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.