Mercury Circuit

The Mercury Circuit (マーキュリー回路) was a device given to Kamen Rider X by Kamen Rider V3 to enhance him.

X was captured and had his energy absorbed by G.O.D.'s villain monster Spider-Napoleon. At the last moment, V3 appeared and challenged him to a battle, only to disappear, escaping with X. Keisuke Jin's life was in danger, and his power wasn't enough to face his opponent, to help the fellow Rider, V3 decided to give him the Mercury Circuit, which could greatly power up X. However, V3 needed to transfer a large amount of his blood to him in the process, weakening himself. Spider-Napoleon soon attacked again, forcing V3 into action before he had recovered. V3 managed to rescue his hostages, but was overwhelmed by Spider-Napoleon and the rebuilt Mach Achilles and Salamander-Gong. He was rescued by the powered up X, who finished off Spider-Napoleon, while V3, even weakened, beat Mach Achilles and Salamander-Gong together.

From then onward, Keisuke used the phrase "Dai Henshin!" (大変身！) to transform while making a "X" sign with his arms.