From the Super Mario Wiki
The Voltage-Proof Coil is a key item from Wario: Master of Disguise. It can be found in a purple treasure chest in Carpaccio's Lab. The chest is in a room with numbered yellow switches that must be pressed in order with Arty Wario's blocks to open a door. Obtaining the Voltage-Proof Coil prevents Wario from taking damage from the electrical currents found in the lab for the remainder of the episode.