Final answer:
A metal envelope does keep a constant electric potential, which is true due to the Faraday cage effect, where charge redistribution in the conductor cancels out external static electric fields.
Step-by-step explanation:
The statement that a metal envelope keeps a constant electric potential is true. This is based on a principle in physics known as the Faraday cage effect, where an external static electric field will cause the electric charges within the conductive material to redistribute themselves, effectively canceling the external field inside and resulting in a uniform potential throughout the surface of the conductor. For this reason, when you have a closed conductive surface like a metal envelope, it will shield the interior from external static electric fields, maintaining a constant electric potential inside.