The metal key case can affect the signal. The metal key shell forms an electrostatic shield inside it.
The electrostatic shielding is to guide the body shell to "protect" its interior, so that its interior is not affected by the external electric field.
In most cases, the shield can be made of copper, aluminum, steel and other metals, but for constant and very low frequency magnetic fields, materials such as ferrite can also be used as the shield. In a system or between different systems, electromagnetic noise or interference often occurs, which causes system performance to deteriorate. Therefore, power lines or magnetic lines of force are required to be limited to a certain area; a certain area is not affected by external power lines and magnetic lines.
Static shielding principle:
If the conductor is placed in an external electric field with an electric field strength of E, the free electrons in the conductor will move against the direction of the electric field under the action of the electric field force. In this way, the negative charge of the conductor is distributed on one side and the positive charge is distributed on the other side. This is the electrostatic induction phenomenon. Due to the redistribution of electric charges in the conductor, these charges form another electric field in the opposite direction to the external electric field, and the electric field strength is within E.
According to the field strength superposition principle, the electric field strength in a conductor is equal to the superposition of E outside and E inside. When the opposite electric field superpositions cancel each other out, the total electric field strength inside the conductor is zero. When the total electric field strength inside the conductor is zero, free electrons in the conductor no longer move directionally.
In physics, the state where no charge moves in a conductor is called electrostatic balance. In an electrostatically balanced conductor, the internal electric field strength is zero everywhere. It can be inferred that in a conductor in an electrostatic equilibrium state, electric charges are distributed only on the outer surface of the conductor. If the conductor is hollow, when it reaches electrostatic equilibrium, there will be no electric field inside.
In this way, the outer shell of the conductor will "protect" its interior and keep its interior from external electric fields. This phenomenon is called electrostatic shielding.