With the discovery of the Geistkraft energy emitting from the hilt of Frederick Barbarossa's Sword, the Nazis began to siphon off the Geistkraft energy from the hilt with the use of the Geistkraft Transfer Device. With this device, it would be able to go from the Emperor's Chamber in the Salt Mine Tunnels to anywhere in the bunker that the rails the device was connected to would go, allowing for an near infinite source of enery for the Nazi's projects at each area of the bunker, such as creating an undead army in both the Laboratory and Morgue and creating various weaponry in the Command Room.
When the four MFAA agents arrive to Mittelburg, they manage to charge up the Geistkraft Transfer Device within the tunnels and help give it energy by killing zombies near it until it reached both the Laboratory and Morgue, where it created the barrel and core of the Tesla Gun with two different machines in both areas. With the two parts, the Geistkraft Transfer Device returned to the Command Room, where it plugged itself into the Weapon Assembly Station and fully created the Tesla Gun with the Geistkraft energy within the device.
When the four other variants of the Tesla Gun were forged, the Geistkraft Transfer Device infused all of them with the same Geistkraft energy for them to be powered.