An automatic watch has an oscillating weight known as a rotor built into the movement. This is visible through a transparent case back in some watches. The rotor spins with the movement of your arm, generating energy, which is then transferred to a metal coil called a mainspring, which is housed in a cylindrical case called a barrel. The energy stored in the barrel is known as the power reserve. The power reserve in our automatics can be anything from around 38 hours to five days (120 hours) if your watch houses one of the next-generation Oris Calibre 400 Series movements.