A coiled steel spring inside a brass mainspring barrel provides 30 hours of power. So early spring-driven clocks slowed down over their running period as the mainspring unwound. The mainspring is accommodated in the bolt carrier assembly slide channel. The force of the mainspring turns the barrel. First, it scales up the speed of rotation of the mainspring or weight pulley. This model uses a barrel and incorporates a grip safety and leaf-type mainspring. This isolated the timekeeping element from the varying mainspring force. Traditional mechanical watch movements use a spiral spring called a mainspring as a power source. There are numerous devices for recording the amount of mainspring power stored in the barrel. If a simple mainspring was used, this would put excessive tension on the mainspring. Fusees became the standard method of getting constant torque from a mainspring. Mechanical watches are powered by a mainspring. Portable timepieces were made possible by the invention of the mainspring. Large mainsprings in clocks are immobilized by "mainspring clamps" before removal. This caused the wheel's period to be very sensitive to the force of the mainspring.