Sigmund Riefler of Munich became the most famous maker of regulator clocks for observatory use, continually striving for perfection of timekeeping. He invented a mercury-in-steel pendulum with mercury almost filling a hollow pendulum rod to avoid temperature differential effects, patented in 1891 but surpassed when Charles Edouard Guillaume invented the alloy invar in 1895. In 1889 Riefler improved his regulators by an escapement which impulsed the pendulum through its suspension spring, making the pendulum virtually 'free' and giving a performance with a daily variation of less than 0. 01 second under good conditions. Riefler's final development was to adopt electrical rewinding and place the pendulum and mechanism in an evacuated cylinder to isolate the clock from atmospheric disturbances, making his regulator the standard timekeeper for most of the world's observatories. Riefler's clock represents the final phase of the development of the purely mechanical regulator.