Any clock which indicates time without possessing its own internal means of measuring time, depending on an external control, may be called a slave clock. The term is usually applied to the controlled clocks in a system operated by an electric master clock which sends out pulses at either 1-second, 30-second, or 1-minute intervals to advance the hands of the controlled clocks. Carl August Steinheil of Munich was the first to use slave clocks, these being driven by Daniell cells switched by contacts on the master-clock pendulum. Reliable slave clocks were not produced until the beginning of the 20th century, although one installed at the gate of the Old Observatory at Greenwich by Charles Shepherd in 1852 has been operating almost continuously since then. Slave clocks may be operated in series or parallel, by pulses in one direction, or by pulses reversing for each step; generally today slave clocks work from single-direction pulse circuits and are connected in series, large systems having groups of series clocks further connected in parallel and energized by a relay circuit to handle the large current required.