On 23 October 1916, an engineer named Henry E. Warren quietly revolutionized power transmission by installing an electric clock in the L Street generating station of Boston’s Edison Electric Illuminating Co. This master station clock kept a very particular type of time: It used a synchronous self-starting motor in conjunction with a pendulum to help maintain the station’s AC electricity at a steady 60-cycle-per-second frequency. As more power stations adopted the clocks, the frequency regulation...