This combined galvanometer and 24 hour clock was in the window of a shop called Hancocks, somewhere in London in the early 20th century.

It was one of many clocks that were electrically connected to London’s Greenwich Observatory. This service started in the 1850s, when Greenwich time was first provided to interested subscribers – the watchmakers, government offices, and railway operators who needed to know the exact time. In 1873, the cost of connecting your apparatus to the nearest Post Office’s time server was £12 a year.

I suppose the needle would have deflected at the start of each hour. More ingenious solutions were tried to indicate the exact moment: most popular was a ball on a pole, that descended at the start of the new hour.

Sometimes, though, nothing can beat a personal service. This picture shows Ruth Belville, the Greenwich Time Lady. Her job was to set her accurate pocket watch at the main Greenwich Observatory 24 hour clock on a Monday morning, get a signed certificate from an official, and then walk around London visiting forty or fifty chronometer makers, who would then transfer the right time to their own most reliable timekeeper. She carried on doing this job from 1892 to the 1930s.

This morning, my computer asked one of Apple’s computers on the other side of the world for the right time, using the Network Time Protocol to set its clock, taking less than a few seconds to do it. Just some of the everyday magic that we take for granted today.