Nollet’s famous demonstration of the Leyden Jar in 1746 for King Louis XV, in which he lined up 180 royal guardsmen and sent an electric shock through the human chain, was afterward repeated with even more people – this time with Carthusian monks. Sources disagree on the actual number involved – some say 900 people joined the line, while others quote the number 900 in reference to the length of the human chain (“900 meters” or “more than one kilometer” are common descriptions that I’ve found). Here’s Joseph Priestley’s account from his History and Present State of Electricity (1769):
Priestley says the line was “nine hundred toises” long and makes it clear that the monks made a much more impressive demonstration than the 180 guards. But what is a toise, and how big is 900 of them?
The toise is a French unit of measure used in pre-revolutionary times that had its origins in the distance between the fingertips of a man with outstretched arms – like a wingspan. It’s (approximately) equal to 1.949 meters, which means Nollet’s chain of monks spanned a distance of 1.75 km, or just over a mile (according to Priestley, at least, who himself cites the Phil. Trans. abridged, v. 10).
Most accounts also mention how impressed the onlookers (including the King) were to see everyone give “a sudden spring” once the circuit was completed. The shock from the Leyden Jar passed through the bodies of the monks and the iron wires arranged between them, which (assuming a negligible deviation from the speed of light in a vacuum) should have taken < 0.00001 seconds. No wonder they thought it was simultaneous!
Further reading: from Priestley: The History and Present State of Electricity: With Original Experiments; more on the toise and other units of measure: “History of Measurement”, “Units of Measurement in France before the French Revolution” (Wikipedia),