confined his duties mostly to demonstration.

He achieved renown by a great experiment--the measurement of the
velocity of electricity in a wire. His method was beautiful and
ingenious. He cut the wire at the middle, to form a gap which a spark
might leap across, and connected its ends to the poles of a Leyden jar
filled with electricity. Three sparks were thus produced, one at either
end of the wire, and another at the middle. He mounted a tiny mirror on
the works of a watch, so that it revolved at a high velocity, and
observed the reflections of his three sparks in it. The points of the
wire were so arranged that if the sparks were instantaneous, their
reflections would appear in one straight line; but the middle one was
seen to lag behind the others, because it was an instant later. The
electricity had taken a certain time to travel from the ends of the wire
to the middle. This time was found by measuring the amount of lag, and
comparing it with the known velocity of the mirror. Having got the
time, he had only to compare that with the length of half the wire, and
he found that the velocity of electricity was 288,000 miles a second.

Till then, many people had considered the electric discharge to be
instantaneous; but it was afterwards found that its velocity depended on
the nature of the conductor, its resistance, and its electro-static
capacity. Faraday showed, for example, that its velocity in a submarine
wire, coated with insulator and surrounded with water, is only 144,000
miles a second, or still less. Wheatstone's device of the revolving
mirror was afterwards employed by Foucault and Fizeau to measure the
velocity of light.

In 1835, at the Dublin meeting of the British Association, Wheatstone
showed that when metals were volatilised in the electric spark, their