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
light, examined through a prism, revealed certain rays which were
characteristic of them. Thus the kind of metals which formed the
sparking points could be determined by analysing the light of the spark.
This suggestion has been of great service in spectrum analysis, and as
applied by Bunsen, Kirchoff, and others, has led to the discovery of
several new elements, such as rubidium and thallium, as well as
increasing our knowledge of the heavenly bodies. Two years later, he
called attention to the value of thermo-electricity as a mode of
generating a current by means of heat, and since then a variety of
thermo-piles have been invented, some of which have proved of
considerable advantage.
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