Charles Wheatstone - Measuring Time

Measuring Time

In 1840, Wheatstone introduced his chronoscope, for measuring minute intervals of time, which was used in determining the speed of a bullet or the passage of a star. In this apparatus an electric current actuated an electro-magnet, which noted the instant of an occurrence by means of a pencil on a moving paper. It is said to have been capable of distinguishing 1/7300 part of a second (137 microsecond), and the time a body took to fall from a height of one inch (25 mm).

On 26 November 1840, he exhibited his electro-magnetic clock in the library of the Royal Society, and propounded a plan for distributing the correct time from a standard clock to a number of local timepieces. The circuits of these were to be electrified by a key or contact-maker actuated by the arbour of the standard, and their hands corrected by electro-magnetism. The following January Alexander Bain took out a patent for an electro-magnetic clock, and he subsequently charged Wheatstone with appropriating his ideas. It appears that Bain worked as a mechanist to Wheatstone from August to December, 1840, and he asserted that he had communicated the idea of an electric clock to Wheatstone during that period; but Wheatstone maintained that he had experimented in that direction during May. Bain further accused Wheatstone of stealing his idea of the electro-magnetic printing telegraph; but Wheatstone showed that the instrument was only a modification of his own electro-magnetic telegraph.

One of Wheatstone's most ingenious devices was the 'Polar clock,' exhibited at the meeting of the British Association in 1848. It is based on the fact discovered by Sir David Brewster, that the light of the sky is polarised in a plane at an angle of ninety degrees from the position of the sun. It follows that by discovering that plane of polarisation, and measuring its azimuth with respect to the north, the position of the sun, although beneath the horizon, could be determined, and the apparent solar time obtained. The clock consisted of a spyglass, having a nicol (double-image) prism for an eyepiece, and a thin plate of selenite for an object-glass. When the tube was directed to the North Pole—that is, parallel to the Earth's axis—and the prism of the eyepiece turned until no colour was seen, the angle of turning, as shown by an index moving with the prism over a graduated limb, gave the hour of day. The device is of little service in a country where watches are reliable; but it formed part of the equipment of the 1875-1876 North Polar expedition commanded by Captain Nares.