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PHILIP HARRIS MIRROR GALVANOMETER, 1950's
The mirror galvanometer was later improved by William Thomson, later to become Lord Kelvin. He would patent the device in 1858.
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Thomson reacted to the need for an instrument that could indicate with sensibility all the variations of the current in a long cable. This instrument was far more sensitive than any which preceded it, enabling the detection of the slightest defect in the core of a cable during its manufacture and submersion. Moreover, it proved the best apparatus for receiving messages through a long cable.
A mirror galvanometer is a mechanical meter that senses electric current, except that instead of moving a needle, it moves a mirror. The mirror reflects a beam of light, which projects onto a meter, and acts as a long, weightless, massless pointer. In 1826, Johann Christian Poggendorff developed the mirror galvanometer for detecting electric currents. The apparatus is also known as a spot galvanometer after the spot of light produced in some models.