History
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Originally the instruments relied on the Earth's magnetic field to provide the restoring force for the compass needle;
these were called "tangent" galvanometers and had to be oriented before use.
Later instruments of the "astatic" type used opposing magnets to become independent of the Earth's field and would operate in any orientation.
The most sensitive form,
the Thompson or mirror galvanometer,
was improved by William Thomson (Lord Kelvin) from the early design invented in 1826 by Johann Christian Poggendorff.
Thomson's design, which he patented in 1858, was able to detect very rapid current changes. Instead of a compass needle, it used tiny magnets attached to a small lightweight mirror, suspended by a thread; the deflection of a beam of light greatly magnified the deflection due to small currents.
Alternatively the deflection of the suspended magnets could be observed directly through a microscope.
The ability to quantitatively measure voltage and current allowed Georg Ohm to formulate Ohm's Law, which states that the voltage across a conductor is directly proportional to the current through it.