Natural Philosophy Collection

Galvanometers were the prime electrical measuring instruments for over a century, only recently being superseded by today's digital meters. All galvanometers measured electrical current. Even those calibrated in volts or ohms (the unit of electrical resistance) really measured current. By including some fixed circuitry inside the galvanometer, the maker was able to convert the measured current to a corresponding voltage or resistance and mark the deal accordingly.

Early galvanometers were based on the fundamental observation that a steady electric current in a wire will deflect a nearby compass needle. This weak effect was soon multiplied by making the current flow in a circular coil of many turns placed around the compass needle. One early development illustrating this principle was the tangent galvanometer, consisting of a distinctive large coil so placed that its influence on the needle was at right angles to that of the Earth's magnetic field. The resulting combination of tugs produced a twist of the needle such that the tangent of the angle of twist was proportional to the current in the coil. Hence by measuring the angle, the current could be deduced.

The most sensitive galvanometers used a pair of compass needles set opposite one another to cancel out altogether the effect of the Earth's magnetic field (so called astatic galvanometers). The pair of needles were suspended by a thin thread and the current carrying coil positioned over only one of the needles. For these galvanometers the twist produced on the needle by the current was set against the restoring twist of the suspension thread.

William Thomson of Glasgow developed the optical lever galvanometer to detect the extremely weak signals transmitted by the first trans-Atlantic telegraph cables laid in the late 1860s. The name optical lever referred to a beam of light that was shone onto a small mirror, mounted on the suspension, and thence onto a distant calibrated scale. The light beam became essentially a huge pointer of no weight. Such instruments became the most sensitive in use for more than half a century.

The familiar panel meter and the electrician's portable meter, with scale and pointer, developed into their modern form just over a century ago. Instead of having a magnetised needle inside a coil, these instruments are made with a small coil that can itself twist on a suspension placed between the poles of a strong magnet. The Earth's magnetic field now has little influence on the coil and these devices can be made more compact and robust than their predecessors.