Back:  A Short History of the Galvanometer

A Short History of the Galvanometer

Trevor L. Cass

Early experiments

In 1820 Hans Christian Oersted (1777-1851) announced his discovery that a compass needle deflected when hung over a wire carrying an electric current. That same year Johann Salomo Christoph Schweigger (1779-1857)of Nuremberg at the University of Hale, turned the wire into a coil around the needle which greatly multiplied the effect and called the device a 'Multiplier' but later started using the term Galvanometer. Further developments were made by others including André-Marie Ampère, until 1837 when Pouillet made the first practical galvanometer.

The word galvanometer is attributed to a professor of anatomy at the University of Bologna Luigi Galvany (1737-1798) who, in the course of some experiments on 'Animal Electricity', hung a dead frog over an iron balcony railing by a copper hook through the frog's back. The creature's body was at once convulsed by lively twitching. Galvani's conclusion was that the frog was a naturally charged Leyden jar, with negative electricity on the exterior surface of the muscles, which discharged through the iron railing, and positive electricity along the nerves of the exterior that discharged through the copper hook. This was just one of his experiments called 'Electricity in Motion', hence the accolade for the instrument. As well as others, for example 'galvanizing', and 'Galvanic', an old term for battery, in honour of the man whose dead frog started the inquiry.

First Practical Examples

Tangent Galvanometer

A Tangent Galvanometer

Claude Servais Mathias Pouillet (1791-1868) described a Galvanometer using the principle of the 'Tangent law of magnetism', this was the first practical use of the instrument for measuring electric current, it was called a 'Tangent Galvanometer'. See later examples from Pye and Philip Harris. The scale is calibrated in four quadrants of 90 degrees, a mirror on the scale was provided to prevent parallax error when reading, i.e. the eye could line up the needle with its reflection to avoid a false reading. The stronger the current through the coil the more the needle will deflect from magnetic north. This means that the instrument needed to be aligned with magnetic north before use.

The solution to the alignment problem had already been solved in 1825 by Leopoldo Nobili (1784-1835) with his Astatic Galvanometer If two compass needles are used one positioned with opposite polarity to the other the effect of Earth's magnetic field is canceled or 'nulled'. One needle is on top of the scale and the other is inside the coil so only one needle is influenced by the current through the coil.

A further development was to hang a mirror on the compass needle and deflect a spot of light onto a distant scale; this would increase the sensitivity by many factors. The Mirror Galvanometer, attributed to William Thomson (1824-1907) later Lord Kelvin, who patented his device in 1858, after Johann Christian Poggendorff (1796-1877) had first developed it in 1826. Instead of magnetic needles, permanent magnets are hung by a silk thread between a coil, fixed to the magnets is a mirror. This is a 'moving iron' type instrument with the advantage that the coils can easily be changed for different sensitivity. A further feature was added by Thomson; above the instrument a magnetic bar was added which could be moved up down and around further cancelling out any effect by Earths magnetic field. Later Thomson made an Astatic type.

Thomson's Mirror Galvanometer

Thomson's Mirror Galvanometer

The moving iron principle was reversed in 1882 by Jacques-Arsene Arsonval (1851-1940) and Marcell Duprez. Moving iron galvanometers are not linear and errors from the earth's magnetic field still exist, in a moving coil version a coil is hung, not by a silk thread, but by a fine wire above and below through which the current for the coil passes, this is then positioned between the poles of a permanent magnet. The mirror is attached to the coil and the movement will give a linear response to the current passing through it.

In 1912 Albert Campbell devised a vibration type Galvanometer.. With this type of unit the mirror can be made to resonate to an applied frequency of alternating current by adjusting the length of the suspension, thus increasing the  Q factor and hence greatly amplifying the effect of the applied voltage. Campbell's galvanometer hung the moving coil and mirror from two wires instead of one from each end, pivoted at the coil. The angle could be varied by moving a block vertically above in the brass tower, the lower the block the wider the angle of the two wires. This unit is also known as a Bi-filar type.

Use in Telegraphy

Inker

A Morse Inker

The first successful transatlantic cable was laid in 1865 by Cyrus West Field (1819-1892). Telegraph cables on land could have relays every few miles to amplify the signal, this was not possible on early undersea cables so that the signal received was very weak and Thomson's mirror galvanometer had to be used to detect the signal. This and capacitative/inductive effects made the data rate very slow and it took 2 minutes to transmit one morse code character.

The stronger signal produced by the relays on landlines allowed insensitive Inkers to be used. The problem was solved for the transatlantic cable by using Thomson's Syphon Recorder invented in 1858. Working similar to a mirror Galvanometer, a tube fed by a reservoir is suspended by the moving coil and insulated from the reservoir, a voltage is used to electrostatically move the ink from the tube on to a moving paper, attracting the ink to a charged plate behind. This is very similar to modern ink jet printers. (See also the History of Telegraphy).

Modern Developments

Modern Mirror Galvanometer

A Modern Mirror Galvanometer

Eventually the moving coil mirror Galvanometer would be replaced by a simple needle over a calibrated scale, the term Galvanometer would remain for many years later to be replaced by the function of the instrument i.e. Voltmeter or Ampmeter (Ammeter), but in laboratories the Spot or Mirror Galvanometer would remain into the 1980's in equipment such as this Cambridge Instruments model.

Other types of Galvanometers include, a Ballistic type, for measuring short electrical impulses, and the String Galvanometer to provide the first practical Electrocardiogram.

More Galvanometers from the Museum of Technology


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