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 Galvanometers & Meters

Galvanometers were the first instruments used to determine the presence, direction, and strength of an electric current in a conductor. All galvanometers are based upon the discovery by Hans C. Oersted that a magnetic needle is deflected by the presence of an electric current in a nearby conductor. When an electric current is passing through the conductor, the magnetic needle tends to turn at right angles to the conductor so that its direction is parallel to the lines of induction around the conductor and its north pole points to the direction in which these lines of induction flow. In general, the extent to which the needle turns is dependent upon the strength of the current.

André-Marie Ampère, (1775-1836), is credited with the invention of the galvanometer in 1824. The earliest galvanometers were literally constructed of a compass surrounded by a coil of wire (see example at right). These meters were called tangent galvanometers because the tangent of the angle of deflection of the needle is proportional to the strength of the current in the coil (at this point in time it was impossible to construct a meter whose needle deflection was directly proportional to the current under measurement).


Struers Tangent Galvanometer


Central Scientific Tangent Galvanometer utilizing compass (1941)

Unfortunately, simple galvanometers such as the Struers model shown above were inaccurate and inconsistent in their readings. By placing the compass at the center of a precisely calculated circle, accuracy could be improved substantially (see left). Other improvements were added later including replacing the compass with a specially designed meter movement, adding leveling screws, etc.

These large stationary-coil type galvanometers were used as the standard current measuring instrument into the last quarter of the 19th century. Additional examples of tangent galvanometers are shown below:


Harris Tangent Galvanometer


Harris Tangent Galvanometer


Eureka Scientific Tangent Galvanometer


University Supply Tangent Galvanometer

Knott Tangent 
Galvanometer
Early Tangent Galvanometer
Early Rectangular Tangent Galvanometer
University Supply Tangent Galvanometer

Reflecting Galvanometers
One of the limitations of early galvanometers was that the length of the needle had to be kept very short in order to minimize the effects of the earth's magnetic field and reduce damping errors introduced by the mass of the needle itself. Unfortunately, the shorter the needle, the less distance the tip will travel as it inscribes an arc, and thus the more difficult it will be to read very small changes in current. This problem was solved ingeniously by using a beam of light as the needle; a shaft was placed through the center of the needle and a very small mirror was attached. A beam of light is reflected off of the mirror and onto a scale located about three feet away. The result is that an extremely small deflection of the mirror will cause a much larger movement of the beam on the scale. Below are some examples of these reflecting galvanometers:


English Mirrored Galvanometer
1910


Harris d'Arsonval Reflecting Galvanometer
1910


Becker Reflecting Galvanometer
c. 1910



Knott Reflecting Galvanometer


Harris Reflecting Galvanometer Scale
c. 1918
Light from the source at the top of the unit was directed at the mirror of the galvanometer. The light reflected from the mirror back to the scale, indicating a reading corresponding to the deflection of the mirror. Reflecting Galvanometers such as these were extremely sensitive.


Moving Magnet Reflecting Galvanometer
Cambridge Instruments
1905


Groves Reflecting Quadrant Galvanometer
1885


Suprecision Mirror Galvanometer
Early 1900's

String Galvanometers
Sensitivity, (called "sensibility" back then), can be increased by suspending the needle at the end of a long string. At right are a couple of string galvanometers. The Braun model is known as an Astatic type. Astatic galvanometers use a combination of two needles of equal size mounted rigidly together in parallel but with their poles pointing in opposite directions. This combination neutralizes the effect of the earth's magnetic field and the needle will remain at rest in any position.


Unknown String Galvanometer
c. 1910


Braun Astatic Galvanometer
1910

d'Arsonval Galvanometers
In 1880, Jacques-Arsene d'Arsonval made a dramatic improvement by attaching a small coil to the meter needle and locating both inside the field of a permanent magnet. This d'Arsonval movement and other rapid changes in electrical technology soon made the tangent galvanometer obsolete.

Early d'Arsonval Meters:


Laboratory Galvanometer
1905



Harris D'Arsonval
Galvanometer
Early 1900's


Weston Model 1 Voltmeter
1912


Weston Model 909
Portable Galvanometer
1898


English Meter
1885
This meter is present in a 1901 photo of Guglielmo Marconi seated at his
laboratory bench.


English Needle Galvanometer
1900


Harris Pot Galvanometer
1920's

 


Thompson AC Ammeter Ratio Meter
1909


Gans & Goldschmidt
Ma Meter
1906


Chauvin & Arnoux
Recording Voltmeter

 
Unidentified Galvanometer


The Chloride of Silver Dry Cell Battery Company
1889


 Drury Bros
D'Arsonval
Galvanometer

  


Siemens & Halske #1371 Needle Galvanometer

Resistance Coils
In testing and in duplex telegraphy, the use of an external source of calibrated resistance is sometimes required. For example, the electrical resistance of a coil or length of wire could be determined by comparing it in a galvanometer  circuit with a known resistance.  Or, in the case of telegraphy, the additional resistance could be used to balance one line against the other.

A Resistance box contains an assortment of coils of known resistance, with the ends of the coils connected together and brought out to brass plates mounted on the top of the box. To configure the box for a specific resistance, brass plugs are inserted into the plates at the appropriate locations. The plugs "short out" one or more of the coils, resulting in the desired resistance at the terminal of the resistance box. 

Some boxes, such as the King Mendham bridge below, contain coils wired into a special circuit known as a "bridge".  The bridge makes very accurate resistance measurements possible by balancing an unknown resistance against three other known values.


King Mendham & Co.
Wheatstone Bridge
Post Office pattern
c. 1885


Electronic Instruments Ltd.
Resistance Coils


Unknown Mfg.
Resistance Coils

  

 

This incredible piece is over 21" tall. The coil is 16" in diameter and can be rotated on two axes around the needle assembly.

 The force on the compass needle decreases as the coil is tilted since that decreases the component of the field in the plane of the needle. These instruments were used in the 1870's to measure the large currents produced by power station dynamos.

 

 

Beautiful Precision Sine Galvanometer



 

  

 

Bibliography:

The Hutchinson Dictionary of Scientific Biography
Hawkins Electrical Guide, Vol 2 pp 431 - 464

 
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