In this opportunity I will share you about one of electronic test instruments. From picture above I will share information about galvanometer. And subtopic will be explained are what is galvanometer, type of galvanometer, working principle of galvanometer, and application of galvanometer. Lets check it out.
1. What is Galvanometer ?
Galvanometer is an electromechanical instrument which is used for the detection of electric currents through electric circuits. Being a sensitive instrument, Galvanometer can not be used for the measurement of heavy currents. However we can measure very small currents by using galvanometer but the primary purpose of galvanometer is the detection of electric current not the measurement of current.
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.
Galvanometer is the historical name given to a moving coil electric current detector. When a current is passed through a coil in a magnetic field, the coil experiences a torque proportional to the current. If the coil’s movement is opposed by a coil spring, then the amount of deflection of a needle attached to the coil may be proportional to the current passing through the coil. Such “meter movements” were at the heart of the moving coil meters such as voltmeters and ammeters until they were largely replaced with solid state meters.
The accuracy of moving coil meters is dependent upon having a uniform and constant magnetic field. The illustration shows one configuration of permanent magnet which was widely used in such meters.
2. Type of Galvanometer
a. Tangent Galvanometer
Struers Tangent Galvanometer
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. 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).
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.
b. Reflecting Galvanometer
Becker Reflecting Galvanometer
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.
c. String Galvanometers
Braun Astatic Galvanometer
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.
d. 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.
e. Resistance Coils
King Mendham & Co.
Post Office pattern
Electronic Instruments Ltd.
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 above, 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.
f. Beautiful Precision Sine Galvanometer
Beautiful Precision Sine Galvanometer
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.
3. Working Principle of Galvanometer
Galvanometer works on the principle of conversion of electrical energy into mechanical energy. When a current flows in a magnetic field it experiences a magnetic torque. If it is free to rotate under a controlling torque, it rotates through an angle proportional to the current flowing through it.
a. Essential Parts Of Galvanometer
There are five essential parts of a Galvanometer :
1. A U-shaped permanent magnet with concave poles.
2. Flat rectangular coil of thin enameled insulated wire ‘C’.
3. A soft iron cylinder ‘B’.
4. A pointer or needle.
5. A scale.
The flat rectangular coil of thin enameled insulated wire of suitable number of turns wound on a light nonmetallic or aluminum frame is suspended between the cylindrically concave poles of magnet by a thin phosphor bronze strip. One end of the wire of the coil is soldered to strip. The other end of the strip fixed to the frame of the galvanometer and connected to an external terminal. It serves as one leas current lead through which the current enters or leaves the coil. The other end of the wire of the coil is soldered to a loose and soft spiral of wire connected to another external terminal. The soft spiral of a wire serves as the other current lead. A soft-iron cylinder, coaxial with the pole pieces, is placed within the frame of the coil and is fixed to the body of the galvanometer. In the space between it and the pole pieces, where the coil moves freely, the soft iron cylinder makes the magnetic field stronger and radial such that into whatever position the coil rotates, the magnetic field is always parallel to its plane.
When a current passes through the galvanometer coil, it experiences a magnetic deflecting torque, which tends to rotate it from its rest position. As the coil rotates it produces a twist in the suspension strip. The twist in the strip produces an electric restoring torque. The coil rotates until the elastic restoring torque due to the strip does not equal and cancels the deflecting magnetic torque, then it attains equilibrium and stops rotating any furthers.
4. Application of Galvanometer
Wheatstone Bridge Circuit
Galvanometer usually uses in Wheatstone bridge circuit. The purpose of Wheatstone bridge circuit is to measure resistance. Whitestone bridge consists of four resistance and galvanometer. It requires fixed, known resistance values for R1 and R2 and a variable resistor for R3. Variable resistor R3 is attached to a calibrated dial that shows its adjusted resistance value. Resistor Rx represents the resistance value to be measured. To determine Rx. The variable resistance R3 is adjusted until the galvanometer indicates zero current. Consequently, the galvanometer is usually termed a null detector. The zero galvanometer reading indicates that the bridge is balanced. When the galvanometer indicates null condition, the voltage on each side of galvanometer is equal. If R1 and R2 are the same value, then calibrated dial reading of R3 indicates the resistance value of Rx. If the values of R1 and R2 are different, the unknown resistance value for balanced bridge can be calculated with this equation :
Rx = R2 . R3 / R1
Hyperphysics web, accessed April 13, 2014. http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/galvan.html
Circuit specialists web, accessed April 13, 2014. http://www.circuitspecialists.com/content/88526/csm-1102a-0.jpg
Spark museum web, accessed April 13, 2014. http://www.sparkmuseum.com/GALV.HTM
City Collegiate web, accessed April 13, 2014. http://www.citycollegiate.com/galvanometer_XIIb.htm
Petruzella, Frank D. Industrial Electronic pg. 126-127. 1996. McGraw-Hill International Editions.
Picture : wordpress web, accessed April 13,2014. http://sandirahmadika.files.wordpress.com/2011/05/capture1.jpg