Monday, May 11, 2009

Tangent galvanometer

TANGENT
GALVANOMETER














TABLE OF CONTENT


CONTENT NAME PAGE NO.

1. INTRODUCTION 1
2. REVIEW OF LITERATURE 2-3
3. THEORY AND WORKING 4-6
4. SUMMARY 7-8
5. BIBLIOGRAPHY 9









INTRODUCTION
A tangent galvanometer is an early measuring instrument used for the measurement of electric current. It works on the basis of tangent law of magnetism.It works by using a compass needle to compare a magnetic field generated by the unknown current to the magnetic field of the Earth. It gets its name from its operating principle, the tangent law of magnetism, which states that the tangent of the angle a compass needle makes is proportional to the ratio of the strengths of the two perpendicular magnetic fields. It was first described by Claude Servais Mathias Pouillet in 1837.
A tangent galvanometer consists of a coil of insulated copper wire wound on a circular non-magnetic frame. The frame is mounted vertically on a horizontal base provided with levelling screws. The coil can be rotated on a vertical axis passing through its centre. A compass box is mounted horizontally at the centre of a circular scale. It consists of a tiny, powerful magnetic needle pivoted at the centre of the coil. The magnetic needle is free to rotate in the horizontal plane. The circular scale is divided into four quadrants. Each quadrant is graduated from 0° to 90°. A long thin aluminium pointer is attached to the needle at its centre and at right angle to it.

Tangent Galvanometer by Claude Servais Mathias Pouillet in 1837.

The instrument has high sensitivity and one of its early jobs was
in the studies of electrophysiology by the inventor.



REVIEW OF LITRECTURE

*Claude-Servais-Mathias Pouillet to verify Ohm’s law.
Scientist’s Name: Claude-Servais-Mathias
Year of discovery: 1837
Title: to verify Ohm’s law.

The tangent galvanometer was first described in an 1837 paper by Claude-Servais-Mathias Pouillet, who later employed this sensitive form of galvanometer to verify Ohm's law. To use the galvanometer, it is first set up on a level surface and the coil aligned with the magnetic north-south direction.

*Professor W.A. Anthony


The Great Tangent Galvanometer
Cornell University. Ithaca, New York

Scientist’s Name: Professor W.A. Anthony
Year of discovery: 1885.
Title: for measurement of heavy currents and direct calibration

This is the great tangent galvanometer of Cornell University, dated 1885. Designed by Professor W.A. Anthony, it was developed to meet the needs of an instrument for the measurement of heavy currents and direct calibration of commercial instruments used for measuring currents in electric lighting, industry, etc.

*James Prescott Joule
Scientist’s Name: James Prescott Joule
Year: 1840
Title: to discover modern absolute system of electric measurements.

In 1840, he graduated his tangent galvanometer to correspond with the system of electric measurement he had adopted. The electric currents used in his experiments were thenceforth measured on the new system; and the numbers given in Joule's papers from 1840 downward are easily reducible to the modern absolute system of electric measurements.

*J. J. Nervander
Scientist’s Name: J.J. Neravander
Year: 1834
Title: to improve the measurements of electric current.

J.J. Nervander designed the more- sensitive tangent galvanometer in 1834, which led to a great improvement in precise measurements of electric current. Because of its ingenuous coiling arrangements, Germander was able to use the tangent busily to prove the validity of the law that the tangent of the deviation angle of the needle of the tangent-bus sol is proportional to the electric current flowing through its coil.

*Lord Kelvin's magneto-static tangent galvanometer
Scientist’s Name: Lord Kelvin
Year: 1887
Title: discovery of tangent galvanometer as lamp.

This form of the tangent galvanometer is designed by Lord Kelvin in c.1887. This is a magneto-static tangent galvanometer used as a lamp counter. The instrument originally consisted of a small magnet on an aluminium pointer suspended at the centre of two loops of heavy copper ribbon positioned above two sets of strong bar magnets.
It is very similar in construction to GLAHM 113325 suggesting that it was designed for use in a lighting system such as the one in Lord Kelvin's laboratory and lecture theatre.



THEORY AND WORKING

Construction
A TG consists of a circular coil of insulated copper wire wound on a circular non magnetic frame. The frame is mounted vertically on a horizontal base provided with levelling screws on the base. The coil can be rotated on a vertical axis passing through its centre. A compass box is mounted horizontally at the centre of a circular scale. The compass box is circular in shape. It consists of a tiny, powerful magnetic needle pivoted at the centre of the coil. The magnetic needle is free to rotate in the horizontal plane. The circular scale is divided into four quadrants. Each quadrant is graduated from 0° to 90°. A long thin aluminium pointer is attached to the needle at its centre and at right angle to it. To avoid errors due to parallax a plane mirror is mounted below the compass needle.



Theory
When current is passed through the TG a magnetic field is created at its centre given by where I is the current in ampere, n is the number of turns of the coil and r is the radius of the coil.
If the TG is set such that the plane of the coil is along the magnetic meridian i.e. B is perpendicular to ( is the horizontal component of the Earths magnetic field), the needle rests along the resultant. From tangent law, i.e.
or
where K is called the Reduction Factor of the TG.

Working
In the tangent galvanometer there is a circular coil having one or more turns of wire, at the centre of which a magnetic needle is either balanced on a point or suspended by a fine fibre of silk or quartz. The instrument is placed so that the plane of the coil is vertical and in the magnetic north and south plane (Figure 5(A)).

FIGURE 5(A)
When a current is sent through the coil the needle turns to one side or the other, and the strength of the current is proportional to the tangent of the angle of deflection. The force due to the current in the coil is at right angles to the plane of the coil at its centre and the strength of the field at that point in a given coil is proportional to the strength of the current (Figure 5(B)).

FIGURE 5(B)
Let G represent the strength of field at the centre due to the coil when unit current is flowing, then IG will be the strength of field when the current strength is I. Let OA in Figure 5(B) represent the plane of the coil and O the point where the needle is placed, then when no current is flowing the needle points in the direction OA, being acted on only by the horizontal component H of the earth's magnetic force. The magnetic force F due to the current in the coil is IG and at right angles to H, therefore, the resultant force R is the diagonal of the rectangle whose sides are IG and H, and

Where x is the angle which the resultant force makes with H. But the needle must point in the direction of the resultant force, and so x is the angle through which the needle turns. Therefore

And if H and G are known the current may be determined by measuring the angle x. In case of a tangent galvanometer the magnetic force F due to the coil is expressed by IG.
But if the current is measured in electromagnetic units,

And since the length of n turns of wire of radius r is,

The galvanometer coil constant G can be calculated from this formula when the coil of the galvanometer has so large a radius compared with the length of the needle that the poles of the needle may be regarded as at the centre, and when the cross section of the coil is so small that all the turns bear nearly the same relation to the needle. If G is determined in this way, r being measured in centimetres, and if H is found in C.G.S. units system, the current will be also found in C.G.S. electromagnetic units by the use of the formula:

To obtain the current strength in amperes, we must take as the value of the coil constant:

By this method the strength of a current is determined in amperes directly from the fundamental units of length, mass, and time, for we have already seen how the measurement of H is based on these units. A tangent galvanometer in which the constant is determined in this way directly from measurements of the coil is known as a standard galvanometer.






SUMMARY

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. The extent to which the needle turns is dependent upon the strength of the current. 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. A tangent galvanometer consists of a coil of insulated copper wire wound on a circular non-magnetic frame. It works on the basis of tangent law of magnetism.It works by using a compass needle to compare a magnetic field generated by the unknown current to the magnetic field of the Earth. It gets its name from its operating principle, the tangent law of magnetism, which states that the tangent of the angle a compass needle makes is proportional to the ratio of the strengths of the two perpendicular magnetic fields.


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

Central Scientific Tangent Galvanometer utilizing compass (1941)


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

Suggestion: One of the limitations of tangent 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 is solved ingeniously by using a beam of light as the needle; a shaft is placed through the centre of the needle and a very small mirror is 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. These types of galvanometers are called Reflecting Galvanometer.



BIBLIOGRAPHY

1. http://physics.kenyon.edu/EarlyApparatus/Electrical_Measurements/Tangent_Galvanometer/Tangent_Galvanometer.html
2. http://en.wikipedia.org/wiki/Galvanometer
3. http://www.historicalprintshop.com/web_pages/S/science/scientific.instruments/scientific.instruments.html
4. http://www.scran.ac.uk/database/record.php?usi=000-000-529-673-C&&
5. http://ieeexplore.ieee.org/Xplore/login.jsp?url=/iel5/5289/4534362/04534374.pdf?arnumber=4534374
6. http://www.scran.ac.uk/database/record.php?usi=000-000-529-497-C
7. http://www.economicexpert.com/a/Tangent:galvanometer.html
8. http://chem.ch.huji.ac.il/instruments/test/galvanometers.htm
9. http://www.sparkmuseum.com/GALV.HTM

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