galvanometer: Definition and Much More from Answers.com

A galvanometer is a type of ammeter — an instrument for detecting and measuring electric current. It is an electromechanical transducer that produces a rotary deflection, through a limited arc, in response to electric current flowing through its coil. The name galvanometer has been applied to devices used in measuring, recording, and positioning equipment.

History

The term "galvanometer" derives from the surname of Luigi Galvani. Many early applications of galvanometers for measuring and recording are associated with William Thomson (Lord Kelvin). The earliest galvanometer was reported by Johann (Johan) Schweigger of Nuremberg at the University of Halle on 16th September 1820. André-Marie Ampère also contributed to the development of the galvanometer.

Operation

The most familiar use is as an analog measuring instrument, often called a meter. It is used to measure the direct current (flow of electric charges) through an electric circuit. Such devices are constructed with a small pivoting coil of wire in the field of a permanent magnet. The coil is attached to a thin pointer that traverses a calibrated scale. A tiny torsion spring pulls the coil and pointer to the zero position. In some meters, the magnetic field acts on a small piece of iron to perform the same effect as a spring.

When a direct current (DC) flows through the coil, the coil generates a magnetic field. This field acts with or against the permanent magnet. The coil twists, pushing against the spring, and moves the pointer. The hand points at a scale indicating the electric current. A useful meter generally contains provision for damping the mechanical resonance of the moving coil and pointer so that the pointer position smoothly tracks the current without excess vibration.

The basic sensitivity of a meter might be, for instance, 100 microamperes full scale (with a voltage drop of, say, 50 millivolts at full current). Such meters are often calibrated to read some other quantity that can be converted to a current of that magnitude. The use of current dividers, often called shunts, allows a meter to be calibrated to measure larger currents. A meter can be calibrated as a DC voltmeter if the resistance of the coil is known by calculating the voltage required to generate a full scale current. A meter can be configured to read other voltages by putting it in a voltage divider circuit. This is generally done by placing a resistor in series with the meter coil. A meter can be used to read resistance by placing it in series with a known voltage (a battery) and an adjustable resistor. In a preparatory step, the circuit is completed and the resistor adjusted to produce full scale deflection. When an unknown resistor is placed in series in the circuit the current will be less than full scale and an appropriately calibrated scale can display the value of the previously-unknown resistor.

Because the pointer of the meter is usually a small distance above the scale of the meter, parallax error can occur when the operator attempts to read the scale line that "lines up" with the pointer. To counter this, some meters include a mirror along the markings of the principal scale. The accuracy of the reading from a mirrored scale is improved by moving the head while reading the scale so that the pointer and the reflection of the pointer are aligned; at this point, the operator's eye must be directly above the pointer and any parallax error has been minimized.

Types

Extremely sensitive measuring equipment once used mirror galvanometers that substituted a mirror for the pointer. A beam of light reflected from the mirror acted as a long, massless pointer. Such instruments were used as receivers for early trans-Atlantic telegraph systems, for instance. The moving beam of light could also be used to make a record on a moving photographic film, producing a graph of current versus time, in a device called an oscillograph.

Galvanometer mechanisms are used to position the pens of analog chart recorders such as used for making an electrocardiogram. Strip chart recorders with galvanometer driven pens might have a full scale frequency response of 100 Hz and several centimeters deflection. In some cases (the classical polygraph of movies or the electroencephalograph), the galvanometer is strong enough to move the pen while it remains in contact with the paper; the writing mechanism may be a heated tip on the needle writing on heat-sensitive paper or a fluid-fed pen. In other cases (the Rustrak recorders), the needle is only intermittently pressed against the writing medium; at that moment, an impression is made and then the pressure is removed, allowing the needle to move to a new position and the cycle repeats. In this case, the galvanometer need not be especially strong.

Tangent galvanometer

A tangent galvanometer is a measuring instrument used for the measurement of electric current. It works on the basis of the tangent law of magnetism. It was first described by Claude Servais Mathias Pouillet in 1837.

A tangent galvanometer consists of a circular coil of insulated copper wire wound on a circular non-magnetic frame. The wire connected to the tangent galvanometer has to be wound, otherwise the field due to the wire will affect the deflection and an incorrect reading will be obtained. 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.^[1]

Theory

When current is passed through the tangent galvanometer a magnetic field is created at its corners 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 B_H (B_H is the horizontal component of the Earth's magnetic field), the needle rests along the resultant. From tangent law, B = B_HTanθ, i.e.

or

or I = KTanθ, where K is called the Reduction Factor of the tangent galvanometer.

The value of θ is taken at 45 degrees for maximum accuracy.

Geomagnetic field measurement

A tangent galvanometer can also be used to measure the magnitude of the horizontal component of the geomagnetic field. When used in this way, a low-voltage power source, such as a battery, is connected in series with a rheostat, the galvanometer, and an ammeter. The galvanometer is first aligned so that the coil is parallel to the geomagnetic field, whose direction is indicated by the compass when there is no current through the coils. The battery is then connected and the rheostat is adjusted until the compass needle deflects 45 degrees from the geomagnetic field, indicating that the magnitude of the magnetoc field at the center of the coil is the same as that of the horizontal component of the geomagnetic field. This field strength can be calculated from the current as measured by the ammeter, the numer of turns of the coil, and the radius of the coils.

Uses

Use of galvanometers declined late in the 20th century, largely replaced by Time-domain reflectometers for finding faults in telecommunications cables.

Mirror galvanometer systems are used as beam positioning elements in laser optical systems. These are typically high power galvanometer mechanisms used with closed loop servo control systems. They can have frequency responses over 1 kHz.

Galvanometers have been replaced as measuring instruments by analog to digital converters (ADC) for most uses. There are, for instance, self contained digital measuring systems, called digital panel meters (DPMs), available to replace most traditional analog meter functions.

A galvanometer appeared in an episode (#12, season 2) of the television medical drama House to function as an electrocardiogram for a patient whose severe and extensive burns prevented use of the normal electrodes.

References

External links

• Selection of historic galvanometer in the Virtual Laboratory of the Max Planck Institute for the History of Science