Faugami When a current, which is steady, is passes through the coil of galvanometer, it is deflected. Such meters are often calibrated to read some other quantity that can be converted to a current of that magnitude. John Wiley and Sons. A major early use for galvanometers was for finding faults in telecommunications cables. It consisted of horizontal magnets suspended from a fine fiber, inside a vertical coil of wire, with a mirror attached to the magnets.
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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. When a direct current DC flows through the coil, the coil generates a magnetic field. This field acts 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.
Careful design of the pole pieces ensures that the magnetic field is uniform, so that the angular deflection of the pointer is proportional to the current. A useful meter generally contains provision for damping the mechanical resonance of the moving coil and pointer, so that the pointer settles quickly to its position without oscillation.
The basic sensitivity of a meter might be, for instance, 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.
These capabilities to translate different kinds of electric quantities, in to pointer movements, make the galvanometer ideal for turning output of other sensors that outputs electricity in some form or another , into something that can be read by a human. 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. Since the s, galvanometer-type analog meter movements have been displaced by analog to digital converters ADCs for many uses. A digital panel meter DPM contains an analog to digital converter and numeric display. The advantages of a digital instrument are higher precision and accuracy, but factors such as power consumption or cost may still favour application of analog meter movements.
Modern uses Edit Most modern uses for the galvanometer mechanism are in positioning and control systems. Galvanometer mechanisms are divided into moving magnet and moving coil galvanometers; in addition, they are divided into closed-loop and open-loop - or resonant - types. Mirror galvanometer systems are used as beam positioning or beam steering elements in laser scanning systems.
For example, for material processing with high-power lasers, closed loop mirror galvanometer mechanisms are used with servo control systems. Closed-loop mirror galvanometers are also used in similar ways in stereolithography , laser sintering , laser engraving , laser beam welding , laser TVs , laser displays and in imaging applications such as retinal scanning with Optical Coherence Tomography OCT.
Almost all of these galvanometers are of the moving magnet type. The closed loop is obtained measuring the position of the rotating axis with an infrared emitter and 2 photodiodes. This feedback is an analog signal. Open loop, or resonant mirror galvanometers, are mainly used in some types of laser-based bar-code scanners, printing machines, imaging applications, military applications and space systems.
Their non-lubricated bearings are especially of interest in applications that require functioning in a high vacuum. A galvanometer mechanism center part , used in an automatic exposure unit of an 8 mm film camera , together with a photoresistor seen in the hole on top of the leftpart. Past uses Edit A major early use for galvanometers was for finding faults in telecommunications cables. They were superseded in this application late in the 20th century by time-domain reflectometers.
Galvanometer mechanisms were also used to get readings from photoresistors in the metering mechanisms of film cameras as seen in the adjacent image. In analog strip chart recorders such as used in electrocardiographs , electroencephalographs and polygraphs , galvanometer mechanisms were used to position the pen. Hans Oersted Edit The deflection of a magnetic compass needle by current in a wire was first described by Hans Oersted in The phenomenon was studied both for its own sake and as a means of measuring electric current.
Early designs increased the effect of the magnetic field generated by the current by using multiple turns of wire. The instruments were at first called "multipliers" due to this common design feature. Poggendorff and Thomson Edit Thomson mirror galvanometer, patented in These were called "tangent" galvanometers and had to be oriented before use.
Main article: Mirror galvanometer An early mirror galvanometer was invented in by Johann Christian Poggendorff. The deflection of a light beam on the mirror greatly magnified the deflection induced by small currents. Alternatively, the deflection of the suspended magnets could be observed directly through a microscope. A mirror attached to the coil deflected a beam of light to indicate the coil position. Weston galvanometer in portable case Edward Weston extensively improved the design.
He replaced the fine wire suspension with a pivot, and provided restoring torque and electrical connections through spiral springs rather like those of a wristwatch balance wheel hairspring. He developed a method of stabilizing the magnetic field of the permanent magnet, so the instrument would have consistent accuracy over time. He replaced the light beam and mirror with a knife-edge pointer that could be read directly.
A mirror under the pointer, in the same plane as the scale, eliminated parallax observation error. This improved linearity of pointer deflection with respect to coil current. Finally, the coil was wound on a light-weight form made of conductive metal, which acted as a damper. By , Edward Weston had patented and brought out a commercial form of this instrument, which became a standard electrical equipment component. It was known as a "portable" instrument because it was affected very little by mounting position or by transporting it from place to place.
This design is almost universally used in moving-coil meters today. The jewel pivots and hairsprings are replaced by tiny strips of metal under tension. Such a meter is more rugged for field use. Tangent galvanometer Edit A tangent galvanometer is an early measuring instrument used for the measurement of electric current. 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 Johan Jacob Nervander in see J. Venermo and A. 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. 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. In operation, the instrument is first rotated until the magnetic field of the Earth, indicated by the compass needle, is parallel with the plane of the coil.
Then the unknown current is applied to the coil. The compass needle responds to the vector sum of the two fields, and deflects to an angle equal to the tangent of the ratio of the two fields. Tangent Galvanometer.
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