..... Click the link for more information. 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 in 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. In the first galvanometers, a freely turning magnetic needle was hung in a coil of wire; in later versions the magnet was fixed and the coil made movable. Modern galvanometers are of this movable-coil type and are called d'Arsonval galvanometers (after Arsène d'Arsonval, a French physicist). If a pointer is attached to the moving coil so that it passes over a suitably calibrated scale, the galvanometer can be used to measure quantitatively the current passing through it. Such calibrated galvanometers are used in many electrical measuring devices. The DC ammeter ammeter (ăm`mē'tər), instrument used to measure the magnitude of an electric current of several amperes or more.
..... Click the link for more information. , an instrument for measuring direct current, often consists of a calibrated galvanometer through which the current to be measured is made to pass. Since heavy currents would damage the galvanometer, a bypass, or shunt, is provided so that only a certain known percentage of the current passes through the galvanometer. By measuring the known percentage of the current, one arrives at the total current. The DC voltmeter voltmeter, instrument used to measure differences of electric potential , commonly called voltage, in volts or units that are multiples or fractions of volts. A voltmeter is usually combined with an ammeter and an ohmmeter in a multipurpose instrument.
..... Click the link for more information. , which can measure direct voltage, consists of a calibrated galvanometer connected in series (see electric circuit electric circuit, unbroken path along which an electric current exists or is intended or able to flow. A simple circuit might consist of an electric cell (the power source), two conducting wires (one end of each being attached to each terminal of the cell), and a
..... Click the link for more information. ) with a high resistance. To measure the voltage between two points, one connects the voltmeter between them. The current through the galvanometer (and hence the pointer reading) is then proportional to the voltage (see Ohm's law Ohm's law (ōm) [for G. S. Ohm ], law stating that the electric current i flowing through a given resistance r
..... Click the link for more information. ).
galvanometer
Instrument for measuring small electric currents by deflection of a moving coil. A common galvanometer consists of a light coil of wire suspended from a metallic ribbon between the poles of a permanent magnet. As current passes through the coil, the magnetic field it produces reacts with the magnetic field of the permanent magnet, producing a torque. The torque causes the coil to rotate, moving an attached needle or mirror. The angle of rotation, which provides a measure of the current flowing in the coil, is measured by the movement of the needle or by the deflection of a beam of light reflected from the mirror.
galvanometer
Galvanometer
A device for indicating very small electric currents. Although the deflection of a galvanometer results from current in the moving coil, the voltage in a closed circuit producing this current is frequently the quantity of interest to the user. In this mode, galvanometers are used to detect a null or an unbalanced condition in a bridge or potentiometer circuit. Electronic instruments that employ amplifying circuits to achieve sensitivities approaching the nanovolt level are also used as balance detectors in bridge circuits. However, in applications where extreme sensitivity and high rejection of ac signals are required, a galvanometer in combination with a photocell amplifier is to be preferred.
Galvanometers may also be used ballistically to integrate a transient current, as from the discharge of a capacitor, or a transient voltage, as produced when a coil moves relative to a magnetic field. See Current measurement, Voltage measurement
The d'Arsonval galvanometer is the most common type and is widely used. Its indicating system consists of a light coil of wire suspended from a copper or gold ribbon a few thousandths of an inch wide and less than 0.001 in. (0.025 mm) thick. This coil, free to rotate in the radial field between the shaped pole pieces of a permanent magnet, carries a small mirror which serves as an optical pointer and indicates the coil position by reflecting a light beam onto a fixed scale. Current is conducted to and from the coil by the suspension ribbons. The torque which deflects the indicating element is produced by the reaction of the coil current with the magnetic field in which it is suspended.
Voltage sensitivity is of importance in applications where the galvanometer serves as the detector of unbalance in a bridge or potentiometer network. The energy of motion in the indicating system must be dissipated and the system brought to rest in order for the deflection to be evaluated. This process, common to all indicating systems in which equilibrium must be achieved between a driving torque and a restoring torque, is called damping. See Damping
The sensitivity of modern galvanometers ranges up to 0.04 in. (1 mm) of deflection, on a scale 40 in. (1 m) distant from the mirror, for a current of 0.00001 microampere. Such a galvanometer may have a coil resistance of 800 ohms and a critical damping resistance of 100,000 &OHgr;. The voltage response of this instrument amounts to 0.04 in./microvolt (1 mm/μV) at critical damping. A galvanometer designed for voltage sensitivity, has a coil resistance of 20 &OHgr;, a critical damping resistance of 30 &OHgr;, and a response of 0.04 in. (1 mm) for 0.05 μV in the critically damped circuit. It will be seen from these examples that a large response to current is associated with large coil resistance and high critical damping resistance, whereas voltage response is associated with low coil resistance and low critical damping resistance.
The limitations reached in improving a galvanometer's resolution by increasing its scale length or using multiple reflections can be overcome by operating the galvanometer in its equilibrium position by using a negative-feedback network. In this system the beam of light reflected from the galvanometer mirror is directed onto a pair of photocells connected in series opposition (see illustration). As the galvanometer coil rotates, the light received on one photocell increases while the other decreases, and an amplified output current is produced whose sign and magnitude depend on the direction and amount of the coil deflection, respectively. The photocell output current is monitored by an external indicator and is approximately 1800 times the galvanometer input current. This photocell output also provides a negative-feedback current to servo the galvanometer at its equilibrium position, so that the linearity and input resistance of the system are increased. In such an arrangement it is frequently possible to approach rather closely the theoretical limit of resolution of angular motion imposed by the brownian motion of the coil, or by the Johnson noise of the circuit connected to the galvanometer. In a low-resistance galvanometer this theoretical resolution may be about 0.001 μV. See Brownian movement, Kinetic theory of matter
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A galvanometer is an extremely efficient low-pass filter, and it is still the most sensitive low-level detector at input impedances of 1 k&OHgr; or less. At higher input impedances, chopper-stabilized amplifiers, called nanovoltmeters, exhibit better resolution, noise, and drift characteristics than galvanometers. These instruments modulate the input dc signal by using a chopper (mechanical contact, transistor, or photocell switching), then amplify the signal by traditional ac techniques to achieve sensitivities of 10 nV or less. State-of-the-art digital voltmeters are now available with nanovolt resolution and are useful as low-level detectors. See Ammeter, Electrical measurements, Electrometer, Voltmeter
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