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Measurements of Current
Absolute measurements of current and potential difference are of importance primarily in laboratory work, so for most purposes relative measurements are sufficient. The meters described in the following paragraphs are all intended for relative measurements.
Galvanometers are the main instruments used to detect and measure current. They depend on the fact that force is generated by an electric current flowing in a magnetic field. The mechanism of the galvanometer is so arranged that a small permanent magnet or electromagnet sets up a magnetic field that generates a force when current flows in a wire coil adjacent to the magnet. Either the magnet or the adjacent coil may be movable. The force deflects the movable member by an amount proportional to the strength of the current. The movable member may have a pointer or some other device to enable the amount of deflection to be read on a calibrated scale.
In the D'Arsonval reflecting galvanometer, a small mirror attached to a movable coil reflects a beam of light on a scale of about 1 m (3 ft) away from the instrument. This arrangement involves less inertia and friction than does a pointer, and consequently, greater accuracy is achieved. The instrument is named after the French biologist and physicist Jacques d'Arsonval, who also conducted experiments with the mechanical equivalent of heat, and in the high-frequency oscillating current of low voltage and high amperage (D'Arsonval current) used in the treatment of certain diseases, such as arthritis. Known as diathermy, this treatment involves the production of heat in, for example, a limb, by passing a high-frequency current between two electrodes placed on the skin. The addition of a scale and proper calibration converts a galvanometer into an ammeter, the instrument used for measuring electric current in amperes. D'Arsonval was also responsible for inventing a direct-current (DC) ammeter.
Only a limited amount of current can be passed through the fine wire of a galvanometer coil. When large currents have to be measured, a shunt of low resistance is attached across the terminals of the meter. Most of the current is bypassed through this shunt resistance, but the small current flowing through the meter is still proportional to the total current. By taking advantage of this proportionality, a galvanometer can be used to measure currents of hundreds of amperes.
Galvanometers are usually named according to the magnitude of the currents they will measure.
A microammeter is calibrated in millionths of an ampere and a milliammeter in thousandths of an ampere.
Ordinary galvanometers cannot be used for the measurement of an alternating current (AC), because the alternation of the current would produce deflection in both directions.
An adaptation of the galvanometer, however, called an electrodynamometer, can be used to measure alternating currents by means of electromagnetic deflection. In this meter a fixed coil, in series with the moving coil, is employed in place of the permanent magnet of the galvanometer. Because the current in the fixed and moving coils reverses at the same instant, the deflection of the moving coil is always in the same direction, and the meter gives a constant current reading. Meters of this type can also be used to measure direct currents.
Another form of electromagnetic meter is the iron-vane meter or soft-iron meter. In this device two vanes of soft iron, one fixed and one pivoted, are placed between the poles of a long, cylindrical coil through which is passed the current to be measured. The current induces magnetism in the two vanes, causing the same deflection no matter what the direction of the current. The amount of the current is ascertained by measuring the deflection of the moving vane.
Meters that depend on the heating effect of an electric current are used to measure alternating current of high frequency. In thermocouple meters the current passes through a fine wire that heats a thermocouple junction; the electricity generated by the thermocouple is measured by an ordinary galvanometer. In hot-wire meters the current passes through a thin wire that heats and stretches. This wire is mechanically linked to a pointer that moves over a scale calibrated in terms of current.
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