Electric Meters

V

Measurements of Current

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 galvanometer, a small mirror attached to a movable coil reflects a beam of light on a scale about 1 m (about 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 devised the first reflecting galvanometer. He 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 (diathermy treatment). 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 must 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.

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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.

VI

Measurement of Voltage

The instrument most generally used to measure potential difference, or voltage, is a galvanometer with a high resistance in series with the coil. When such a meter is connected across a battery, or to two points in an electrical circuit between which a potential difference exists, a small current (limited by the series resistor) will pass through the meter. The current is proportional to the voltage, and the latter quantity can be measured if the galvanometer is calibrated appropriately. By using the proper values of series resistors, one galvanometer can be used to measure a large range of voltages. The most accurate instrument for the determination of voltage, resistance, or direct current is the potentiometer, which indicates an unknown electromotive force by comparing it with a known value.

For the measurement of AC voltages, AC meters having high internal resistance, or similar meters with high series resistance, are employed.

Other methods for measuring the value of voltages depend on vacuum tubes and electronic circuits (see Electronics) and are especially useful in measurements at high frequencies. One such device is the vacuum-tube voltmeter. In the simplest form of this meter an AC voltage is rectified by a diode tube, and the rectified current is measured by an ordinary galvanometer. Other such voltmeters employ the amplifying characteristics of electronic tubes to measure extremely small voltages. The cathode-ray oscilloscope can also be used for voltage measurements because the deflection of the electron beam is proportional to the voltage impressed on the deflection plates or coils.

VII

Miscellaneous Measurements

The most accurate measurements of resistance are made with a galvanometer in a circuit called a Wheatstone bridge, named after the British physicist Charles Wheatstone. This circuit consists of three known resistances and an unknown resistance connected in a diamond pattern. A DC voltage is connected across two opposite points of the diamond, and a galvanometer is bridged across the other two points. When all four of the resistances bear a fixed relationship to each other, the currents flowing through the two arms of the circuit will be equal, and no current will flow through the galvanometer. By varying the value of one of the known resistances, the bridge can be made to balance for any value of unknown resistance, which can then be calculated from the values of the other resistors. Similar bridges, substituting known inductances and known capacitances for the resistance arms of the bridge, are employed in the measurement of the inductance and capacitance of circuit components. Bridges of this type are usually known as AC bridges, because AC sources are used rather than DC sources. These bridges are often balanced by means of a telephone receiver rather than a galvanometer. When the bridge is unbalanced, a tone will be heard in the receiver corresponding to the frequency of the AC source, but when the bridge is balanced, no tone will be heard.

The power consumed by any part of an electric circuit is most easily measured by a wattmeter, an instrument resembling the electrodynamometer. The wattmeter has its fixed coil connected so that the whole current of the circuit passes through it, and the moving coil is connected in series with a high resistance so that the current passing through it is proportional to the voltage of the source. The resulting deflection of the moving coil depends on both the current and the voltage and can be calibrated directly in power, because power is the product of voltage and current. The watt-hour meter, also known as a service meter, is a device to measure the total energy consumed in a circuit such as a home electrical circuit. It resembles the wattmeter except that the movable coil is replaced by a motor armature. The armature, which is regulated by a magnetic governor, revolves at a speed proportional to the amount of power consumed. The armature shaft is geared to a series of dials that indicate the total energy consumed.

See also Electricity; Electroscope.