Moving-coil instruments

Thermocouple-type instruments measure the temperature produced when a precision resistor is heated by the signal. The electrical current produced by the thermocouple is used in a moving coil instrument to measure the signal. Since the heat produced in a resistance is proportional to the RMS value of the signal, accuracy is maintained while measuring waveforms containing high-frequency components. 

This instrument was developed from the hot-wire ammeter, some examples of which can still be found. In the modem equivalent, the current to be measured (or a known proportion of it) flows through a small element that heats a thermocouple, so producing a rms voltage at its terminals, which is a function of the current. This voltage then supplies a current to a permanent magnet, moving-coil movement. 

The voltage output of the more common types of thermocouple is of the order of 50V/C and the output is either read on a sensitive moving-coil meter or on a digital voltmeter. The reading is converted to temperature using a calibration chart supplied with the thermocouple. Some commercial units are available in which the thermocouple and instrument is supplied as an integral unit with the scale directly calibrated in temperature. If a separate instrument is to be used then it should be noted that the thermocouple resistance is only of the order of 10 and... 

The various types are dealt with in some detail in the following pages. Except where noted, the instruments have a moving-coil movement. 

By the use of suitable shunts, the basic moving-coil movement can be adapted to measure m almost unlimited range of currents. Figure 10.46 illustrates a direct-indicating instrument with shunt, to measure current up to 5 A d.c. To ensure that the resistance of the circuit is not materially altered by the insertion of an ammeter, it is usual to install either a shunt or the meter itself (usually a moving-coil meter with internal shunt) permanently in the circuit. Ammeter shunts are normally of the four-terminal type, to avoid contact resistance errors, i.e. two current terminals and two potential terminals, as shown, in fig. J0.46. 

Harrison-Foote Compensated Indicator.—This instrument manufactured by the Brown Instrument Co. is illustrated in simple form by Fig. 5. The circuit CDGF is an ordinary millivoltmeter in which G represents the moving coil. 

The application of cathodic protection generally involves the use of direct current. The movement in nearly all instruments used in cathodic protection is therefore of the moving-coil permanent-magnet type, which gives coil deflection (and thus pointer deflection) proportional to the current in the coil. 

The main types of instruments are the nwving iron (d.c. and a.c.) (Fig. S.l 1), the permanent magnet moving coil (d.c. and a.c.) (Fig. S.12), and the moving coil dynamometer type (a.c.), all analogue devices whereby the deflection shown is proportional to the quantity being measured. 

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