Moving coil

Spulen-galvanometer, n. moving-coil galvanometer. -lack, m. (Elec.) insulating varnish (Textiles) bobbin finish, -ol, n. spooling oil. -wicklung, /. (Elec.) coil winding, -wider-stand, m. (Elec.) coil resistance. 

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. 

In the coating of continuous metal coils, reverse roller coating is often used. In this technique the web is moving counter to the application roller direction, so that the paint is partly wiped off by the moving coil. Shear leads to better flowout. Another type of reverse roller coating is used for the application of stiff paste fillers to chipboard. Application is by forward roller, but this is immediately followed by a reverse roller, which presses the filler into the board and doctors it smooth. 

Ferrite magnets are used in the form of flat toroids in moving coil loudspeakers (Fig. 9.59(b)), and large pieces for magnetic separators for mineral beneficiation and water filters. 

Moving coil and magnet supplied by Rank-Wharfedale Ltd., Idle, U.K. 

DArsonval galvanometer — Historic moving-coil - galvanometer where a rectangular metal coil is hanging between the horseshoes of a permanent magnet. 

Loudspeakers. The conventional design of a fixed magnet and moving coil can be inverted. 

Calorimeter temperature was measured with a Leeds and Northrup G-l Mueller bridge used in conjunction with a d.c. Null Detector (No. 9834) or with a moving coil galvanometer (No. 2284-D) and lamp and scale. 

The temperature changes were measured using the thermistor located inside the calorimeter and its leads were taken out and connected to a Wheatstone Bridge comprising of Decade Resistance Boxes (fine and accurate set of electrical resistances) and a moving coil galvanometer. 

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. 

This adjustment is made in the following manner. By depressing the key K, a portion of the galvanometer resistance T2 is short circuited and the rest of the resistance rz containing the moving coil, is shunted by a resistance r. If e represents the potential drop across DF when the key is open and e" represents the potential drop when the key is closed, we obtain ... 

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