VISUAL INDICATORS 



a false indication. Whilst the error as a percentage in volts might not be 

 very great, since light output changes very rapidly with applied voltage, 

 the error in light output could be serious. 



Moving-coil meter 



In the moving-coil meter the field due to the moving-coil reacts with a 

 steady field due to a powerful permanent magnet to twist the coil and 

 deflect the pointer against the action of a spring {Figure 33.25). The torque 

 depends on the product of the coil field and the permanent magnet field, 

 and by making the latter large a very sensitive movement can result. The 

 pointer deflection is proportional to the average coil current, and the scale 

 is linear. 



In a range of moving-coil ammeters and voltmeters of a particular size 

 and made by a particular manufacturer it is possible that they all contain 

 the same basic movement, perhaps 1 mA full-scale deflection and of 100 ohms 

 coil resistance. Higher current ranges are then obtained by the addition of 

 appropriate shunts, and the voltmeter range by suitable series resistors. 

 The sensitivity of voltmeters is often given in terms of 'ohms per volt', a 

 figure which is merely the reciprocal of the current for full-scale deflection. 

 Thus the 1 mA full-scale deflection movement yields a range of voltmeters 

 of 1 ,000 ohms/ V, and the resistance of the 300 V full-scale deflection member 

 of the range would be 300,000 ohms. Since the resistance of a voltm.eter 

 should be as high as possible, the ohms/V figure gives a measure of the 

 virtue of the instrument in this respect. 



For lower full-scale deflection currents than 1 mA it is necessary to have 

 a better movement, i.e. one with a stronger magnet, more turns of thinner 

 wire on the moving-coil, more friction-free pivots and weaker control 

 springs. The device is therefore more fragile both mechanically and 

 electrically. A 1 mA movement is probably as mechanically robust as 

 many other electronic components, but a 50 //A F.S.D. instrument should 

 be handled with circumspection. The most sensitive industrial grade 

 movement known to the writer is the Taylor, model 500, 0-5 /uA. He has 

 no experience of one of these, but would suggest it be afforded the same sort 

 of care in handling one would devote to, say, a large cathode ray tube. 



Sizes of meters 



Meters are described by the arc length of the scale, in inches. The 2, 

 2^ and 3| in. sizes are commonest, but tiny 1 in. models are available where 

 panel space is limited, and 5 in. movements may be used where the scale 

 has to be read with great precision. 



Accuracies of meters 



Accuracies for meters are laid down in a rather complicated British 

 Standard Specification, BS 89/54. The limits of error are expressed as 

 percentages of the full-scale deflection at 20°C. In the important case of 

 moving-coil meters not of the multi-range type, the following errors are 

 allowed over pointer indications in the range 10 to 100 per cent of full 

 scale. 



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