METERS 



Similar accuracies are decreed for moving-iron voltmeters, and moving- 

 iron ammeters are allowed slightly larger errors, but in both cases the 

 limits only have to be met between 20 and 100 per cent of full scale. 



Non-linear moving-coil movements 



Particularly in war surplus equipment, one occasionally comes across 

 moving-coil movements having non-linear relationships between current 

 and deflection, often approximately logarithmic, and these are often useful. 



Figure 32.2 



In the case of end-zero meters, the law may be obtained by shaping the 

 permanent magnetic pole pieces as shown in Figure 32.2. With the coil 

 in the position shown the permanent-magnet field encountered is strong, 

 but weakens as the coil turns clockwise. This gives a scale which is expanded 

 on the left and cramped at the right. Centre-zero movements for null- 

 detection may be made sensitive in the region of the null-point by using 

 pole pieces as shown in Figure 32.3, or by using conventional pole pieces 

 and an elliptical central armature {Figure 32.4). 



Figure 32.3 



Figure 32.4 



Forms of construction such as these afford mechanical protection to 

 the movement from excessive currents, since a current much in excess of 

 normal is required to fling the pointer hard enough on the end stop to 

 cause damage : they cannot, of course, prevent the coil itself from accidental 

 burn-out. 



449 



