METERS 



coupled with the fact that they do not consume a steady current, makes 

 them appropriate for monitoring the values of EHT power supplies, where 

 the regulation is commonly so poor that the current demands of a meter 

 of some other type would produce a reading much too low. 



Moving-iron meters 



In one form of these, two pieces of soft iron are magnetized so that they 

 repel one another, under the influence of a solenoid which encircles them 

 both {Figure 32.1). The intensity of magnetization produced in each is 

 proportional to the current in the solenoid, and the repulsive force is pro- 

 portional to the product of the two intensities. One piece of iron is fixed, 





N 



N 



x»* 



Figure 32.1 



and the other is pushed away from it, against the action of a spring, to 

 deflect the pointer. Since the repulsive force is proportional to the square 

 of the current in the coil, the moving-iron movement is another square-law 

 device, may be used on alternating currents and reads true R.M.S. values. 

 The scale is cramped on the left and extended on the right, though matters 

 may be improved by suitable kinematic design. 



Moving-iron movements are not very sensitive devices, and the field 

 of application is therefore in the measurement of appreciable currents, 

 say greater than 50 mA. A certain firm produce a moving-iron movement 

 with full-scale deflection 5 mA, but to achieve this a large number of turns 

 of wire is necessary on the solenoid. The wire has therefore to be thin, and 

 the resistance is 12,000 ohms. Clearly the introduction of such a resistance 

 into many circuits in order to measure the current flowing will seriously 

 upset the status quo. Similarly, when voltmeters are made by connecting 

 the movement in series with a suitable resistor, the current drawn may 

 pull down the voltage being measured very seriously unless the source is 

 of low internal resistance. Hence, moving-iron voltmeters are suitable for 

 checking the supply mains, or the outputs of a mains transformer, but 

 would be quite unsuitable for finding the anode potential of a valve in an 

 R-C coupled amplifier. 



The value of a moving-iron movement in reading true R.M.S. may be 

 illustrated by an example : suppose a 6 V motor car headlamp is being 

 used as a light source and is fed from the mains via a constant-voltage 

 transformer. The colour temperature of the lamp depends upon the R.M.S. 

 applied voltage. The output of the constant-voltage transformer is not 

 sinusoidal in form. Then a moving-iron voltmeter across the lamp terminals 

 is the only appropriate meter movement; for an electrostatic instrument 

 would be two orders too insensitive, and a moving-coil and rectifier instru- 

 ment — which reads the average of the modulus of the input — would give 



447 



