32 



VISUAL INDICATORS 



P. E. K. DONALDSON 



Visual indicators are those devices which permit the magnitude of electrical 

 quantities to be read off, usually by converting them into mechanical 

 movement along a scale. The most important are: the meter, for measuring 

 quantities which change only slowly, or not at all; and the cathode ray 

 tube, which displays quantities which change rapidly, usually in a system 

 of rectangular co-ordinates in which time is the independent variable. 



METERS 



In this section we are not concerned with galvanometers, in which the 

 over-riding consideration is sensitivity, or with precision meters, in which 

 accuracy is the principal object. We are interested in the familiar instrument, 

 of the grade known as 'Industrial', which is mounted on electronic equip- 

 ment as a fixture. 



The relevant meter movements are the electrostatic, the moving-iron and 

 the moving-coil, of which the latter is the most important. Hot wire, 

 dynamometer (wattmeter), ratiometer (Megger) and frequency meters are 

 not of great interest to the electrophysiologist; the latter is for checking 

 the frequency of the supply mains, and would not be appropriate for 

 measuring, for example, the spike-discharge rate of a nerve. 



Electrostatic voltmeter 



The structure of an electrostatic movement resembles an air-dielectric 

 variable capacitor. The potential difference to be measured is applied 

 between the two sets of plates, which are in consequence attracted together 

 against the action of a spring, to produce a movement dependent upon 

 the potential difference. The application of the voltage to the instrument 

 displaces a charge round the circuit so that one set of plates is charged 

 to +^, and the other to —q. Since the attractive force is proportional to 

 minus the product of the charges on the two sets of plates, = q^, the force 

 is proportional to the square of the applied voltage and the instrument is 

 'square-law'. Two important properties follow: 



(1) The scale of the instrument will be cramped near zero and extended 

 near maximum, though the effect can be to some extent offset by special 

 shaping of the plates. 



(2) If the applied voltage be alternating, or direct with an added ripple, 

 the instrument reads true R.M.S. values. 



Electrostatic voltmeters of the industrial grade are not very sensitive 

 devices and commonly have full scale readings of the order of kV. This, 



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