TRANSDUCERS 



Unfortunately the maximum change of resistance which can be obtained 

 before the gauge wires pass their elastic limit, and hence change their calibra- 

 tion, is about 4 per cent. Furthermore, the power dissipated in the gauge 

 must not exceed, typically, about 25 mW lest its characteristics be affected. 

 Thus, with an unmounted strain gauge of 1,000 ohms in a suitable bridge, a 

 maximum output of 0-2 V would be obtained for a force of about 1 kg wt. 

 Their sensitivity to humidity, however, severely limits the applications of 

 unmounted strain gauges in biological research. 



Steel electrode coated 

 with insulating varnish 

 except at extreme tip 



Output 



Phosphor-bronze 

 diaphragm 



Tension thread 



Perspex 

 body 



Silver electrode 

 shrouded in 

 insulation except 

 at tip 



Glycerin 

 electrolyte 



^Energizing 

 voltage 



1 ia 



Figure 33.15 Liquid potentiometer for force measurement 



Another type of transducer is based on the properties of carbon contacts. 

 If a circuit includes a number of contacts between pieces of carbon, the 

 resistance of the circuit depends on the mechanical pressure at the contacts. 

 Devices based on this effect include the carbon pile, a stack of discs under 

 end-wise pressure, and the carbon microphone, with granules trapped 

 between a fixed and a moving electrode. All such devices tend to be non- 

 linear, noisy and subject to drift, and are therefore best considered for 

 qualitative work only. 



Variable inductance transducers 



The inductance of an iron-cored inductor may be varied by moving some 

 part of its magnetic circuit mechanically. This is the principle of the opera- 

 tion of the variable inductance transducer. 



Like the moving-iron transducer, to which it bears a superficial resem- 

 blance, the variable inductance transducer may occur in many different 

 configurations. Some of these are ilUustrated in Figure 33.16. Unlike 

 moving-iron devices, no permanent magnets are used and magnetic forces 

 are small. Armature control springs may be correspondingly lighter, and in 

 some cases it is possible to attach a light armature directly to the preparation 

 and use no springs at all. Alternatively, by using very stiff armature control 

 springs and very small movements, transducers of a high mechanical imped- 

 ance suitable for force measurement may be constructed. 



488 



