STATIC MECHANOELECTRICAL TRANSDUCERS 



entirely on the suspension of the moving electrode. In some cases it is 

 possible to attach this electrode directly to the preparation, in which case the 

 loading effect of the transducer can be made negligible. When the trans- 

 ducer is operated through a thread or chain, some form of spring suspension 

 is necessary ; the compliance of this spring determines the impedance of the 

 transducer. 



Torsion wire 

 control spring 



Operating arm with 

 platinum wire tip 



Output 



Perspex trough 



containing 



glycerin 



Energizing 

 voltage lOOV ac. 



Figure 33.14 Liquid potentiometer for displacement measurement 



Two examples of liquid potentiometers will serve to illustrate their 

 possibilities. The first, due to J. A. Popple, is illustrated in Figure 33.14. 

 An output of 50 V per cm movement can be obtained with an input com- 

 pliance of more than 10"^ cm per dyne (10 cm per g wt). This transducer has 

 been used to study the gill movements of fishes. The second design {Figure 

 33.15) is due to Dr. D. W. Kennard, and has several novel features. First, a 

 very stiff suspension is used as the transducer is designed for the measure- 

 ment of muscle forces under isometric conditions. Secondly, d.c. energiza- 

 tion is used. It is found that, provided the voltage across the electrolyte is 

 low enough ('->-'l V), polarization effects stabilize after about an hour, and 

 that no bubbling occurs. The suspension compliance is about 10"'^ cm per 

 dyne (1 fx per g wt), giving a usable response up to about 5 kc/s. A sensitivity 

 of 1 /iV/dyne (ImV per g wt) is obtained. 



Resistance transducers based on deformation — If a length of wire is subject 

 to stress the resulting elongation and change of area alters its resistance. 

 This principle is used in the resistance strain gauge, which consists of many 

 turns of resistance wire wound on a paper former. In the engineering 

 applications for which these gauges were designed the unit is cemented to a 

 structure under test; the exceedingly small extensions in the structure can 

 then be measured electrically. Strain gauges can, however, be used un- 

 mounted, or bonded on to flexible materials such as rubber. In this way 

 transducers with a very low compliance (10~^ cm per dyne, OT /^ per g wt), 

 suitable for measuring force without any appreciable extension, can be made. 



487 



