. Electronic apparatus for biological research . Energizing voltage lOOV ac. Figure 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 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 ) is due to Dr. D. W. Kennard, and has several novel features. First, a very stiff
. Electronic apparatus for biological research . Energizing voltage lOOV ac. Figure 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 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 ) 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, 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
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