. Electronic apparatus for biological research . Silver electrode shrouded in insulation except at tip Glycerin electrolyte ^Energizing voltage 1 ia Figure 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 mov
. Electronic apparatus for biological research . Silver electrode shrouded in insulation except at tip Glycerin electrolyte ^Energizing voltage 1 ia Figure 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 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
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