THE USE OF TRANSDUCERS IN BIOLOGY 



the form of an amplitude-modulated strip, whose envelope described the 

 vibrations experienced at the transducer. 



While some results were obtained by this method, it was found that the 

 transducer was much too stiff (i.e. was of too high mechanical impedance) 

 and as a result the web near the point of contact was soon torn. However 

 delicately the transducer armature was constructed, the impedance it 

 presented at the web was always so much greater than the impedance of 

 the web fibres that the resulting stresses damaged the web. Here, then, is 

 a case where the biological material is so delicate and the forces involved 

 so small that not even the most refined m-e transducer is likely to be of 

 value. 



A similar result was obtained when an attempt was made to induce 

 vibrations in the web with a moving-coil e-m transducer (a moving-coil 

 loudspeaker with the cone removed, fitted with a stylus). The relatively 

 high output impedance of the transducer again caused damage to the web. 

 However, a moving-iron transducer of very low mechanical impedance, 

 and of great simplicity, finally proved successful. A very small iron turning 

 was placed on the web, and subjected to an alternating magnetic field from 

 an iron-cored solenoid driven from an oscillator. This placed virtually 

 no constraint on the web and made it possible to originate controlled 

 vibrations at any point in it. 



A variable mechanical load 



It is known that the flight muscles of certain insects, when stimulated, 

 can display cyclical changes of length at a much higher frequency than 

 that of the stimulus. The frequency and amplitude of these oscillations 

 is dependent on the external mechanical load applied to the muscle; this 

 load in the living insect is of course the wing, and the oscillation produces 

 the wing beat. 



The first experiment demonstrating this effect in an isolated muscle 

 preparation was due to Boettiger; he attached the muscle to a lever 

 carrying a mass to provide inertia, and had a friction arrangement to give 

 damping. To gather more data in the relatively short life-time of the 

 preparation, the author has devised an apparatus^^ whereby mechanical 

 loads can be generated electronically. Any combination of mass, viscosity 

 and compliance can be produced, the amount of each being instantaneously 

 variable by potentiometers in the electronic circuits. In addition any of 

 the components may be made negative, so that mass, compliance or viscosity 

 present in the preparation can be cancelled out. 



Figure 33.41 shows the block diagram of the equipment. The muscle 

 preparation is attached to the output shaft of a moving-coil e-m transducer. 

 A vane is attached to the same shaft and moves in the path of a beam of 

 light to a photocell. The output voltage of the photocell is thus made 

 proportional to the displacement x of the output member. This voltage is 

 then differentiated twice, giving voltages proportional to x and x; voltages 

 proportional to —x, —x and —x are generated by inverting amplifiers. 

 With the arrangement shown, fractions a, /3, y (which may be positive or 

 negative) of the three voltages representing x, x and x are selected and 



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