TRANSDUCERS 



likely to be sufficient for measuring pressure variations in nearly all bio- 

 logical systems. 



In addition to describing the manometer, Hansen^" also reviews previous 

 designs, most of which are m-e transducers. The same reference contains 



Coaxial 



output 



cable 



Insulation 



Fixed capacitor 

 plate 



Phosphor-bronze 

 diaphragm 

 (moving capacitor 

 plate) 



Pressure 



applied 



here 



Figure 33.40 Hansen capacitance manometer 



some very sophisticated examples of the use of electrical equivalent circuits 

 in the solution of mechanical problems. 



Investigation of the vibration sensitivity of spiders 



In an attempt to discover how a spider detects the presence of its prey 

 on the web, Dr. D. A. Parry has carried out some preliminary experiments 

 on vibrations of the web, using transducers designed and constructed by 

 J. A. Popple*. These experiments illustrate well some of the difficulties 

 encountered in the application of transducers to biological material ; they 

 also illustrate how a simple transducer, designed specifically for the one 

 purpose, may lead to results otherwise obtainable only with delicate and 

 complex apparatus. 



The first experiments sought to measure the vibrations induced in a web 

 by a fly caught on it. A very small and light differential transformer of the 

 type shown in Figure 33.17 was used. The armature, which was brought 

 into contact with the web, was a mumetal wire about 0-1 mm diameter and 

 5 mm long, flexibly mounted through a sheet of rubber 0-05 mm thick. 

 For easy recording of vibration frequencies up to 1 ,000 c/s an energizing 

 frequency of 10 kc/s was used, with the output of the transducer feeding 

 the Y plates of an oscillograph. A moving-film camera gave a record in 



* I am indebted to Dr. Parry and Mr. Popple for allowing me to describe this preliminary 

 (and unpublished) work, particularly as both successful and unsuccessful applications of 

 transducers are mentioned. 



506 



