Sec. 1-2] MECHANICAL IX PUT TRANSDUCERS 23 



square centimeter) is equal to that of the gas layer of the thickness 

 Ai?. Test specimens of nonuniform thickness cause an absorption 

 characteristic as shown in curve c, so that thickness nonuniformity 

 can be detected and measured. 



The accuracy is about 2 per cent for 25- /u films of polystyrene and 

 4 per cent for o-/ti films. 



With the exception of the alpha gauge, the error of radiation-gauge 

 measurements is, in general, between 1 and 2 per cent, sometimes 

 less. The operation is not affected by temperature or pressure varia- 

 tion nor by a variation of the position of the test object between 

 the source and the detector. Only if very thin foils are measured 

 are errors likely to arise from absorption in the air gap, which varies 

 with temperature and pressure. The mass absorption equivalent of 

 a 1-cm air gap is 1.18 mg/cm 2 . Variation of the source intensity with 

 time must be corrected. 



A principal advantage of the radiation gauge is the fast and direct 

 indication without the need for a mechanical contact between the 

 gauge and the test object. Radiation hazards and toxicity can, in 

 general, be avoided by adequate shielding and sealing of the source. 



1-2. Transducers for Displacement or Force 



The following transducer systems furnish an electric signal in 

 response to a displacement or a force. 



Every displacement transducer requires a force for its actuation. 

 If the force required to produce a useful output is small or negligible 

 compared to the forces available from the actuating system, the 

 transducer acts as a true displacement transducer. If the displace- 

 ment required by the transducer is small or negligible compared to 

 the displacement of the actuating system, the transducer acts as a 

 true force transducer. Practical transducers are neither pure force 

 nor pure displacement transducers. 



Force- or displacement-transducer systems are based either on 

 resistance variation (1-21), inductance variation (1-22), or capaci- 

 tance variation (1-23). A group of electrothermal systems in which 

 the temperature and the resistance of an electrically heated w r ire 

 vary in response to a displacement is described in 1-24. Systems 

 based on the piezoelectric effect will be found in 1-25. A number of 

 systems based upon physical mechanisms other than those described 

 above are compiled in 1-26. 



Displacements and forces can sometimes be converted into electric 



