72 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



particular when such a motion leads to a local separation between the 

 conductor and the insulator. The phenomenon has been described by 

 Perls. 1 The effect can be considerably reduced by the application of a 

 conductive layer (carbon compound) which firmly adheres to the 

 inside and the outside of the dielectric. 



The capacitive displacement transducer is primarily useful where 

 very small forces are available. The transducer has the principal 

 advantage that the physical mechanism involved in its action does 

 not depend upon any physical property of materials, with the excep- 

 tion of a few constructions involving the use of dielectric materials 

 between the plates. Therefore, a high degree of stability and repro- 

 ducibility can be obtained. A disadvantage of the capacitive trans- 

 ducer is its relatively large output impedance, which requires careful 

 shielding and short connections to subsequent stages. 



The transducer has been vised for a wide variety of measurements of physical 

 magnitudes, such as pressure, thickness, acceleration, etc., which, as far as they 

 fall into the scope of this book, are described under the respective headings. 

 For the application of the capacitive transducer for the measurement of tem- 

 perature (on the order of 6 x 10 _5 °C) see W. Sucksmith, Phil. Mag., (6) 43, 

 223 (1922). The construction of a microbalance with the use of a capacitive 

 transducer is described by R. Whiddington and F. A. Long, Phil. Mag., (6) 49, 

 113 (1925). The smallest detectable mass (noise level) is about 10~ 9 gram. A 

 capacitance-type torque meter and a capacitance transducer with toothed disks 

 for it are described by H. G. Mills, J. Sci. Instr., 25, 151 (1948). For construc- 

 tional details on capacitive transducers, see also E. A. Holmes, III, in John F. 

 Blackburn (ed.), "Components Handbook," M.I.T. Radiation Laboratory 

 Series, vol. 17, chap. 9, McGraw-Hill Book Company, Inc., New York, 1949. 



1-24. Thermal Displacement Transducers 



a. Bolometer Systems. The operating principle of these transducers 

 is the following: A spiral made of platinum wire is heated by a con- 

 stant current. When the spiral is elongated, the cooling rate is in- 

 creased, and thus the temperature of the spiral and, in further 

 consequence, its resistance decreases. 



A complete transducer system is shown schematically in Fig. 

 (l-2)56. 2 Four platinum-wire spirals are electrically connected to 

 form a bridge and mechanically suspended between posts mounted 

 on a base plate and on a movable lever L\ a rotary displacement of 

 the lever in the direction of the arrow elongates the spirals A and D 

 and compresses the spirals B and C. The spirals are heated by the 

 bridge current; their temperature varies with their elongation. The 

 variation of temperature leads to a resistance variation and causes 



1 T. A. Perls, J. Appl. Physics, 23, 674 (1952). 



2 See also E. B. Moss, J. Sci. Instr., 7, 393 (1930). 



