24 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



signals by one of the thickness-gauge systems described in 1-1. 

 Liquid-level displacements are to be found in 1-5. 



For summarizing references see H. C. Roberts, "Mechanical Measurements 

 by Electrical Methods," The Instruments Publishing Company, Inc., Pitts- 

 burgh, 1951 ; and Paul M. Pflier, "Elektrische Messung mechanischer Grossen," 

 3d ed., Springer Verlag, Berlin, 1948. A compilation of commercial transducers 

 can be found in D. B. Kret, "Transducers," published by Allen B. DuMont 

 Laboratories, Inc., Clifton, N.J. 



1-21. Resistive Displacement Transducers 



a. Slide-wire Resistors. A resistance wire or ribbon with a sliding 

 contact, as shown in Fig. (1-2)1, represents the simplest form of a 



Resistance wire 



Mec_hqnjcol_ 

 input 



^A 



■oC 



^B 



Fig. (1-2)1. Slide-wire resistance dis- 

 placement transducer. 



Fig. (1-2)2. Mercury-filled resistive 

 displacement transducer (from W. 

 Geyger, Arch. tech. Messen,V 3821-1, 

 November 1935; by permission). 



displacement transducer for the continuous conversion of longi- 

 tudinal or rotational displacements into electric signals, but lends 

 itself in general only to the construction of transducers with low 

 resistance, up to about 10 ohms. A special mechanical construction 

 of a potentiometer with a continuous setting permits the use of thin 

 wires with a resistance up to 250 ohms for a 360° shaft movement. 1 

 A modification of the slide-wire transducer is illustrated in Fig. 

 (1-2)2. It consists of a doughnut-shaped glass tube containing a 

 platinum wire (resistance about 15 ohms) or a carbon filament (100 

 ohms) . The tube is partly filled with mercury in a hydrogen atmos- 

 phere. Rotation of the tube about its axis P causes a variation of the 

 resistances R l and R 2 . This transducer is useful primarily for the 

 conversion of large angles ; the torque requirements are several gram- 

 centimeters. The accuracy is limited by the surface tension of the 



1 Spiralpot, trademark, G. M. Giannini & Co., Inc., East Orange, N.J. 



