30 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



require precision machining and, with the exception of special con- 

 struction, their resolution is limited. The mechanical force and 

 torque requirements are not always negligible. The transducers are 

 primarily applicable for large linear movements in the order of inches 

 or for angular movements of at least ten or more degrees. The life- 

 time is in general at least 10 6 full cycles of movement of the contact 

 shaft over the potentiometer travel. A potentiometer filled with a 

 lubricant liquid, manufactured by the Helipot Corporation, has a 

 lifetime of 10 7 shaft revolutions. 



b. Spring Transducer. A spring is wound so that its initial tension 

 increases uniformly over its length, Fig. (1-2)11. It may be wound as 



Force 



"Expansion 

 Useful range 



Extension 



Fig. (1-2)11. Spring transducer. 



Fig. (1-2)12. Calibration curve (resis- 

 tance versus extension) of a spring 

 transducer (private communication from 

 W. A. Wild hack). 



a cylindrical spring either with variable feed angle or variable initial 

 tension, orfrom wire with uniformly increasing cross section ; oritmay 

 be wound in the form of a tapered, i.e., cone-shaped helix. When the 

 spring is closed it represents, electrically, a cylinder surface, and the 

 current passes from one turn to the next in the direction of the arrow 

 A. When a force is applied which elongates the spring, the single 

 windings become separated one by one; the current then passes 

 through the separate turns in the direction of the arrow B. 



The resistance increases with the elongation, as shown in Fig. 

 (1-2)12; the ratio of the "open" to the "closed" resistance for gold- 

 plated high-resistivity springs ma} 7 be as high as 50 to 1. 



Spring transducers have been applied for elongations varying from 

 0.006 to 0.6 in. for full-scale operation. The forces required for such 

 displacements range from less than 1 to 100 grams, and the appli- 

 cation for higher forces presents no difficulty in principle. Displace- 

 ments as small as 10~ 5 in. (0.25 [x] may be measured with simple 

 equipment and without the use of amplifiers. 



