98 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



changes rapidly as the liquid sodium moves into the plane of the coil. 

 The method is successful because the tube material is but feebly 

 magnetic, and the liquid metal is a good electrical conductor. The 

 transfer characteristic is nonlinear. A similar system for the detec- 

 tion of conductive liquids (acids, etc.) is described by Broadhurst. 1 



A different construction for the conversion of the level variation 

 of a mercury column into an electric signal is shown in Fig. (1-4)6. 



d 



U 



Fig. (1-4)5. Inductive-level trans- 

 ducer for liquid metals. 



Fig. (1-4)6. Alternative arrangement 

 of a level transducer for liquid metal 

 (mercury). 



The mercury column surrounds the iron core and forms the secondary 

 winding of a transformer. Depending upon the level of the mercury, 

 the resistance of this winding (and, thus, the inductance, the power 

 consumption, or the current on the primary side of the transformer) 

 varies and furnishes a signal which is a function of the mercury level. 2 



1-43. Capacitive Methods 



a. Direct Capacitance. Direct capacitive methods for liquid-level 

 detection are applicable primarily to nonconductive liquids, such as 

 liquid gases, oil, or gasoline. The liquid forms the dielectric of a 

 capacitor; its dielectric constant is higher than that of the gas phase 

 above the liquid. Any rise of the liquid level increases the capaci- 

 tance between the two electrodes. The electrodes are frequently 

 arranged in the form of concentric cylinders, as shown in Fig. (1-4)7. 

 At the lower end of the external cylinder are openings B which permit 

 hydraulic communication with the outside liquid. If these openings 

 are sufficiently small, they will cause a mechanical damping of the 

 surface variations of the liquid, which is sometimes desirable. 



The capacitance of a cylinder is 



€h (1) 



C = 



2 In (rjr^ 



1 J. W. Broadhurst, J. So'. Instr., 21, 108 (1944). 



2 G. Ruppel, Arch. tech. Messen, V 1244-1, June, 1933. 



