226 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 4 



4-33. Electrolytic Trough 



Space-potential distributions in electrostatic fields can be found 

 from model experiments in the electrolytic trough. For similar elec- 

 trode configurations, the distribution of the stationary field in a 

 homogeneous conductor (electrolyte) is analogous to that of an 

 electrostatic field in a homogeneous dielectricum. 



The electrolytic trough 1 consists of a shallow insulating container, 

 as shown in Fig (4-3)3, filled with an electrolyte. Metal electrodes A 



and B are inserted so that the field con- 

 figuration of this model is analogous to 

 that of the simulated system. The elec- 

 trodes are connected to an a-c source S 

 (to avoid polarization). If a conductive 

 stylus P is inserted into the electrolyte, 

 its potential can be found by comparison 

 with that along the calibrated resistor 

 R. M is a zero indicating system (e.g., 

 a headphone). By setting the movable 

 contact C at fixed points on the resistor 

 R and moving the stylus P so that the 

 meter M does not indicate any potential 

 difference, equipotential lines can be 

 lines are the trajectories to the equi- 



Fig. (4-3)3. Electrolytic 

 trough: A and B, electrodes 

 immersed in electrolyte; P, 

 stylus; C, variable contact on 

 resistive voltage divider R; M, 

 zero indicator. 



drawn. The electric-force 

 potential lines. 



Conductive paper (Teledeltos Paper, Western Electric Company) 

 can be used in two-dimensional systems, instead of the electrolytic 

 trough, but it does not furnish as accurate results as an electrolyte 

 because of local variation of the conductivity of the paper. 



The electrolytic trough chosen should be as large as is practical so 

 that errors resulting from the influence of the walls upon the field 

 distribution are avoided. Polarization can be avoided by making all 

 the electrodes from the same material and as large as possible. The 

 resistivity of the electrolyte should be reasonably high. The stylus 

 usually consists of a thin wire; good results have been obtained with 

 a stylus consisting of a tungsten wire of 0.001 in. diameter fused into 

 the end of a glass capillary. Unavoidable capacitances between 

 electrodes and probes cause an incomplete zero reading. The effect 

 can be reduced by capacitive balance, i.e., inserting of variable 

 capacitors between A and P, and B and P. 



For review, see R. Strigel, Arch. tech. Messen, V 312-1, February, 1943. 

 1 C. L. Fortescue and S. W. Farnsworth, Proc. AIEE, 32, 757 (1913). 



