390 E. A. POLYAKOV 



coefficient with the growth of the fluids resistance is a fact illustrated by the 

 data shown in Fig. 1. 



It is necessary to point out that the contact resistance also depends on the 

 purity of the metallic surface concerned. Oxidation and pollution of the 

 metallic parts as well as their corrosion can lead to a rearrangement of the 

 electric field and to a change of the coefficient of the device. This explana- 

 tion evidently applies to the majority of sharp oscillations of the resistivity- 

 meter coefficient such as are observed in practice. In order to achieve a con- 

 stant coefficient it is necessary to remove the influence of the instrumental 

 metallic parts on the probe. 



SELECTION OF THE ELECTRICAL ARRANGEMENT. 



Various probes, differing in their sizes, shapes and mutual positions of 

 the electrodes, will obviously have different degrees of coefficiental inconst- 

 ancy. By using a suitable probe one can attain a constant resistivity meter- 

 coefficient, and ensure the possibility of getting accurate results in meas- 

 uring the specific resistance of the drilling fluid. 



In order to select the most suitable probe a series of electrical devices 

 involving probes with different shapes of electrodes were constructed and 

 investigated. 



The probe electrodes were mounted on ebonite discs. Thus, as is usually 

 done in resistivity-meters, a gradient -probe was used in every instance. 

 Fig. 4 shows some typical electrode devices, which can be divided into 

 three groups. 



1. The electrodes are discs in a linear arrangement (i in Fig. 4). 



2. The measuring electrodes are concentrically arranged rings {2 and 3 

 in Fig. 4). 



3. The measuring electrodes are rings with loops. It can be considered 

 that a spherical surface with large cut-outs is adopted for one (A^) or both 

 {N and M) of the potential electrodes. Consequently, the electrodes of the 

 latter group of devices (4—8 in Fig. 4) can be called three dimensional. 



With the exception of the first device the current electrode is placed in 

 the centre of the rings, which are the potential electrodes of the probe 

 represented by the spheres formed from the loops. 



Using these electrode arrangements measurements were made in a tank 

 having (1) a metallic body as the second current electrode B, (2) a body 

 made of insulating material with the second current electrode being repres- 

 ented by a metallic disc at the bottom of the tank. The tank was filled with 

 sodium chloride solutions of varying concentrations. The electrode device 



