A good method would be to get a plastic tube (eg electrical conduit) and drill a few holes in the side. Then fill it with Play Doh and poke electrodes in the ends. As the V and I are increased, you can measure the voltage drop across two voltmeter probes placed at a measured separation in the Play Doh. I tried an external voltages from 0.1V to 1.0V and measured voltages between the probes at different distances. I found resistivities of about 0.2 Ωm. You could investigate the conditions (and reason) under which it becomes non-ohmic. If you want to see how the resistivity changes over the day, the ends need to be sealed with cling-wrap or something else to stop it drying out (and resisting the movement of charge). You may like to try different diameters of pipes as well. It all goes towards seeing if the measurement of resistivity of this funny stuff is subject to different lengths, areas and voltages. Chris Fuse said about the time factor:
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New measurements have been made of the electrical resistivity of high quality pyrolytic graphite. It is shown that the very large anistropy of the resistivity, near T = 0, can be qualitatively understood in terms of the scattering of carriers from the highly anisotropic strain fields produced by ribbons of stacking fault.
Physics II I- Electrical Resistivity
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Mineral grains comprised of soils and rocks are essentially nonconductive, except in some exotic materials such as metallic ores, so the resistivity of soils and rocks is governed primarily by the amount of pore water, its resistivity, and the arrangement of the pores. To the extent that differences of lithology are accompanied by differences of resistivity, resistivity surveys can be useful in detecting bodies of anomalous materials or in estimating the depths of bedrock surfaces. In coarse, granular soils, the groundwater surface is generally marked by an abrupt change in water saturation and thus by a change of resistivity. In fine-grained soils, however, there may be no such resistivity change coinciding with a piezometric surface. Generally, since the resistivity of a soil or rock is controlled primarily by the pore water conditions, there are wide ranges in resistivity for any particular soil or rock type, and resistivity values cannot be directly interpreted in terms of soil type or lithology. Commonly, however, zones of distinctive resistivity can be associated with specific soil or rock units on the basis of local field or drill hole information, and resistivity surveys can be used profitably to extend field investigations into areas with very limited or nonexistent data. Also, resistivity surveys may be used as a reconnaissance method, to detect anomalies that can be further investigated by complementary geophysical methods and/or drill holes.
Insulator (electricity) - Wikipedia
Surveys of lateral variations in resistivity can be useful for the investigation of any geological features that can be expected to offer resistivity contrasts with their surroundings. Deposits of gravel, particularly if unsaturated, have high resistivity and have been successfully prospected for by resistivity methods. Steeply dipping faults may be located by resistivity traverses crossing the suspected fault line, if there is sufficient resistivity contrast between the rocks on the two sides of the fault. Solution cavities or joint openings may be detected as a high resistivity anomaly, if they are open, or low resistivity anomaly if they are filled with soil or water.
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Layout of electrodes should be done with nonconducting measuring tapes, since tapes of conducting materials, if left on the ground during measurement, can influence apparent resistivity values. Resistivity measurements can also be affected by metallic fences, rails, pipes, or other conductors, which may induce spontaneous potentials and provide short-circuit paths for the current. The effects of such linear conductors as these can be minimized, but not eliminated, by laying out the electrode array on a line perpendicular to the conductor; but in some locations, such as some urban areas, there may be so many conductive bodies in the vicinity that this cannot be done. Also, electrical noise from power lines, cables, or other sources may interfere with measurements. Because of the nearly ubiquitous noise from 60-Hz power sources in the United States, the use of 60 Hz or its harmonics in resistivity instruments is not advisable. In some cases, the quality of data affected by electrical noise can be improved by averaging values obtained from a number of observations; sometimes electrical noise comes from temporary sources, so better measurements can be obtained by waiting until conditions improve. Occasionally, ambient electrical noise and other disturbing factors at a site may make resistivity surveying infeasible. Modern resistivity instruments have capability for data averaging or stacking; this allows resistivity surveys to proceed in spite of most noisy site conditions and to improve signal-to-noise ratio for weak signals.