Theory for the electrical sensing zone method

The resistance of an element with a particle included, 57 , is that of two resistors 5/ , 57 2 in parallel 

Thus the change in resistance of the element, 6(A/ ), due to the presence of the particle is given by  

The external resistance of the circuit is sufficiently high to ensure that a small change, SR, in the resistance of the aperture due to the presence of a particle will not affect the current / the voltage pulse generated is therefore ISR. 

In practice, it is found that the response is independent of the resistivity of the particle. If this were not so, the whole technique would break down since a different calibration factor would be required for each electrolyte-solid system. This independency is attributed to oxide surface films and ionic inertia of the Helmholtz electrical double layer and associated solvent molecules at the surface of the particles, their electrical resistance becoming infinite [19]. The terms involving p/p may therefore be neglected and the preceding equation becomes  

The response therefore, is not proportional to the volume of the particle, but is modified by the a/A term. For rod-shaped particles whose length is smaller than the aperture length, this leads to an oversizing of about 6% in 

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