prior practical circuits and probes have not been capable of measuring relatively small volumes v accurately in fig5 it can also be observed that the dash line intersects the 0 voltage line at some equivalent negative value v p 39 which is the effective probe volume of the equivalent probe circuit it is of course desired to make this effective value to be as reasonably small as practical furthermore it will be seen in fig5 that the relationship of signal voltage e s 39 and volume v is nonlinear above a certain critical value of volume v it may be appreciated at this point that this departure from proportionality is caused by the air mass m pm of fig2 as represented as an electric quantity c pe in fig3 approaching resonance with the equivalent electrical inductance l ve representing the volume v to be measured it has been discovered that probes with relatively wide necks can be designed to have a resonance with volume v and neck air mass m p of the probe 13 always above the test frequency then the curve of fig5 is repeatable and therefore the nonlinear circuit shown in fig6 could be developed to compensate for effects of such resonance referring to fig6 input signal e s 39 appearing at terminals.