space between the turns of the coil changes with the intensity of the reaction, the self-resonance frequency of the coil is proportional to the reaction. The resonant frequency can be easily and inexpensively read with high accuracy. The advantage of using this method over using small parallel electrodes is that the capacitance is greater and this, as mentioned above is easier to measure with a low cost instrument. Embodiment #4: So far, the tests and the instrument are separate, which results in lower cost per test. However, the instrument could be incorporated into the test, as shown in Figure 9: