The nutrients and salts in the nutrient solution conduct tiny amounts of electricity, which can be measured by passing a small current between two electrodes in the solution.

Pure water is an insulator and will not conduct electricity. However, once an element is in solution in the water, it will then conduct. The more elements that are added, the higher the level of conductivity will become.

Therefore, the measure of conductivity indicates the amount of nutrients in the solution, that is, how ‘strong’ it is.