Inside of a TDS meter is an electrical circuit which measures the resistance between its two electrodes when immersed in a liquid. This circuit applies an AC voltage to the electodes, and then measures the AC current which flows between the two electrodes. This reading is then corrected for temperature (if its a high quality instrument), and electrode geometry. The result is a measure of Total Ionic Content. The name Total Dissolved Solids is actually a misnomer, as a lot of dissolved solids will not read at all on a TDS meter since they do not ionize when they dissolve. Sugar is such a substance. If you were to dissolve a tablespoon of table sugar in a cup of distilled water and read it with a TDS meter, the resulting reading would be zero.The reason AC is used instead of DC voltage on the electrodes is to keep the metals in the solution from plating onto the electrodes. With AC, anything that plates onto the negative electrode will in theory come off when the polarity reverses. AC also keeps the ions from migrating from one electrode to the other, thereby keeping the solution homogeneous.