When using a hydrometer, where is the specific gravity of a cell measured?

The specific gravity of a cell is measured at a point that is a third of the way down the side of the cell because this location provides a representative sample of the electrolyte concentration within the cell. This positioning helps to minimize the effects of variables such as stratification that can occur in a cell due to temperature gradients, variations in density, or agitation from use. By taking the measurement at this specific point, it reflects a more accurate reading, allowing for better assessment of the cell's performance or health.

Measuring at the top of the cell may lead to an inaccurate reading as the concentration can be affected by evaporation and could miss out on the denser solution below. Measuring at the bottom could give misleading results due to sediment or higher concentration layers forming there. The center of the cell does not account for any variations along the vertical axis that may influence overall electrolyte density. Thus, measuring a third of the way down strikes the best balance for accuracy in evaluating the cell’s specific gravity.