Weston cell

As shown in the illustration, the cell is set up in an H-shaped glass vessel with the cadmium amalgam in one leg and the pure mercury in the other.

Electrical connections to the cadmium amalgam and the mercury are made by platinum wires fused through the lower ends of the legs.

Reference cells must be applied in such a way that no current is drawn from them.

The original design was a saturated cadmium cell producing a 1.018638 V reference and had the advantage of having a lower temperature coefficient than the previously used Clark cell.

However, an unsaturated cell's output decreases by some 80 microvolts per year, which is compensated by periodic calibration against a saturated cell.