Standard Cell Voltage Reference 9152T6

Six Weston standard cells assembled inside a temperature controlled oven. The thermostatic oven is normally connected to the AC mains, but can be connected to a 12 V storage battery when the standard has to be moved to another site for calibrating. The temperature controller board uses discrete semiconductors. Mercury thermometer on the front panel, 'T' option.
Each mercury cell has separate output terminals. Cells are built according the process first devised by Edward Weston in 1892, based upon a chemical reaction between mercury (cathode) and a mercury-cadmium amalgam (anode). Nominal output voltage of each cell is 1,01830 V at 20 degrees centigrade. Cells are individually calibrated with an accuracy of about 1 p.p.m.

The Weston cell was approved as primary voltage standard since 1911. At any room temperature, other than 20°C, the output voltage of asuch a cell could be computed according to the formula:
  V(t) = 1,01830 – 0,0000406 ( t – 20 ) – 0,00000095 ( t – 20 )² + 0,0000001 ( t – 20 )³
where t was the actual room temperature.

Here you find 2 models, 2 with images and 0 with schematics for wireless sets etc. In French: TSF for Télégraphie sans fil.
All listed radios etc. from Guildline Instruments Ltd.; Smiths Falls/Ontario