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Originally Posted by Cpl.George
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This has mostly been answered, but....
The original battery was a pair of "Cells, Dry, X, Mk.II" or "Cells, Inert, S" which were 1.5" square x 5.25" tall (including the terminals). The 'S' cell was manufactured with completely dry ingredients and had a cork in the top so that water could be added to activate the cell when it was required for use. (This gave it a very long shelf life - vital in the days when shipping stores around the world could take several months, and high temperatures reduced the shelf life of normal batteries to weeks.
The original cells had square cases folded from sheet zinc, wrapped in bitumen impregnated fabric and then dipped in pitch or wax to seal them.
Later (post-WW2) cells had card cases dipped in microcrystalline wax as a waterproof "conformal coating", the cell inside was a standard 'F' size as used in the 996 lantern battery, and the name changed to Cell, Dry, 1.1/2V No.12.
The telephone requires 3 volts to power the carbon microphone insert, C or D cells are fine for this. (Ericsson manufactured a holder that took two 'D' cells for the Telephone Set J (and their commercial derivatives of that and the 'L').
Telephone Sets D and L require a handset with a switch to activate the microphone, otherwise the battery will be exhausted very quickly as there is no cradle switch to disconnect it when not in use.
Chris.