WWII HYDROMETER SECONDARY CELL PORTABLE No 1 FOR WIRELESS SET No 19, 1940's

Hydrometers are used for the measurement of specific gravity.The unit can also approximate the charge of a secondary cell (chargeable type) that has a liquid electrolyte by, in this case, the floating or sinking of the plastic balls in the barrel of the tube, (the liquid is sucked up from the battery by depressing the bulb and releasing).
If all balls float the battery is fully charged.

If the white ball sinks three quarter charge.
Green ball sinks half charge.
Red ball sinks quarter charge or less.

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