No. 3 Mercury Hearing Aid "A" Battery

The No. 3 1.4 volt mercury hearing aid "A" battery was made by an unnamed manufacturer. It was used in vacuum tube hearing aids in the late 1940s and early 1950s. The No. 3 battery was the same size as the RM-3 battery manufactured by Mallory.

The No. 3 mercury battery (and the RM-3) were the second largest "button" batteries made. It weighed 0.8 oz. (24 g) and measured ⅝" high by 1" in diameter (1.7 x 2.5 cm).

Mercury batteries are no longer sold today because of the large quantities of mercury in them and the consequent environmental concerns.

Side view of the No. 3 mercury hearing aid battery.

Bottom view of the No. 3 mercury hearing aid battery showing the  patent number. The bottom was the positive terminal (+) of the battery.

The small hole at the right was a pressure-relief valve that opened to relieve excess pressure that could be built up when the battery was being used.