No. 4 Mercury Hearing Aid "A" Battery
The No. 4 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. 4 battery
was the same size as the RM-4 battery manufactured by Mallory.
The No. 4 mercury battery (and the RM-4) were the largest "button" batteries
made. It weighed 1˝ oz.
and measured ⅝" high
by 1 3/16" in diameter.
Mercury batteries are no longer sold today because of the large
quantities of mercury in them and the consequent environmental concerns.