The Hearing Aid Museum

Hearing Aids of all types—Ear Trumpets, Carbon Hearing Aids, Vacuum Tube Hearing Aids, Transistor Hearing Aids, Body Hearing Aids, Eyeglass Hearing Aids and much more!

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Click on the "General Information" button (top button above) for an overview and general information on this category of hearing aid.

Hearing Aid Batteries

No. 4Z Mercury Hearing Aid "A" Battery

The 4Z 1.4 volt mercury hearing aid "A" battery was made by an unnamed battery manufacturer. It was used in a few vacuum tube hearing aids manufactured around 1947 and 1948.

This battery was extremely rare and, as far as I know at this point, was only used in the Acousticon A-90 and in some A-90M vacuum tube hearing aids.

It was the same diameter as the RM-4 but was not as high.

The 4Z battery weighed 1.1 oz. compared to the RM-4 at 1.5 oz. It measured ½" high by 1 3/16" in diameter as compared to the RM-4 at ⅝" high.

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Bottom view of the 4Z mercury hearing aid battery showing the model number (4Z) and 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.

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Side view of the 4Z mercury hearing aid battery.

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Notice the difference in thickness between the standard-sized RM-4 battery (left) and the 4Z battery (right).

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