Mercury Buttons

These originated in the 1930s by Mallory who developed them as “built in” long life solution for tube (valve) grid bias. The Battery radio sets from the 1920s had used 3 to 12 “B” cells as grid bias. Though almost no current is consumed the “shelf life” (self discharge) means that a Winner 9 Grid battery in the 1920s or 1930s had to be replaced every 18 months or less. The hermetically sealed mercury based “button” cell might last much more than 10 years.  The company Mallory founded is today known as Duracell. During WWII they were in great demand to power Proximity Fuzes (Fuses) in Shells and Bombs as well as some later portable radio HT packs. Two types of construction existed with standardization of package by 1946 and application as hearing aid tube (valve) filament supply and later transistor hearing aids. Use as “grid bias” was short lived as by late 1930s most battery valve (tube) sets used only a resistor in the HT- feed. Mains radio sets used a resistor in series with cathode for “automatic” grid bias as this provides DC negative feedback if “decoupled” with a suitable capacitor The Silver Oxide (1.6V peak, nominal 1.55V), Alkaline (1.5V drooping to 1V) and Zinc Air (1.4V peak, nominal 1.35V)  button cells have replaced  Mercury Cells which are now no longer made due to toxic effect of Mercury in the environment. The Silver Oxide  are more expensive and last longer with flatter discharge than Alkaline  cells. For sealed or very intermittent operation more than a couple of weeks the Zinc Air button cells are unsuitable. There is a particular problem with some Camera Meters designed for Mercury cells as the reading will be too high. The meter reading depends on light on the CdS cell and the battery voltage which with a Mercurry cell is 1.35V till 95% of capacity is consumed. If the camera has a Bridge type meter the reading will be the same with Siver Oxide, Mercury or Alkaline, if it’s not a bridge type then only Silver Oxide can be used unless you replace the Zinc Air type two to three weeks after peeling seal, Alkaline  varies too much. Some meters can be recalibrated and thus can be adapted for Silver oxide. If the meter isn’t a bridge type nor has calibration then a Schottky diode needs to be added which drops the voltage by about 0.25V to 0,3V at the low current of the meter. Thus  the 1.55V volts is 1.35 to 1.4V depending on current draw.
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