History
The PP3 appeared when portable transistorized radio receivers became common, and is still called a "transistor" battery by some manufacturers. The Eveready company claims that it introduced this battery type in 1956. Early transistorized radios and other equipment needed a low voltage battery, but the lowest voltage commonly available, small battery at that time was a 22.5V battery made for vacuum tube/thermionic valve hearing aids and for photo flash gun (using flash bulbs). The 22.5V voltage was at the upper limit of the transistor voltage ratings, and it was clear that what was needed was a battery of lower voltage and high enough capacity to run the transistor radios for a reasonable time.
Read more about this topic: Nine-volt Battery
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