Eighty years ago, Eveready introduced the “Mini-Max” B battery, model number 412. The 22-1/2 volt battery measured only 2 x 1-1/32 x 5/8 inches, and weighed only 1-1/2 ounces. It was about the same size as the still-in-production 9 volt battery, although rather than having both terminals on one end, the 412 had positive on one end and negative on the other.

It was originally intended for use as the B battery in tube portable radios, but it was also used in instruments, and even in some early transistor radios, such as the Regency TR-1. The carbon-zinc battery remained in production through at least the 1980s.

If you’re looking for a modern replacement, a replacement using alkaline cells is available on Amazon.  The ad above appeared in the October 1945 issue of Radio Service Dealer.