The UM-1 battery is also called D, R20 or Mono cell.
The round battery with a diameter of 34.2 mm and a height of 61.5 mm is used in many portable radios.
The nominal voltage of zinc-carbon batteries or R20 or of alkali-manganese batteries or LR20 is 1.5 volts.
Nickel-cadmium accumulators (NiCd, resp. KR20) or nickel-metal hydride accumulators (NiMH, HR20) have a slightly lower nominal voltage of 1.2 volts. Not in all cases a set can be operated with accumulators instead of batteries, as the lower total voltage might be insufficient for reliable operation.