Nuclear weapons effects computers one and two were produced by Blundell Rules Limited [BRL] of Weymouth, England. They have BRL’s name on the back. Number 1 dates from about 1960.
They could be used calculate the consequences of a nuclear attack on a British target, most likely a city. The magnitude of the burst is set (in kilotons or megatons) and the calculator estimates, for various distances from ground zero, the damage to buildings, the crater dimensions, the percent of the population killed, trapped, and seriously injured. The back of the calculators have tables that estimates the extent of the resulting fires.
Number three’s design is significantly different and does not have BRL’s name on its back. Number one was issued to the Royal Observer Corps during the Cold War. I do not know if versions two and three were also used by the ROC.
For more posts about Cold War calculators click on the Cold War Calculators category on the right.