I have always had a soft spot for mechanical computers. There is something fascinating about rotating dials to get answers, its tactile and understandable in a way that electronic devices are not. Mechanical computers were an essential tools in the days before hand held electronic devices. If you wanted to work out a formula rapidly they were invaluable and saved lengthy calculations on a piece of paper. Today we are looking at a Radiac Calculator No1, used to work out the levels of contamination after a nuclear explosion. This calculator consists of three discs of different sizes, two of which are reversible to allow for the difference in contamination on land or sea. On land, the two pink discs are chosen:

These discs are secured with a central metal rivet which they pop over. They can be gently removed and turned over to the blue side to calculate levels at sea:

The rear of the calculator has detailed instructions on its use, together with the NSN number and /|\ mark indicating military ownership:

Whilst made of celuloid, this calculator is relatively fragile, so a dark green protective case is issued, again with the NSN number and /|\ mark printed on the front in black:

The RADIAC Calculator No. 1 was produced by Blundell Rules Limited of Weymouth, England. Earlier examples can be found dating to the 1950s that are marked as having been made in Luton, but the company left this town in 1956, so any with a trademark indicating Weymouth date to after this point. The calculator is based on the Radiation Dosage Calculator designed by William Orr in 1951. It works as follows: if the exposure rate (roentgens/hr) is known at a given time after a nuclear explosion, the calculator predicts the exposure rate at any other time. It also estimates the dose to personnel who are in the area at specified periods of time after the explosion.