had to operate in any weather, which made conventional lead-acid batteries unsuitable due to their poor cold-weather performance. The system also had to be stored for long periods of time before being activated, which again argued against lead-acid. The solution was found to be small nickel-iron batteries that could be repeatedly and rapidly recharged in the field, and operated across a wide range of temperatures. To protect the system from capture, it was fitted with small explosives that would destroy enough of the circuitry to make it impossible to determine the exact frequencies being used. The transponders were mounted in suitcases, selected on a case-by-case basis to match common types in use in that area. A dozen transponders were supplied to the SOE in 1942, at which point the TRE's involvement largely ended. Brown later learned that these were used extensively throughout the war. One example, dropped in Norway, was used on seven separate drops, in spite of being buried for the better part of a year in a biscuit tin. Airborne use The Airborne Forces Equipment Committee took up development of the system in the summer of 1942, funding low-priority development of a Mark II system intended for use on glider tugs and paratroop aircraft. At the time, it was decided that each Eureka should be able to handle interrogation from up to 40 aircraft at a time. They also selected a design based on several sub-units that would allow the equipment to be changed simply by swapping sub-units from a common chassis. Both Rebecca II and Eureka II were developed by Murphy Radio, with early pre-production of Rebecca II by Dynatron Radio. A system using a selection of tuned capacitors was used to select the operational frequency from a set of four. Looking for a controller, Murphy selected a General Post Office 5-position electromechanical system used in their telephone exchange systems. A similar selection of four channels was available in the Eureka units, but these were selected manually. Rebecca was powered off the aircraft mains, whilst Eureka was battery powered with a lifetime of about six hours. In testing, Eureka II proved to be too heavy for practical use, so A.C. Cossor was selected to build a Mk III version. They used US miniature 9000-series tubes for this version and a much smaller battery with a three-hour life. US use In December 1942 Brown was flown to the