It was noted in Table 3 of the first paper that the failure rate of common types of valves is about one half per cent per thousand hours of operation. Some other components are as unreliable as this, but they are not employed in such large numbers in typical electronic equipment so that (at least until very recently) valves are much the commonest cause of electronic equipment failure. The mechanical and electrical tests which valves are required to pass before the Armed Services grant type approval are given in Joint Service Specification K1001, Ministry of Aviation, June 1958. In an analysis of failure reports made in 1953 in the Royal Navy, the Royal Air Force, civil aviation, and under typical laboratory conditions, the proportion of faults traced to valves ranged from 45 to 55 per cent of the total. Under conditions of rough handling the average life of valves is likely to be further reduced, more so than for any other common component, so that the number of faults and the proportion of faults due to valves, will be even higher.