I believe the proximity fuse started with the British? At least the original idea/circuits. The Americans were able to productionize it ( make it by the hundreds of thousands and have a shelf life/storage of a number months without without maintenance).
The British were a bit busy at the time
But as I am saying, there is more to "technology" than the basic idea.
Operations research is important to work out what is effective and what is ineffective but it can't really come up with ideas. It can wean the good ideas from the bad.
Operations research told us that night bombing by celestial navigation was inaccurate and that attacking tanks with rockets was inaccurate.
Every country had the idea of implementing a proximity fuse in the early 1930s Germany, UK, US, France. Most attempted to use optical methods ie focused light or infrared but there was some idea of using radio.
The Germans were the ones that persisted and got somewhere first.
The time delays used in some of their bomb fuses were electronic since the mid 1930s. The bomber could arm the bomb electrically by charging a capacitor and some bombs could have a variable bursting time delay determined by the amount of charge. The bombs could be armed and disarmed during flight. The fuses were so stable that the Germans were the only combatant that shipped bombs with fuses installed. The aircraft was safe from bombs detonating in a landing crash.
The core of these devices was copper oxide or selenium diodes and a type of vacuum tube (or vacuum valve if your English) called a cold cathode tube or cold cathode thyratron. Basically the cold cathode tube was a very sensitive 3 electrode switch that triggered at a very precise voltage, very fast and let through a large current that could blow a fuse link.
When the British started recovering bombs with these shock hardened fuses they re-examined their earlier work and passed the idea across desks of their best electronic and radar engineers and figured out that it should be possible to toughen a vacuum tube/valve to handle shock.
The man in charge John Cockroft had other important work to do with Tube Alloys (nuclear bomb) but the UK work was passed on to the USA. US engineers at the USN Buro of Ordinance immediately began dropping 37 shells backwards onto concrete and then firing 37mm shells with a hearing aid triode that generated a RF tone to prove the valve could survive.
The circuit used on the US fuse ultimately came from W.A.S Butement the New Zealand/British engineer who developed much Chain Home radar.
It used 5 or 6 valves and turned out to be unexpectedly easy to jam and predetonate so secrecy was important.
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Those of British extraction might be pleased to know that UK development of the proximity fuse did continue. In 1942 test rounds were fired in which a a corner reflector in the base of he shell was tracked by radar. When the shell echo merged with the target echo the shell was detonated by a double pulse. The longer term goal had been to track the shell, compare the miss distance and adjust the aim but it was fund that the dispersion of the shell was much to great to make this realisable. It would have worked for a close in weapons system where the shell dispersal was much less. The program was cancelled in favour of the American fuse which was entering production in 1942.
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The German fuse work was suspended in 1940 apparently along with a lot of other programs that could not produce a fieldable weapon within 6 months due to the need to defeat France. Like a great many of the other similarly suspended programs they took a lot of time to reconsititute.
Fuse work recommenced in 1942 and produced test firings by 88mm shells in 1943. The shells could burst at 1m, this was improved to 2m and finally by 1944 to 4m with over 95% reliability That should have been enough since a 88mm shell was lethal to 3.5m. (10m-15m had been hoped for but was not really so) and work continued to improve the detection range to 10m and finally 15m though these were not tested. The shell was essentially ready for production.
The shell worked by detection the distortion in the electrostatic field on one side of the shell versus the other as it spun. This tech had been looked at by Buro or Ordinance but rejected due to its degradation in rain.
The other way the Germans planed to use their cold cathode tubes was to create electrically programmable time fused shells that could be programmed in the barrel or doing loading. They had some success by aging and then grading and matching components.
The German electrostatic fuse had a nose contact fuse. Interestingly the Germans found that when the standard mechanical time delay fuse was replaced by a double fuse (doppelzunder) with a nose contact fuse the lethal hit rate could be increased 4 fold by simply biasing the shells to burst 100m beyond the target. This was because many shells burst early and many late whereas a direct hit would pass through the aircraft seldom destroying it.
Hence the electrostatic fuse would have worked well with a programmable time delay as a backup and with a nose contact fuse. It was cheap and didn't need batteries.
One of the issues may have been that the cold cathode tubes may have needed radium to slightly ionise the gas to get a faster switching as tetrode tubes were developed to avoid this for some reason.
