Into this technological melee they dropped William Alan Stewart Butement. In 1931 he and P. E. Pollard, invented a shipboard radio device for the detection of ships. Today we call it radar. It operated at 600 MHz and using pulse modulation was able to detect ships100 yards away. The Navy wasn't big on it, but others saw potential. By 1938 Butement had ramped up his invention into large scale devices that could be used from land to protect the coast.
In 1939 Butement attempted to improve anti-aircraft guns. It's hard to hit a moving target. He had a better idea. His plan was a very compact Radio Direction Finder (RDF) unit placed on the projectile. It would then trigger the detonation when near the target. This was not simple. The RDF had to be small and also durabel enough to be fired out of a cannon. This circuit included glass vacuum tubes. Somehow just over a year the United States was manufacturing projectiles with a proximity fuse. This was also called a VT (variable-time) fuse.
This was not an induction trigger. It wasn't sensing a ferrous body. It was detecting the reflection of radio signals. A later improvement was The transmitter which used the shell body as an antenna and sent out a continuous wave at around 200 MHz. As the shell approaches a reflecting object, (a ship, a plane, the ground) the reflected signal created interference. That pattern changes with proximity. As the objects get closers the signal moves in and out of resonance as the reflected signal length changes; half a wave is resonant, so is a quarter, an eighth etc.
This causes a small oscillation of the radiated power and consequently the oscillator supply current of about 200–800 Hz, the Doppler frequency. This signal is sent through a band pass filter, amplified, and triggers the detonation when it exceeds a given amplitude. Later in life, Butement said that he considered the proximity fuse as his most significant accomplishment.