How A Proximity Fuse Works

(Above) Newsreel on the proximity fuse in the National Archives
Dick Mann, Navy Fighter Pilot and Engineer (35 years at Sylvania) explained the radio proximity fuse at the September 21, 2011 luncheon. He saw it in use in the Pacific against Kamikaze attacks, both while flying and on the deck of his carrier.
The written description below comes from a Yank magazine article written by Donald Nugent (Source: old magazine
Comments in brackets [ ] were added for those unfamiliar with electrical terminology. Be forewarned that although incredibly ingenious and clever, the sequence of events is quite complex (a little like a Rube Goldberg cartoon) and not necessarily easy to follow the first time around. It helps to examine the cutaway diagram (from Wikipedia) at the bottom of the webpage.
"Here's how the VT [the radio proximity fuse for WWII ammunition] works:
   The shock of fire [that is to say the shock of firing the artillery piece or antiaircraft gun] breaks a small glass vial [or container inside the artillery shell] filled with liquid electrolyte [the liquid chemicals inside a wet battery--like your car battery] near the base of the fuse [which is the top part of the shell]. Centrifugal force [the force pushing outward when an object spins] in the rotating projectile causes the liquid to flow towards the outside of a cylindrical cell [or round battery container] through a stack of thin, ring-shaped plates insulated from each other. Contact between the electrolyte and the plates instantly makes it an active wet battery [like your car battery], charging a firing condenser with electricity [The condenser is able to store up electricity, and is also sometimes called a capacitor] .
   This electricity activates a radio vacuum tube [a device that generates radio signals, a form of light well below what we can visibly see with our eyes on the electromagnetic spectrum], which sends a continuous radio frequency signal [away from the shell] at the speed of [light, or] 186,000 miles per second. This signal will be reflected back by any target that gives a radio reflection, such as airplanes, ships, or other metal objects, water, or earth.
   The reflected [radio] signal, received by an oscillator [a coil of wire that creates alternating current] interacts with the outgoing signal to create a "ripple pulse."  When the projectile approaches within 70 feet of a reflecting object, the ripple pulse (amplified by audio tubes) becomes powerful enough to trigger a thyratron tube [which is a device that when it receives a poweful enough signal, sets off a trigger. ]
    This sets off a chain of reactions, all accomplished in a fraction of a second. Energy stored in the charged condenser is released, an electrical detonator exploded, an auxiliary ("booster") explosive charge set off, and finally the explosive filling in the projectile detonated [meaning the shell expoded].
   Since the shell is designed to explode on making radio contact with its target, what prevents it from bursting in the muzzle itself as a result of the nearness of the gun or the ship or the earth from which it was fired? The inventors took care of this danger by designing two safety switches, described below, which are not entirely released until the projectile has traveled about 400 yards at the appropriate rate of 2,600 feet per second. Only then is the projectile ready to detonate.
    The first of the safety devices is a a flexible reed switch [like a flexible piece of a metal used to close a circuit] placed in the circuit so as to keep the firing condenser [which stores electricity] discharged when the projectile is at rest. Upon firing, centrifugal [or spinning] force opens the switch and permits the firing condenser to charge.
    The second safety device is a mercury unshorter switch, composed of two chambers: an inner one filled with mercury [the silvery liquid metal which can conduct electricity and open or close a switch and complete a circuit], which maintains an electrical short [electricity follows the shortest route] between firing condenser and case, and an outer chamber, empty prior to the shell's spinning. A porous diaphram [filled with holes] separates the two chambers. When the projectile starts spinning as it is fired, mercury seeps through the diaphram into the outer chamber. This removes the short circuit and arms the projectile.
     If the projectile misses its target, the reed spin switch gradually closes as the projectile's spin decreases; then with the mercury in the outer chamber, it establishes the circuit that explodes the shell, thus preventing the projectiles from falling intact into enemy hands. " 
The diagram below is from Wikipedia's article on the proximity fuse.
[According to one person's webposting at
"Technically the Allied fuse was not radar: it did not send out a pulse
and listen for an echo. It had 4 tubes. One tube was part of the
oscillator. When a 'target' that was about a ½ wavelength in size
came within a few wavelengths it would load the amplifier and the anode
current would increase. Two additional amplifiers would detect this
change and then triggered the 4th valve (a gas filled thyraton) to set
off the detonator. Contrary to other reports, it apparently did not
trigger on Doppler shift either or on frequency change. There were
many shock hardening techniques including planar electrodes and packing
the components in wax and oil to equalize the stresses."]