The magnetic amplifier. What is it? This device, using saturable reactors and the non-linearity curve of specific transformer cores are used in HVDC measurement and certain types of welders. And of course, pre-digital age we had the arithmentic logic unit (ALU) which utilised tubes and transformers for summing in early mainframes – this was the 1950s folks so everything was big, bulky and of course heavy. Go to Wiki or tpub.com for more information.
Magnetrons and Klystrons – a guide to early radar. The magnetron is not only used in microwave ovens, it’s also the device used in radar for radiating RF power. Early radar also used Klystrons for the receiver front end (this was the reflex Klystron used as the local oscillator) – both devices use resonant properties of their cavity. Klystrons are also used as particle generators and in UHF transmitters. I’m sure there must be many readers out there that worked in the radar industry that can recall the time taken to tune in the klystron local oscillator to get maximum output in the radar front end. I do. Klystrons are no longer used in radar (to my knowledge) in the receiver front end haviong been surpassed by semiconductor devices. Wiki has an interesting article on klystrons.
TR Tubes. A problem with radar is of course that the same waveguide carrying the output RF is fed into the front end. The front end is a very sensitive device so what stops all this enrgy from blowing the front end to pieces. Situated in the waveguide at carefully calculated lengths from the transmitter and receiver are ATR and TR tubes which switch the energy to the required path. (actually blocking or passing through a short circuit) There are fundamental issues with this as explained in this interesting article by this Radar Tutorial. Radar engineers will recall that a TR tube never failed without one having to replace the Xtal Diodes in the front end as well. 
. The TR tube containes Tritium gas (radioactive isotope of hydrogen = H3) and also had a keep alive voltage applied to an electrode. Radar engineers will recall that this keep alive voltage used to also fail.
Klystrons were been superceded by Gunn oscillators in the local oscillator stage when I stopped radar repair twenty years ago. Gunn oscillators were strictly made of Gunn Diodes (a Transferred Electron Device. Gallium arsenide devices are practical up to about 200GHz) which were practical for radar usage in the X-Band spectrum (3 cm). I didn’t find much about Gunn Ocillators on the web without having to pay for a book so if readers can lead me to an interesting website I’d love to hear about it.
Submarines have a very serious problem: Communication. If you feel you are up to it and can develop a submarine radio telecommunications system under cover without your neighbours getting wind of it you should be serioulsy looking into this technology to make some serious money. Up to now submarines receive radio traffic at a very low frequency (actually ELF – extra low frequency 3 – 3000Hz) and if they wish to transit have to release a buoy to the surface to communicate effectively. Water, especially salty water is not known for it’s friendly radio communication ability. Of course the US navy is not going to tell you about their latest communication techniques, neither the Russians, French or Brits but you can bet your bottom dollar that there have been advancements made in this field but possibly not much. A new system? Read here. Sounds like an old system to me.
And last but not least but not in the electronics field. Did you know that the human body weighs 40 times more than the brain. Does that make fat people more intelligent?