this is the heart of the microwave oven, its called a magnetron. It consists of a vacuum chamber that contains a filament that is heated up by putting power to wires here
to a point where it will emit electrons, and around this filament is an anode which is connected to these fins which take away the heat that doesn't get used in making power
and this end here is the power output, it connects into the oven cavity. We call the oven a cavity.
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so that is what the heart of a microwave oven is, it is the power generator, very high frequency.  Basically the tube itself is simple.
And here if you cut away all the outside, you see that I've taken away the outsides and we have the connections to the hot filament I was talking about
and we have the connection to take the power out, then the question is how do you make power?
so we took one of these apart, and you can see what is inside of it. This filament is broken, but there is usually a spiral of tungsten, or thoriated alloy tungsten.
connecting between this point and this little shell here, and thats the thing I was talking about getting hot
and the electrons that are emitted by that filament, when you put voltage between that filament and this anode, would ordinarily just zing across and nothing would happen.
but there is a magnet here, and when the electrons start to move in one direction the magnet turns them, and depending on the voltage it turns faster or slower
so now we got the electrons whirling around the filament, and they are not going over to connect with the copper anode, now you notice the anode is not a solid piece of metal
its got some veins, these are designed to be resonant at the frequency that we want.
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Resonance is like when you have a person on a swing and you want them to go higher you push them at the right time so that it makes them go faster.
The electrons go by each of these veins at a time such that they can kick another electron inside this copper at the right time such that they can build up a voltage.
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And thats what happens in a magnetron
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electrons whirl around at the right speed to generate wave energy in an anode which is designed as a bunch of resonators.
So now you got this thing making microwaves but you have to do something about getting the energy from there out.
They go around and around such that they are synchronous with the high voltage, voltage that is changing rapidly back and forth, like how a synchronous motor would be
they generate electric fields that are stored in these veins, between the veins as electromagnetic energy, that is a resonant cavity, now how do we get that power to come out to the oven so we can use it?
there is a cap on the end and an insulator, and a small rod from this cap connects to one of these veins, connects the power out and by mounting this probe in the sidewall of the oven cavity 
which you can see here, you mount that to the wall of the cavity, stick this inside the oven, you radiate energy into this oven
now the oven itself is also a cavity, waves bouncing back and forth across all the walls, and if there is a piece of material in the cavity like a food item
they impinge on the food, and in the process the food absorbs the energy and gets hot. and because the wave is coming in from so many different directions it heats all around and penetrates into the food
and makes it cook relatively uniformly inside as well as outside unlike other forms of cooking which require heat to come in from the side and conduct in, instead of coming in as an electromagnetic wave
and the primary mechanism is that you are heating the water content or fat content rather than heating the dry materials, like the fiber materials
This is the older version of how you connect microwave power into an oven, this is a vintage 1960s, late 60s. this flange would connect to the wall of the oven, through a hole you'd radiate the power
into, in an oven there is a wave guide, which is a square piece of tubing that goes down to a hole in the oven wall.  
in the modern design people have figured out how to make joints in the ceramics which low loss and stronger than glass, they don't break, but the same concept exists. 
this is the output end of a new magnetron, it shows the evolution of magnetrons over some 20 years.
The advent of ceramic to metal joining which people like Jim Bags? and Ralph Bondly? pioneered made a whole world of difference, its mechanically stronger its electrically less lossy
you see its the way to go, … technology advances as time goes on.
  
