Microwave ovens are ubiquitous in modern kitchens

Microwave ovens are ubiquitous in modern kitchens. They cook food by microwave radiation, a process that starts with the magnetron.
The magnetron is a key component in every microwave. The device was invented essentially by accident when a Raytheon engineer testing one accidentally melted his snack in 1946.
The Electrons
Microwave oven magnetrons are the key to the microwave radiation that cooks your food. They are also a major player in radar, though solid-state power amplifiers have made them less of a household name.
Electrons have three main properties: their electric charge, their mass, and their spin. The spinning of electrons causes magnetic fields, which affect how electrons arrange themselves inside atoms and how atoms interact with each other.
The microwave oven magnetron has been around a long time, with an early form appearing in 1910. It became important in radar during World War II as the work of Randall and Boot led to more powerful magnetron tubes, which were crucial in enabling the US Navy to detect enemy submarines faster than they could with earlier conventional systems.
The Waveguide
The magnetron is a key part of the microwave oven. It generates microwaves using the interaction of a stream of electrons with a magnetic field while moving past a series of open cavities in a metal block. This arrangement works similar to the way a whistle produces a tone when air is blown past it.
Electrons whiz by the open cavities and cause microwaves to oscillate within the cavity. This radiation is then directed into the microwave oven cooking cavity to warm your food.
Waveguides are a huge topic in microwave engineering, there are entire books written about them. They can be very difficult to manufacture. If you ever visit a microwave waveguide factory you'll see old bearded guys pounding away with hammers and files at tubes of rectangular cross-section.
The High Voltage Control Circuit
The magnetron is a high-powered vacuum tube which oscillates when electrons are manipulated to pass through open cavities in the metal block. This process produces microwave radiation which is then directed into the oven chamber to heat food.
Boot and Randall developed stronger magnetrons which became the basis for radar equipment used by American submarines during World War II. Dr. Percy Spencer was also experimenting with these tubes and discovered that when he placed a popcorn kernel near one it caused it to pop.
When removing the magnetron from your microwave, make sure you discharge the capacitor by touching it with an insulated tool or using a multimeter to read its voltage. The wires and components inside the microwave still have a high voltage.
The Anode
The magnetron is a short copper cylinder fitted with resonant chambers, like the ones shown above. High voltage direct current from the cathode causes electrons to jump ship and whiz by these open resonant cavities, producing electromagnetic waves in the microwave range. These waves are guided by the waveguide to the oven cavity to heat your food.
But the days of the magnetron in microwave ovens may be numbered; manufacturers are starting to introduce solid-state power amplifiers into the market that can replace them. These devices offer advantages over magnetrons, including improved RF efficiency and output control, shorter heating times, and lower DC voltage needed.
The Cathode
In the 1930s Albert Hull invented a device called the magnetron, which is basically at the heart of every microwave oven. He was trying to create vacuum tubes at the time and discovered that electrons, when heated, can circulate radially between the cathode and anode without hitting either of them. This is known as cyclotron radiation and it gives off energy in the radio frequency spectrum.
The cathode is a cylindrical piece of refractory metal such as tungsten or thoriated tungsten. It's heated red-hot by an electric current passing through it to produce electrons by thermionic emission. This is the same process that is used to illuminate CRT televisions and computer monitors, as well as x-ray generators and fluorescent lamps.
The cathode is also surrounded by several coupled resounding cavities that help drive the oscillations of charges. This is what creates the microwave radiation that we can detect.