Tuesday, March 19, 2013

The Magnetron's Heritage and Progeny

I'm still catching up from the midterm yesterday.  Consequently, today's post is just a few brief scattered history of physics/engineering notes inspired by a ham radio license exam question.  The extra class ham radio exam asks: "What is a magnetron oscillator?"  The simple answer is "A UHF or microwave oscillator consisting of a diode vacuum tube with a specially shaped anode, surrounded by an external magnet"... But wait, there's way more!

The first reference I found to the magnetron was in relation to radar systems in volume six of the MIT Radiation Laboratories Series "Microwave Magnetrons".  The series of books accumulates all the knowledge gained at MIT during the war time development of radar.  This is the same MIT Radiation labs where David Tressel Griggs was piloting radar test flights in  the plane he purchased with the insurance settlement from his car crash with Agnew Hunter Bahnson, (it's a bit of a long story[2]).  The reference was to the work of Albert Hull of General Electric.  The magnetron he built is known as a cyclotron magnetron and a drawing of the device is shown below,(picture 1).

Interestingly, Hull didn't mention using the tube to generate microwave radiation he was simply using it to study the behavior of electrons between the cathode and anode in the presence of a magnetic field.  The article immediately following Hull's[2] mentions using the same tube at high voltages to experimentally verify that the mass of the electron increases as it accelerates to relativistic speeds.  There are no citations to this article regarding an actual experiment, so it seems the experiment never took place.  That's all for today, but tune in soon for more on the magnetron including how it's related to the accelerating tubes of today's particle accelerators.

1.  Hull's first 'magnetron' paper
Hull A. (1921). The Effect of A Uniform Magnetic Field on the Motion of Electrons Between Coaxial Cylinders., Physical Review, 18 (1) 31-57. DOI:

2.  Follow-up on finding the relativistic mass of the electron
Page L. (1921). Theory of the Motion of Electrons Between Co-axial Cylinders Taking into Account the Variation of Mass with Velocity., Physical Review, 18 (1) 58-61. DOI:

3.  David Tressel Griggs, radar test pilot


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