Mercury Rectifier – Cambridge Museum of Technology

After many years of sitting in a rack gathering dust, one of the volunteers at Cambridge museum of technology has resurrected their mercury rectifier.

It used to run a large DC Motor from the new-fangled AC mains. But this was soon superseded by a replacement.

The Rectifier has lain dormant since at least the 70’s. The Museum has managed to get a supply to the anodes and enough current via it’s ignitor circuit to get the tube lit up and working (although at much lower power).

A fantastic achievement that shows the engineering (and beauty?) of these rare devices. Much better than an odd shaped dust covered glass thingy in a rack.

Cambridge Teslathon 2015

My Stuff

My Van De Graff and a Moving Static Display.



Alex’s Stuff


Dave’s Coil

Phil T and Phil S’s Coil(s)



John and Steves’s stuff

Steve’s coil with rotating Breakout

Marks battery powered coil.

(sorry no photo)


Other interesting stuff from Cambridge 2015

Also: Slow Motion Sparks from Cambridge Teslathon 2015

With thanks to all of the Coilers (and wives) who brought stuff and supported us, Staff and volunteers at Cambridge Museum of Technology who let us abuse their electricity supply for the day and the visitors who asked some great questions this year.

Watch this space for details of next years Teslathon at the end of October 2016.


Tesla coil Slow motion Video using a raspberry Pi

After a suggestion by Phil T at Cambridge Teslathon I tried to make a slow motion video using my EMIpi

After finding the right command line for high frame rate from I tried the 90 FPS at 640/480 pixels

As I had no way to view the videos I had no idea what I’d captured. Until after the Thon.

Returning home I found a problem. First I hadnt got a .264 player, and then when I played the videos they were at normal speed.

Reading up on .264 the frame rate isn’t encoded in the video, so the player will try to play them at normal speed. A quick google revealed the answer if you use a program called GPac you can re-encode the videos at mp4 and re-sample them at whatever viewing rate you like.

This is my test video shot at 90fps and re-sampled at 5fps (1/6 real speed see below)

and the same video re-sampled at 1fps (1/30 real speed see below)

The actual speed in these videos is open to debate.

The first one should be 18 times slower than “real” and the second 90 time slower than real. But if that were true the frames would change one a second, and that is clearly not the case.

So my guess is that MP4box is assuming that the original video was 30fps (not 90fps) so setting a frame rate in MP4box of 30fps would give an actual speed of 1/3rd real. 10fps therefore would be 1/9th real, and 1fps would be 1/30th real.

I should be able to prove this as I know the break rate of the coil, and should be able to analyse the arc progression with each bang.

Downside is obviously the resolution.


I need to try the other modes.

  • 2592×1944 1-15fps, video or stills mode, Full sensor full FOV, default stills capture
  • 1920×1080 1-30fps, video mode, 1080p30 cropped
  • 1296×972 1-42fps, video mode, 4:3 aspect binned full FOV. Used for stills preview in raspistill.
  • 1296×730 1-49fps, video mode, 16:9 aspect , binned, full FOV (width), used for 720p
  • 640×480 42.1-60fps, video mode, up to VGAp60 binned
  • 640×480 60.1-90fps, video mode, up to VGAp90 binned

Although the higher resolution ones are probably not fast enough to capture anything interesting in a TC discharge.