Magneto Test Bench Electrode Design

John,
I am building a magneto test stand and I'm wondering why there are typically 3 electrodes coming together at each spark gap?  I would think only 2 are needed.  For example, see the third contact at each spark gap in the photos below of other test sets.  I am currently planning to only have two contact points at each spark gap unless there is a good reason to add the third.

Teaser Electrode extends from back of bar to lower electrode

The teaser electrode is used to solve the problem of time lag when using pointed electrodes (versus a spherical electrode). Time lag results in inconsistent voltage to produce an arc (voltage tends to be higher or overshoot without the teaser). This third electrode is universal used when measuring arc voltage across pointed electrodes.

The teaser electrode is insulated from the other two electrodes and is typically set to 0.1 mm gap from the main electrode (5 mm gap). As voltage rises on the main electrode a tiny arc (invisible) travels to the teaser which produces ionization in the main spark plug gap which helps set off the main arc. 

Without the third electrode you have no method of comparing your results with book values since the spark voltage will be different and variable. I would add that I believe that the 0.1 mm gap distance has to be accurately set on all electrodes in order to get even sparking across leads.

A properly designed magneto tester with a properly operating magneto should produce such even sparks that the sound of the sparks is as regular as music. You should be able to detect a bad magneto from across the room with your back turned to the tester just from the sound of the arching. 

See the following document for a more thorough analysis

http://naca.central.cranfield.ac.uk/reports/1925/naca-report-202.pdf