3 lead spark testers ?

The tester below has 3 connection points. The clip goes to the block/gnd, the spark plug wire connects to the to bare stud. I curious what the insulated stud is for. This tester is based on an old Briggs and Stratton design.

 

I'd guess it is a point you can optionally touch to ground.

That would allow the cylinder to be cut off for testing the power change, or to see if there were any output if there was not enough to cause spark across the main gap.

It still looks to have a gap, just a very small one which would spark before the main gap or plug.

 

Thanks guys.

Given how little of the 3rd stud extends outside the insulation I don't see it being a place to put a plug wire.

These are sold by Briggs & Stratton for small engines. The original Briggs design used screws which made all gaps adjustable. You can easily ground out the engine by touching gnd to the lead the spark attaches too.

Generally this sort of thing is made as inexpensively as possible and still function. There has to be a better reason for the 3rd stud.

 

OK fair enough.