General question about spark testers

[benhogan]

I have an in-line spart tester that I am using to track down a misfire in a cylinder. I do not know if it is caused by a bad spark plug or a bad ignition wire.

If I put the spark tester between the plug and the ignition wire and I see a spark. Does this mean that the spark plug is firing or does this just mean that electricity is just getting to the plug.

This is the tool.

I guess I could check the resistance of the ignition wires. Does anyone know what the ballpark acceptable resistance should be for ignition wires?