Then attach the positive clamp to the threaded end of the glow plug. Be careful to not let the positive and negative clamps touch.
Pictured is a good, brand new Bosch glow plug lighting up nice and hot at the very tip. This is how the plugs should all perform.
Be extremely careful during this process! Never ever touch the tip, or place it on a flammable surface just after testing. The tip can reach temperatures over 1000F!