I Don't Understand Spot Welding

I found a free spot welder and can't get anything to weld. Is it broken or am I just useless at welding? It definitely goes zap, but doesn't seem to join anything together. The manual is long on metallurgy of exotic alloys and short on garage-level how-to instructions. Should I turn this into a railgun or try to actually use it as a spot welder? What would I even use a spot welder for? Feel free to leave suggestions in the comments!

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