We used spot welding transformers with a low voltage output to give us 
very high currents to test contact resistance. We put a variac on the 
primary side to control the input voltage. We clamped the device under 
test in the welder tongs and measured the voltage across the device 
under test. Of course, we were measuring AC millivolts, but the 
resistance of the device could be calculated. In your instance, 200 amps 
through the device, measured with a clamp on AC meter,  would yield 
0.020 millivolts for a resistance of 0.0001 ohms measured across the 
device.

I believe the transformers we used had a winding ration of 50 to 1. The 
secondary, low voltage side was a solid copper bus.


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