What is this black "wire" type thing clamped between my cold and hot water pipe? What is its purpose? 
Plumbing – this wire clamped between the cold and hot water pipes
plumbing
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Best Answer
It is a ground cable. It completes the ground between the hot and cold water pipes.
The ground path through the water heater might not be complete. This cable will make sure there is a ground path between hot and cold water pipes.