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When doing an ECG we measure the electrical voltage between two points on a person's skin (for example for Lead II it's right arm and left leg).

I am confused as to how the heart's potential difference can travel all the way to the skin. It seems like the potential difference is not that big and I would be very surprised to see that small charge difference make it through blood which is constantly moving.

I would like a more physics heavy explanation if possible.

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  • The body is permeated with salt water, which is a pretty good conductor. It's everywhere, not just in the bloodstream. – Carey Gregory Jul 24 '20 at 19:46

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