First, the tip is brought very close to the surface of thing being studied. This distance is about half of a nanometer. Then the tip is moved very carefully back and forth across the surface. The electrical current is measured while the tip is moved back and forth (constant height method). The STM can also work by adjusting the tip so the tunnel current stays the same (constant current method). Using the constant-height method is faster. Using constant-current method helps avoid bumping the tip into things on the surface, so it can study things that are rougher.