Digital confusion - There are two main reasons:
A. The surface of the protective film is damp or stained with cutting fluid (or other conductive particles).
Treatment: Wipe the surface of the ruler and the protective film with a dry cloth slightly dampened with absolute ethanol or aviation gasoline.
B. The copper clad plate under the protective film is affected by corrosion and blackening.
Treatment method: Uncover the protective film, polish the surface of the copper strip with high-grade rubber or metallographic sandpaper, and clean it.