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Figure 3. Types of modern sealsClick to Enlarge Image

Prehistoric evidence indicates that people have always been concerned with detecting whether others have tampered with their belongings. Early human beings may have swept the ground in front of their dwellings to detect trespassers' footprints. At least 7,000 years ago, intricate stone carvings were pressed into clay to seal jars and later, writing tablets. What is the most secure way to ensure that people are not messing with your things? Roger Johnston's tests have covered everything from ancient clay seals to metal flange seals used to secure cargo containers and electronic seals used on nuclear material. He has found that high-tech, expensive seals are often no more reliable, and factors such as properly training inspectors to know what to look for are often just as important as the seal itself. Johnston has also developed some new electronic seals that are harder to defeat because they use "anti-evidence": They provide the correct passcode only when they are not tampered with, and the passcode is erased if they are interrupted.

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