Barcol hardness test the Barcol impressor was developed by Walter Colman as a hand-held, portable means of assessing the hardness of a material during World War II. The United States Army Air Corps required a hand-held method of checking the hardness of rivets due to concerns that aircraft could be sabotaged by replacing normal rivets with soft lead or wooden ones which would fail during flight. The impressor operates when the tip is pressed against the material in question. The hardness of the material determines how far the tip indents and this is transferred by a tension spring and lever to be read
Walter Janka 1932 he undertook a type setting apprenticeship. In 1930 Walter Janka became an Organisation Leader, and then a Political leader of the Young Communists (KJVD / "Kommunistischer Jugendverband Deutschlands") for the Chemnitz sub-region. After his elder brother, , had been murdered by the Nazis, Walter himself was imprisoned by the Gestapo. He was remanded in custody in Chemnitz and in Freiburg before being convicted of preparing to commit high treason. After 1 years of imprisonment in Bautzen prison, he spent a six-month term in Sachsenburg concentration camp. Finally, in 1935 he was deported to Czechoslovakia. In 1936 Janka went to