| 摘要 |
A magnetic recording indicator on banknotes (10) or other valuable documents uses a low Curie temperature magnetic material, for example, as a magnetic pigment, such as CrO2. CrO2 is very black in color and is an excellent magnetic recording medium, and has a Curie temperature of 128 degrees C. A region (12),(14), (16) of a banknote or other valuable document is printed with an ink containing CrO2 particles. To test the validity of the document, the magnetic media on the document is subjected to magnetic field having a characteristic spatial pattern; the field of a permanent magnet (18) having alternating magnetic poles is a convenient field source. The banknote, and its magnetized region, is then brought to a temperature of at least 128 degrees C, which is readily accomplished by use of a heat lamp (22), and the region inspected with a magnetic field sensitive optical reader (20). If it is a genuine bill whose magnetized region was printed with an ink containing CrO2, the recorded magnetic pattern will have disappeared as the media becomes non-magnetic above its Curie temperature. A counterfeit, if recorded with an ink containing Fe3O4, will retain the recorded pattern when heated to 128 degrees C, as its Curie temperature is 585 degrees C. <IMAGE> |
