Howard Robert Johnson developed a permanent magnet motor and, on April 24, 1979, received U.S. Patent 4,151,431.[The United States Patent office main classification of his 4151431 patent is as a "electrical generator or motor structure, dynamoelectric, linear" (310/12).] Johnson said that his device generates motion, either rotary or linear, from nothing but permanent magnets in rotor as well as stator, acting against each other.[39] He estimated that permanent magnets made of proper hard materials should lose less than two percent of their magnetization in powering a device for 18 years.[40]