Abstract
This thickness gage measures the force necessary to overcome the magnetic attraction between the precision formed magnetic needle in the gage and the magnetic base material. The essential part of this gage is a magnetic needle, which is made of iron of high magnetic permeability and is fixed so as to be perpendicular to the coating to be measured. This needle is magnetized and attracted upward by the moving permanent magnet placed at the other end of the needle. To measure the thickness first the position of the permanent magnet is changed by turning the barrel. Then, the needle over the film is magnetized gradually by the moving permanent magnet. When the magnetic attraction of the magnet for the needle becomes larger than the force holding the needle to the surface to be tested, the needle with the indicater rod connected to it jumps saddenly apart separately from the film and the indicater extends above the top of the gage, which provides accurately the definite end point indication. It was found that the moving distance of the magnet is proportional to the thickness of the film.
The gage is calibrated with standard coating of known thickness. From the graduation marks on the barrel of the gage, the thickness is read directly. To rest the needle, depress the rod with the gage-end firmly against the surface to be measured.
This gage will determine coating thickness in the range from 0.002mm up to 0.5mm. In this report the effect of the magnetic properties of the base metal, the aging effect of the needle and the magnet and the error problem are also discussed.
