GB542968A - Improvements in magnetic apparatus for measuring the thickness of thin layers - Google Patents

Abstract

542,968. Magnetic tests. TAIT, W. H., and CHALMERS, B. July 1, 1940, No. 11119. [Class 37] Magnetic apparatus for measuring the thickness of thin coatings, such as tin, chromium, paint, or lacquer, on magnetic bases comprises a magnet adapted to be attracted to the base, a standard non-magnetic layer adapted to be inserted between the magnet and the coated base, and means for measuring the force required to detach the magnet both with and without the layer. The latter should be about 100 times as thick as the coating to be measured, and is arranged as a bronze layer over one pole 1 b of a magnet 1 which is mounted to rotate about the axis of a counter-weighted beam 2 so as to bring either the pole 1b or the uncoated pole 1a against the magnetic base. The beam 2 can swing on bearings 3 and carries a leafspring 5 engageable by a pin 7a on a pivoted lever 7 which carries a toothed segment 9 engaging a pinion 10 on a shaft 11 carrying an indicator drum 13 and ratchet wheel 12. The lever 7 is engageable by a pin 6 on a rod of a spring-dashpot 8, the arrangement being that by depressing the rod, where it is held by the pawl 15 engaging wheel 12 and depressing magnet 1 so as to free the latter and engage the base with pole 1b, the dashpot spring slowly rotates lever 7 and strains the spring 5 until the magnet is torn off the base. At this point the pawl 15 arrests wheel 12 again and an indication is given on the drum 13. The latter is rotated by mean of a friction brake 16 to zero or other given mark, magnet 1 is rotated, and the above operation repeated with the pole 1a. In this operation an eccentric 18 engages and tensions a spring 17 which assists in providing the greater force necessary. The drum-reading is then a measure of the coating thickness.