Or the arm itself has a slide which makes the arm able to be moved. Usually this is only possible by eccentrically drilled tonearm bases that can be turned. It has to be moved so that the correct alignment of the cartridge can be achieved given the size of the headshell slots. In this case the tonearm has to be moved to a different distance between spindle and pivot. Sometimes the correct alignment can be limited by the mounting slots in the headshell of the tonearm. A stylus contacting the record with only a few thousands of a millimetre needs an extraordinary precise aligning. With interpolations and even more by finding the pivot by simply looking along a line on the protractor, there occur too many deviations, at least added by inprecise printings of the simple protractors. With this protractor there is no interpolation necessary, which makes it fast to precisely align the cartridges and tonearms. Due to mirroring over this offset the outer null point is equally adjusted by only adjusting the cartridge precisely on the inner null point. This offset is half of the difference between outer and inner null point 121 – 66 mm. One special thing with this protractor is the offset between the guideway of the bar and the hole for the spindle.
Evaluations have shown that the IEC rules are good and lead to good results. There have been many discussions about different possible null points, but here we will not go into this. Geometrically the protractor uses the IEC rules for the both null points : 66.04 and 120.9 mm, in following given as 66 and 121 mm. With our method and tooling the protractor allows for a precision alignment of the cartridges and tone arm but it still requires very careful use ofthe protractor to achieve best results.
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