Arc Protractors

Regarding arcs, there is an argument that says that a second null is pointless for arcs, as differences in mounting distance lead to tiny unobservable variations in the outer null.

However this makes the erroneous assumption that the inner null will be set exactly. It is almost certain this will not be the case, due to observation errors in offset, therefore a second null allows errors in offset to possibly become apparent and allows a mean setting.

It also avoids a false sense of security being generated in the user, as it acknowledges the possibility of an erroneous setting.

While I do not quibble with the ease of use of arcs, I have yet to see any proof of their increased accuracy. The usual argument is that if the new arc shows a difference in alignment  from the old alignment using a two point,  the old alignment was inaccurate (rather than vice versa). Or that the new alignment with an arc sounds better, which simply begs the question of whether the old alignment was not so well set up.

All arc protractors should have two nulls as a simple means of cross checking, as, by definition, a correct alignment should be square to both nulls. If not, then something is wrong with the arc, the mounting distance or the cartridge offset.