How does the 'open circuit' flux density, the remanence, and the shape of a magnet relate to each other? I'm searching for magnets to buy on Alibaba for a magnetic circuit I'm building. One seller measured 264 mT, 225 mT and 80 mT using a Teslameter, for block shaped magnets she claims are of grade N35, for which she also measured the sizes $28.8\ \text{mm}\times 18.8\ \text{mm}\times 9.5\ \text{mm}$, $28.9\ \text{mm}\times 9.4\ \text{mm}\times 4.5\ \text{mm}$, and $29.0\ \text{mm}\times 9.3\ \text{mm}\times 1.7\ \text{mm}$, respectively. The last measurement in each magnet size is the distance between the poles.
According tho this page, it seems that N35 neodymium magnets should have magnetic flux densities that lie within the interval 1170–1220 mT. Clearly, all of the measurements the seller gave me are much lower than the lower limit for the interval given on this page, which led me to believe that what she was selling was not N35 magnets even though she says they are.
On the other hand, according to another page on the same domain, what you measure with a Gauss meter—which to my understanding is the same thing as a Teslameter—is "an 'open circuit' value which will be substantially lower than the Br value" (the remanence) "and will be directly related to the material and the length to diameter ratio of the magnet." It also states that "long magnets with small diameters will have a much higher open circuit flux density than short magnets with relatively large diameters, even when they are manufactured from the same grade of magnetic material." Intuitively, this makes sense to me, since a very short magnet should reasonably have a very weak effect on any measuring instrument (although I must say that my intuition when it comes to magnets and magnetic fields at large is very rudimentary, and at the same time, if a magnet becomes thinner, my intuition tells me that it would also yield a lower reading on the measuring instrument, but a thinner magnet means a magnet that has a larger length-to-"diameter" ratio, which according to the cited statement seems like it maybe should yield a higher reading, not a lower reading...).
What I'm wondering is:

*

*What is the relationship between the remanence of a magnet and the 'open circuit' flux density as measured by a Teslameter held at one of its poles? Is the open circuit flux density always (much) lower than the remanence? Can the ratio between the two be calculated from the shape of the magnet somehow?


*Can I say anything about the grade of the magnet from the Teslameter readings and the size measurements she gave me? Can I use them to calculate the likely remancences of the magnets?
 A: N35 is realatively weak. There are N42, N48, N52 magnets and they are all very cheap so there is no motive to lie about N35.
How much magnetic flux density the magnet can provide is not related to N35, N42 or N52, but related to what type of magnet it is and whether it is fully magnetized.
If a magnet is too short, its magnetic flux densities will de-magnetize itself. Nxx is a measure of how strong the magnet is to resist such a de-magnetization force. With N52 material you can make shorter magnets than N35 that can still keep themselves magnetized.

How much magnetic flux densities you measure will depend on the shape of the magnetic circult. Think this way:The N pole is a hose opening spilling out a flux (think of gas) and the same amount of flux will go back into the S pole. The opening is not directed; the flux coming out spills "forward" as well as "backward" and expand to go everywhere. So with a "fully" open circult like the left one in the figure, you measure at most half of the density possible (half if the magnet is very long; less if it is shorter). If the magnet is in a "closed" circult like the right one in the figure, it is possible that you measure the full flux this kind of magnet material can provide.
