See the chart on the second page of this:
http://www.cotorelay.com/RI-01B_Series.pdf
Can somebody tell me what the unit AT is? Amp-tesla? Google doesn't seem to recognize that phrase, and it sounds a little funny anyway. Judging from the way that it's being used, it seems like some unit of magnetic force or field strength. How does this translate into some function of the strength of a magnet and the distance away? I'm interested in using a permanent magnet, but I suspect that AT has something to do with coils.