The disadvantage of the cold exchangers ist that you never see,
whether your magnet has enough nitrogen. To prevent the dripping of
the ice from the silicon rubber tube, we use the following cheap
device:
Cut away the upper conical part of a typical 1l polyethylene lab
bottle. Drill a hole into the bottom, which is a bit smaller in
diameter than the nitrogen ports of the magnet. Put the bottle over
these ports. The water will drip into the bottles and evaporate from
there. You always are visually aware, that the nitrogen dewar is
cold.
stb