Hot Plate Magnetic Stirrer: Ring Heater over a Relay Magnet on a Shielded Spindle (Hug, US 3,028,476)

"Attempts have been made to couple conventional heating means to magnetic stirrers, but the same have proved to be unsuccessful. Thus, the base of a laboratory vessel must be positioned relatively close to the driving rotating magnet ... Moreover, if an appreciable amount of heat from the heating means is transmitted to the motor of the magnetic stirrer, it will drastically reduce the useful operating life of said motor."

Make a housing of aluminium (or another non-magnetic material) and mount an electric motor inside with its shaft upright. Fix a permanent bar magnet horizontally on the top of the shaft, just under the housing top. Wire the motor through a rheostat to its cord so the speed can be varied. Sizes, motor rating and how the housing is formed are not given.

Shape a radiation shield of non-magnetic stainless steel so it straddles the top of the housing, on stainless-steel legs (a poor conductor of heat) that stand on the housing top. Bore its centre and fit a threaded spindle there with a nut on each side of the shield. Tap a blind thread in each leg.

Bore a blind cavity in a hub of non-magnetic material so it turns freely on the spindle. Lay a permanent bar magnet 1/4 in. by 1 in. by 1 3/4 in. on top of the hub and hold it with a strap screwed to the hub. The hub and strap material is not given.

Make three support members of non-magnetic stainless steel, each an upright with flanges. Screw the flanges down into the legs, with small tubular spacers of insulating material round the screws between the shield and the flanges, so air can circulate and carry heat away. Fit a grommet in one support for the heater cord. The spacer material is not given.

Mount an annular ring heater (elements in a non-magnetic sheath) coaxial with the spindle; join its cord wires to its threaded stud between two nuts. Above it lay an aluminium heat sink, a flat plate wider than the heater, so the relay magnet is about 1/2 in. below it. Cover with a top plate of aluminium or stainless steel whose edge is spun into an inverted dish, and screw the edge to the tops of the supports. The patent claims a blackened outer surface on the heat sink and top plate (how is not given). In use, set the hot plate on the stirrer and put a stirring bar (a short bar magnet sealed in glass or coated with PTFE) in the vessel: the drive magnet turns the relay magnet, which turns the bar.