Re: Who invented the magnetic stirrer?

I am not in a position to give you a definitive answer on this one; I 
certainly do not know of anyone who disputes your grandfather's claim. But 
it would be quite a significant research project in its own right to find 
out.

Here are some relevant points and suggestions:

A very good place to start would be with a patent search. Check a few of 
your local friendly magnetic stirrers and see if they disclose a patent 
number. Or it is possible to search the patent database by subject to find 
any patents for magnetic stirrers (but very tedious!). If you can find a 
patent, any previous published work about related inventions should be 
referenced. If you are lucky, you will find a patent that you can connect 
with that rep from the chemical equipment supplier that you talk of (in 
which case your grandfather's case is looking good), or you will find a pre 
world war 2 patent that seems to have no connection with your grandfather, 
in which case his claim is looking bad.

Have a bit of a look into the history of plastics. Prior to world war 2 
there was not nearly the range of plastics available that there is today. A 
magnetic stirrer is not a great deal of use unless it is chemically 
resistant. That is why you coat the iron in an unreactive plastic. Were 
unreactive plastics available? I do not think that magnetic stirrers really 
came into their own until at least polyethylene was available for the 
coating, and probably not until teflon could be used. Plasticised PVC, or 
nylon, would erode or degrade too readily in fairly ordinary solutions 
(e.g. dilute sulfuric acid, aqueous ethanol) to be of much practical use. 
The type of chemical stirrer that was most used around the time of the 
second world war (there were still some in use in the University where I 
trained in the 1960s) consisted of a motor which rotated a vertical glass 
rod around a vertical axis. There was a loop in the end of the glass rod, 
and a shorter glass rod, about an inch long, was attached with an 
interlocking loop. The overall setup was not unlike a milk-shake machine 
(but it rotated much more slowly).

Good luck with your search. It will not be easy to prove or disprove your 
grandfather's claim. Unless an invention is published and/or patented, it 
is practically impossible to prove 50 or 60 years later.