What is it?

I bought this thing at an estate sale because no one running the sale knew what is was.  They took my $5 offer.  The base is about 2.75" x 5.5" and seems to be made of bakelite.  The +/- terminals indicate it would be connected to a direct current power source (battery?).  Turning the knob at the top raises it up and down.  There is a plastic cap over the top, left removable probe type thing.  When lowered all the way down the probe is still about 3/4" from touching the left pedestal.  A magnet will not stick to any of the metal surfaces which indicates the metal might be stainless steel.