| This apparatus, in the collection at Hampden-Sydney
College in Virginia, is listed in the 1900 Max Kohl catalogue as
"Apparatus for showing the difference of the two electricities" and cost
20 Marks. It has suffered somewhat over the years: the insulating columns
were once straight, and the catalogue illustrations shows a point facing
toward a disk, with one point on the left and one point on the right.
The apparatus was designed to illustrate the asymmetry
of discharges from positive and negative points. The 1923 Leybold catalogue
calls this "Gaugains's valve apparatus for showing that positive electricity
passes more easily from a point to a plate, negative on the contrary passes
more easily from a plate to a point."