Plate XV. The constellations ‘Telescopium’ and ‘Microseopium’ from Adams’
Celestial Globe (Cat. 93).
E L E C T R I C I T Y
280 ELECTROSTATIC GENERATOR 1774 (51-2) Fig. 248, 249
by Martinus van Marum & Gerhard Kuyper, Groningen
Baseboard 716 x 255, height to top of disk 620, diameter of disk 410.
The friction disk is mounted on an axle with a crank handle, being supported by
brass bars that are provided with slots so that the extent to which the disk dips into
the wooden trough may be varied. The trough holds mercury, which is the rubbing
agent in this machine, and the novel feature of its design. The trough is supported by
a block of pitch. The original disk was made of shellac, and became broken in the
course of time (some portions are preserved); in 1930 a replacement disk was made
of black-painted wood of the same size as the original disk. At either end of the baseboard
are the brass electrodes mounted on glass pillars. One is connected to the mercury,
and so takes a positive charge, while the other could be connected to the collector,
and so become negatively charged. The collector is mounted on a glass pillar attached
to an axial support.
While at the University of Groningen, Van Marum became a convinced exponent
of Franklin’s one-fluid theory of electricity, and this led him to construct a frictional
electrical machine in collaboration with the local philosophical instrument-maker,
Gerhard Kuyper. This instrument shows that Van Marum had an early gift for the
design of experimental apparatus. Van Marum published an account of the instrument
in 17 76, and his engraving is reproduced as fig. 32 in volume iii of the present work.
Van Marum sold this generator to Teyler’s on 19 November, 1790, for ƒ 120. At that
time it was equipped with two lacquer and one glass disks.
Van Marum (1776); Hackmann (1971a); (L & W iii 1971b).
I™. 5313