285 THREE LEYDEN JARS c. 1785 (491/1,^ Fig. 272
Possibly by John Cuthbertson, Amsterdam
Three jars, (height 250, diameter 160): a, b have high, turned wooden caps, through
which pass brass rods carrying on top a sphere and below a ‘plunger’ ; c has
a flatter cap, a thinner rod and a chain on the inside. An earthing band is fixed
to the outside of the jars. The coated area is 0.1 m2, which corresponds to the area on
the jars used by Van Marum in his first battery. Van Marum ordered from John
Cuthbertson on 21 February, i785,asetof 135 Leydenjars (MV), and these examples
may be survivors from that order.
The Leyden jar takes its name from Petrus van Musschenbroekliexperiment of
January 1746.
Van Marum (1785); (1787); Dorsman & Crommelin (1957); Heilbron (1966).
286 LEYDEN JAR % 18th C. i ;|n52) Fig. 258
The glass jar (height 100, diameter 70) is foiled for most of its height, and has a-,
boxwood cap. The cap supports the brass electrode terminating in a chain below and
a sphere above that projects 50 mm above the cap.
287 LEYDEN JAR 4/4 18th C. (489) Fig. 259
The interior of the glass jar (height 270, diameter 120) is filled with crumpled foil,
and the top is sealed. Above the level of the foil, the jar is painted red;
288 INSULATED TABLE % 18th C. (1
A turned mahogany base supports a pillar of pitch-like material, with a circular
wooden table (outer diameter 180) on top (overall height 315J, This is provided with a
rim (inner diameter 162), and a channel is cut into the wood to the centre. This table
was probably used to isolate a Leyden jar.
Inv. 37(4
The electrical discharge in a partial vacuum
7. Picard (1620—1682) noticed, in 1676, a glow at the end o f a barometer tube when it was being
carried at night. This phenomenon was later explained by Francis Hauksbee Snr (d. i 7,3)>
as caused by friction between the mercury and the glass wall, and the effect was noticed by others
when working with evacuated glass vessels. Van Marum proposed that the Aurora Borealis was
produced by an electrical discharge in rarified air. In fact, the effects are most pronounced when
the pressure in the vessel is between 1 and 10 to n ; Benjamin Wilson (1721-1788j discovered
that there is no luminous discharge in a strong vacuum.
289 THREE AURORA FLASKS 4/4 18th C. (485/1-3) Fig. 259
a,b,c Three closely similar flasks (height 280—300, maximum diameter 185). The glass
flask, the lower half of which is covered in foil, is evacuated and sealed with a brass
knob, from which a pointed glass rod descends into the middle of the flask. There
is a connector in the cap for attachment to an air-pump.
290 AURORA TUBE % 18th C. (532)
The glass tube (overall length i,55cHength of glass 1,390, diameter 50), is terminated
at each end by a brass collar, one of which contains a valve for evacuation, and a
spherical cap, from which a brass rod and ball project into the tube.
Inv. 56/1
291 AURORA TUBE 4/4 18th C. (1156) Fig. 265
The glass tube- (overall length 340, length of glass 260, diameter 26), is terminated
at each end by a brass collar, in one of which is a valve for evacuation, and a
spherical cap. A rod and ball project into the tube from one end, and a pointed rod
from the other. The character of the discharge shows whether the conductor is
positively or negatively charged.
Adams^1784) IV, fig. 61, 62.
292 AURORA TUBE pre 1790 531) Fig. 260
A thin, straight glass tube mounted on a circular mahogany base (overall height 550),
is surrounded by a thin helical glass tube; both are broken at the top.
On ig November. 1790, Van Marum sold to Teyler’s an evacuated spiral tube, and
two evacuated tubes containing phosphorus (Cat. 331), for ƒ n (MV).
Inv' ^ |l|f
293 AURORA JAR AND STAND % 18th C. (538) Fig. 261
The glass jar (height 330, diameter 180), has a wide neck on which rests a brass
disk. In the centre of the disk is a boss holding a length of glass tube, through which
slides a brass rod, the lower end having a Small sphere and the upper end a
terminal.
There is a stand for use with this and similar jars. It consists of a turned mahogany
base with a brass threaded socket to take a turn-cock fixed to a brass disk (height
of stand 250, disk diameter 250). The turn-cock can be attached to an air-pump, the
jar seated to the disks top and bottom by grease, and a vacuum created in the jar.
On removal from the pump, the wooden base is fitted for the demonstration of the
glow discharge.
Inv. 36(10