Leyden Jar, Sir William Thomson's.
An especially efficient form of Leyden jar. It consists of a jar with
outer tinfoil coating only. For the interior coating is substituted a
quantity of concentrated sulphuric acid. The central rod is of lead with
a foot, which is immersed in the acid and from which the rod rises. A
wooden cover partly closes the jar, as the central tube through which
the rod passes is so large as not to allow the wood to touch it. Thus
any leakage from inner to outer coating has to pass over the inside and
outside glass surfaces. In the common form of jar the wooden cover may
short circuit the uncoated portion of the inner glass surface. In the
cut a simplified form of Thomson's Leyden jar is shown, adapted for
scientific work.
Lichtenberg's Figures.
If the knob of a Leyden jar or other exited electrode is rubbed over the
surface of ebonite, shellac, resin or other non-conducting surface it
leaves it electrified in the path of the knob. If fine powder such as
flowers of sulphur or lycopodium is dusted over the surface and the
excess is blown away, the powder will adhere where the surface was
electrified, forming what are called Lichtenberg's Figures, Lycopodium
and sulphur show both positive and negative figures, that is to say,
figures produced by a positively or negatively charged conductor. Red
lead adheres only to negative figures. If both positive and negative
figures are made and the surface is sprinkled with both red lead and
flowers of sulphur each picks out its own figure, the sulphur going
principally to the positive one.
The red lead takes the form of small circular heaps, the sulphur
arranges itself in tufts with numerous diverging branches. This
indicates the difference in the two electricities. The figures have been
described as "a very sensitive electrosope for investigating the
distribution of electricity on an insulating surface." (Ganot.)
Life of Incandescent Lamps.
The period of time a lamp remains in action before the carbon filament
is destroyed. The cause of a lamp failing may be the volatilization of
the carbon of the filament, causing it to become thin and to break; or
the chamber may leak. The life of the lamp varies; 600 hours is a fair
estimate. Sometimes they last several times this period.
The higher the intensity at which they are used the shorter is their
life. From their prime cost and the cost of current the most economical
way to run them can be approximately calculated.
[Transcriber's note: Contemporary incandecent buls are rated for 1000
hours; flourescent bulbs up to 24000 hours; LED lamps up to 100000 hours.]
Lightning.
The electrostatic discharge to the earth or among themselves of clouds
floating in the atmosphere. The discharge is accompanied by a spark or
other luminous effect, which may be very bright and the effects, thermal
and mechanical, are often of enormous intensity.
The lightning flash is white near the earth, but in the upper regions
where the air is rarefied it is of a blue tint, like the spark of the
electric machine. The flashes are often over a mile in length, and
sometimes are four or five miles long. They have sometimes a curious
sinuous and often a branching shape, which has been determined by
photography only recently. To the eye the shape seems zigzag.