Leyden Jar.

A form of static condenser.

In its usual form it consists of a gla.s.s jar. Tinfoil is pasted around the lower portions of its exterior and interior surfaces, covering from one-quarter to three-quarters of the walls in ordinary examples. The rest of the gla.s.s is preferably sh.e.l.lacked or painted over with insulating varnish, q. v. The mouth is closed with a wooden or cork stopper and through its centre a bra.s.s rod pa.s.ses which by a short chain or wire is in connection with the interior coating of the jar. The top of the rod carries a bra.s.s k.n.o.b or ball.

If such a jar is held by the tinfoil-covered surface in one hand and its k.n.o.b is held against the excited prime conductor of a static machine its interior becomes charged; an equivalent quant.i.ty of the same electricity is repelled through the person of the experimenter to the earth and when removed from the conductor it will be found to hold a bound charge. If the outer coating and k.n.o.b are both touched or nearly touched by a conductor a disruptive discharge through it takes place.

326 STANDARD ELECTRICAL DICTIONARY.

Fig. 213. LEYDEN JAR WITH DISCHARGER.

If one or more persons act as discharging conductors they will receive a shock. This is done by their joining hands, a person at one end touching the outer coating and another person at the other end touching the k.n.o.b.

From an influence machine a charge can be taken by connecting the coating to one electrode and the k.n.o.b to the other.

Fig. 214. SULPHURIC ACID LEYDEN JAR.

327 STANDARD ELECTRICAL DICTIONARY.

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 subst.i.tuted a quant.i.ty 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 pa.s.ses is so large as not to allow the wood to touch it. Thus any leakage from inner to outer coating has to pa.s.s over the inside and outside gla.s.s surfaces. In the common form of jar the wooden cover may short circuit the uncoated portion of the inner gla.s.s surface. In the cut a simplified form of Thomson's Leyden jar is shown, adapted for scientific work.

Lichtenberg's Figures.

If the k.n.o.b of a Leyden jar or other exited electrode is rubbed over the surface of ebonite, sh.e.l.lac, resin or other non-conducting surface it leaves it electrified in the path of the k.n.o.b. 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 princ.i.p.ally 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.

328 STANDARD ELECTRICAL DICTIONARY.

In the case of a mile-long flash it has been estimated that 3,516,480 De la Rue cells, q. v., would be required for the development of the potential, giving the flash over three and one-half millions of volts.

But as it is uncertain how far the discharge is helped on its course by the rain drops this estimate may be too high.

There are two general types of flash. The so-called zigzag flash resembles the spark of an electric machine, and is undoubtedly due to the disruptive discharge from cloud to earth. Sheet lightning has no shape, simply is a sudden glow, and from examination of the spectrum appears to be brush discharges (see Discharge, Brush) between clouds.

Heat lightning is attributed to flashes below the horizon whose light only is seen by us. Globe or ball lightning takes the form of globes of fire, sometimes visible for ten seconds, descending from the clouds. On reaching the earth they sometimes rebound, and sometimes explode with a loud detonation. No adequate explanation has been found for them.

The flash does not exceed one-millionth of a second in duration; its absolute light is believed to be comparable to that of the sun, but its brief duration makes its total light far less than that of the sun for any period of time.

If the disruptive discharge pa.s.ses through a living animal it is often fatal. As it reaches the earth it often has power enough to fuse sand, producing fulgurites, q. v. (See also Back Shock or Stroke of Lightning.)

Volcanic lightning, which accompanies the eruptions of volcanoes, is attributed to friction of the volcanic dust and to vapor condensation.

[Transcriber's note: The origin of lightning is still (2008) not fully understood, but is thought to relate to charge separation in the vertical motion of water droplets and ice crystals in cloud updrafts. A lightning bolt carries a current of 40,000 to 120,000 amperes, and transfers a charge of about five coulombs. Nearby air is heated to about 10,000 ?C (18,000 ?F), almost twice the temperature of the Sun?s surface.]

Lightning Arrester.

An apparatus for use with electric lines to carry off to earth any lightning discharge such lines may pick up. Such discharge would imperil life as well as property in telegraph offices and the like.

Arresters are generally constructed on the following lines. The line wires have connected to them a plate with teeth; a second similar plate is placed near this with its teeth opposite to those of the first plate and nearly touching it. The second plate is connected by a low resistance conductor to ground. Any lightning discharge is apt to jump across the interval, of a small fraction of an inch, between the oppositely placed points and go to earth.

Another type consists of two plates, placed face to face, and pressing between them a piece of paper or mica. The lightning is supposed to perforate this and go to earth. One plate is connected to the line, the other one is grounded.

The lightning arrester is placed near the end of the line before it reaches any instrument. (See Alternative Paths.)

329 STANDARD ELECTRICAL DICTIONARY.

Fig. 215. COMB OR TOOTHED LIGHTNING ARRESTER.

Fig. 216. FILM OR PLATE LIGHTNING ARRESTER.

Lightning Arrester, Counter-electro-motive Force.

An invention of Prof. Elihu Thompson. A lightning arrester in which the lightning discharge sets up a counter-electro-motive force opposed to its own. This it does by an induction coil. If a discharge to earth takes place it selects the primary of the coil as it has low self-induction. In its discharge it induces in the secondary a reverse electro-motive force which protects the line.

Lightning Arrester Plates.

The toothed plates nearly in contact, tooth for tooth, or the flat plates of a film lightning arrester, which const.i.tute a lightning arrester. Some advocate restricting the term to the plate connected to the line.

Lightning Arrester, Vacuum.

A gla.s.s tube, almost completely exhausted, into which the line wire is fused, while a wire leading to an earth connection has its end fused in also.

A high tension discharge, such as that of lightning, goes to earth across the partial vacuum in preference to going through the line, which by its capacity and self-induction opposes the pa.s.sage through it of a lightning discharge.

It is especially adapted for underground and submarine lines.