The Boy's Playbook of Science Part 23

A great variety of experiments, depending on the proper arrangement of discs of tinfoil on various tubes of coloured gla.s.s are manufactured, and some in the form of windmills, the sails being made luminous by the pa.s.sage of the electricity. The names of ill.u.s.trious electricians, beautiful crescents, stars, and even profile portraits, have been produced in continuous streams of electric sparks.

_Thirty-first Experiment._

When an electrified body is brought towards another which is not electrical, the latter is thrown into the opposite state of electricity as long as the excited body remains in its neighbourhood; and this condition of electrical disturbance, set up without any contact or supply of electricity, is called _induction_, and involves a vast number of interesting facts, which are thoroughly discussed in Dr. Noad's excellent work on electricity, but can only be briefly alluded to here.

[Ill.u.s.tration: Fig. 172. The lengths of bra.s.s wire supported on gla.s.s rod pillars indented by blowpipe, so as to retain the bra.s.s wires with the pith b.a.l.l.s hanging from each series, the letters P and N mean Positive and Negative, and the signs for these terms are placed above.

The letters P and N are painted on the blocks which support the gla.s.s rods.]

If a number of lengths of bra.s.s wire, supplied with b.a.l.l.s at the extremities, are supported on gla.s.s legs and arranged in a line, with a little pith ball attached to a thread hanging from each end of the length of bra.s.s wire, the effect of induction is shown very nicely; and when an excited gla.s.s rod is brought towards one end of the series, the rising of the pith b.a.l.l.s to each other betrays the change which has occurred in the electrical state of the bra.s.s wires by the mere neighbourhood of the excited gla.s.s tube. The gla.s.s tube is electrified positively, and attracts the negative electricity from the bra.s.s wire towards the end nearest to it; [Page 188] the other extremity of the bra.s.s wire is found to be in the positive state, and this re-acting on the next, and so on throughout the lengths, completes the electrical disturbance in the whole series. (Fig. 172.)

_Thirty-second Experiment._

[Ill.u.s.tration: Fig. 173. A A. Large circular tin or bra.s.s disc with turned-up edge half an inch deep, and containing the resinous mixture B, which is rubbed with the warm flannel. C C. The upper plate supported by the gla.s.s handle D, a pith ball attached to a wire shows the electrical excitation, and the spark is supposed to be pa.s.sing to the hand E.]

If an insulated bra.s.s rod (such as has been described in the last experiment) is touched by the finger whilst under induction, it remains permanently electrified on the removal of the disturbing electrified body; and it is on this principle that the useful electrical machine called the Electrophorus is constructed. This _constant_ electrical machine--for it will remain in action during weeks and months if kept sufficiently dry--was invented by Volta in the year 1774, and has been brought to great perfection by Mr. Lewis M. Stuart, of the City of London School; so that with a little additional apparatus the whole of the fundamental principles of electricity can be demonstrated. It consists of a flat bra.s.s or tin circular dish about two feet in diameter and half an inch deep, which is filled with a composition of equal parts of black rosin, sh.e.l.l-lac, and Venice turpentine; the rosin and the Venice turpentine being first melted together, and the sh.e.l.l-lac added afterwards, care of course being taken that the materials do not boil over and catch fire, in which case the pot must be removed from the heat, and a piece of wet baize or other woollen material thrown over it.

Another tin or bra.s.s circular plate of twelve inches diameter, and supported in the centre with a varnished gla.s.s handle nine inches long, is also provided, and the resinous plate being first excited by several smart blows with a warm roll of flannel, the plate held by the gla.s.s handle is now laid upon the centre of the resinous one, and if removed directly afterwards, does not afford the electric spark; but if, whilst standing upon the excited resinous plate, it is touched, and then removed by the gla.s.s handle, a powerful electric spark is obtained; and this may be repeated over and over again with the like results, provided the plate with the gla.s.s handle is touched with the finger just before lifting it from the resinous plate. (Fig. 173.) [Page 189] The electricity excited on the resinous plate is not lost, and by induction sets up the opposite condition in the plate with the gla.s.s handle. The resinous plate, being excited with negative electricity, disturbs the electrical quiescence of the upper plate, and positive electricity is found on the surface touching the resinous plate, and negative electricity on the upper one, so that when it is removed without being touched, the two electricities come together again, and no spark is obtained; but if, as already described, the upper plate is touched whilst under induction, then positive electricity appears to pa.s.s from the finger to the negative electricity on the upper side of the plate, when the two temporarily neutralize each other, and then, when the plate is removed, the excess of electricity derived from the earth through the finger becomes apparent. Induction requires no sensible thickness in the conductors, and can be just as well produced on a leaf of gold as on the thickest plate of metal; and it should be remembered that non-conductors do not retain their state of electrical excitation when the disturbing cause is removed, whereas conductors possess this power, and this fact brings us to the consideration of the Leyden jar.

_Thirty-third Experiment._

If one side of a dry gla.s.s plate is held before and touches a bra.s.s ball proceeding from the prime conductor of an electrical machine whilst in action, the other side is soon found to be electrical; this does not arise from the conduction of the electricity through the particles of the gla.s.s, but is produced by induction, the side nearest the ball being in the positive state, and the other side negative: as gla.s.s is a non-conductor of electricity, the effect is much increased by coating each side with tinfoil, leaving a margin of about two inches of uncovered gla.s.s round the covered portion, then, if one side of such a plate is held to the prime conductor of the electrical machine, and the other connected with the ground, a powerful charge is acc.u.mulated; and if the opposite sides are brought in contact with a bent bra.s.s wire, a loud snapping noise is heard, and the two electricities resident on either side of the gla.s.s come together with the production of a brilliant spark, or if the hands are subst.i.tuted for the bent bra.s.s wire, that most disagreeable result is obtained--viz., an _electric shock_; hence these gla.s.s plates are sometimes fitted up as pictures, and when charged and handed to the unsuspecting recipient, he or she receives the electric discharge to the great discomfort of their nervous system.

Mica is sometimes subst.i.tuted for gla.s.s, and the late Mr. Crosse, the celebrated electrician, constructed a powerful combination of coated plates of this mineral. It consisted of seventeen plates of thin mica, each five inches by four, coated on both sides with tinfoil within half an inch of the edge. They were arranged in a box with a gla.s.s plate between each mica plate, all the upper sides were connected by strips of tinfoil to one side of the box, and all the under surfaces in the same manner with the opposite extremity of the box. They were charged like an ordinary Leyden battery.

[Page 190]

_Thirty-fourth Experiment._

If the gla.s.s plate coated with tinfoil is charged, and then placed upright on a stand, it may be slowly discharged by placing a bent wire on the edge with the extremities covered with pith b.a.l.l.s. The wire balances itself, and continues to oscillate with noise until the electricities of the two surfaces neutralize each other. (Fig. 174.)

[Ill.u.s.tration: Fig. 174. A A. Gla.s.s plate or stand coated with tinfoil on each side, B. C. Wire with pith b.a.l.l.s oscillating during the discharge of the gla.s.s plate.]

_Thirty-fifth Experiment._

It is easy to imagine the gla.s.s plate of the last experiment rolled up into the more convenient form of the Leyden jar, which consists of a gla.s.s vessel lined both inside and out with tinfoil, leaving some two or three inches of the gla.s.s round the mouth uncovered and varnished with sh.e.l.l-lac; a piece of dry wood is fitted into the mouth of the jar, through which a bra.s.s wire and chain are pa.s.sed, and the end outside is fitted with a ball. The Leyden jar is charged by holding the ball to the prime conductor of the electrical machine until a sort of whizzing noise is heard, caused by the excess of electricity pa.s.sing round the uncovered part of the jar and not through it, as the smallest crack in the gla.s.s of the Leyden jar would render it useless. Electricity is sometimes called a fluid, and the fact of collecting it like water in a jar, helps us to understand this a.n.a.logy. The noise, the bright spark, or the shock are obtained by grasping the outside with one hand and touching the ball with a bra.s.s wire held in the other. (Fig. 175.)

[Ill.u.s.tration: Fig. 175. The Leyden jar and bra.s.s wire discharger.]

_Thirty-sixth Experiment._

The jar is silently discharged if the b.a.l.l.s are removed from the discharger and points used instead; so, also, the whole of the electricity produced by an electrical machine in full action may be readily drawn off by a pointed conductor, such as a needle, placed at the end of a bra.s.s wire. Electricity pa.s.ses much more rapidly through points than rounded surfaces, hence the reason why all parts of electrical apparatus are free from sharp points and rough asperities.

[Page 191]

_Thirty-seventh Experiment._

Extremely thin wires may be burnt by pa.s.sing the charge of a large Leyden jar through them. The show jars, called specie jars, usually decorated and placed in the windows of chemists' shops, make excellent Leyden jars, when not too thick; and with two of the largest, all the interesting effects produced by acc.u.mulated electricity may be displayed. To pa.s.s the discharge through wires, nothing more is required than to strain them across a dry mahogany board, between two bra.s.s wires and b.a.l.l.s, and if a sheet of white paper is placed under them, most curious markings are produced by the fine particles of the deflagrated metal blown into the surface of the paper. An arrangement of two or more Leyden jars is usually called a Leyden Battery, just as a single cannon is spoken of as a gun, whilst two or more const.i.tute a battery. (Fig.

176.)

[Ill.u.s.tration: Fig. 176. A. Mahogany board with a sheet of white paper and three pairs of bra.s.s wires and b.a.l.l.s fixed in the wire, three on each side. The thin wires are stretched between the b.a.l.l.s, and the lower one is in course of deflagration. B B. Charged large Leyden battery of two jars; the arrows indicate the path of the electricity.]

_Thirty-eighth Experiment._

Little models of houses, masts of ships, trees, and towers are sold by the instrument makers, and by placing a long balanced wire on the top of the pointed wire of a large Leyden jar, having one end furnished with wool to represent a cloud, a most excellent imitation of the effects of a charged thunder-cloud is produced. The mechanical effect of a flash of lightning has been a.n.a.lysed, and it has been stated, in one instance, that the power developed through fifty feet was equal to a 12,220 horse-power engine, or about the power of the engines of the _Great Eastern_, and that the explosive power was equal to a pressure of three hundred millions of tons. (Fig. 177.)

[Page 192]

[Ill.u.s.tration: Fig. 177. A. Charged Leyden jar with balanced wire and wool at B, representing a thunder-cloud. C. The obelisk overturned with the discharge. D. Another model of the gable end of a house; the square pieces of wood fly out when the continuity of the conductor is broken.]

It was the learned but humble minded Dr. Franklin who established the ident.i.ty between the mimic effects of the electrical machines (such as have been described), and the awe-inspiring thunder and lightning of nature. A copper rod, half an inch thick, pointed and gilt at the extremity, and carried to the highest point of a building, will protect a circle with a radius of twice its length. The bottom of the rod must be pa.s.sed into the earth till it touches a damp stratum.

[Ill.u.s.tration: Fig. 178. A storm.]

[Page 193]

CHAPTER XIV.

VOLTAIC ELECTRICITY.

In describing the various means by which electricity may be obtained, it was stated that "Chemical Action" was a most important source of this remarkable agent; at the same time it must be understood that it is not every kind of chemical action which is adapted for the purpose; there are certain principles to be rigidly adhered to--first, in the generation of the force; and secondly, in carrying it by wires so as to be applicable either for telegraphic purposes, or for the highly valuable processes of electrotyping and electro-silvering, plating, and gilding.

A lighted candle, or an intense combustion of coal, c.o.ke, or charcoal, no doubt involves the production of electricity, but there are no means at present known by which it may be collected and conducted; when that problem is solved, the cheapest voltaic battery will have been constructed, in which the element decomposed is charcoal, and not a metal, such as iron or zinc. The first and most simple experiment that can be adduced in proof of electrical excitation by chemical means, is to take a bit of clean zinc and a clean half-crown, and placing one on the tongue and the other below it, as long as they remain separate no effect is observed, but directly they are made to touch each other, whilst in that position, a peculiar thrill is rendered evident by the nerves of the tongue, which in this case answers the same purpose as the electroscope already described, and in a short time a peculiar metallic taste is perceptible.

It has been stated over and over again that it was to a somewhat similar circ.u.mstance we owe the discovery of voltaic electricity, and the story of the skinned frogs agitated and convulsed by an accidental communication with two different metals, or, as some say, with the electricity from an ordinary machine, has been repeated in nearly every work on the science. Professor Silliman, however, a.s.serts that the galvanic story is doubtful, and is a fabrication of Alibert, an Italian writer of no repute, and that greater merit is due to Galvani than that of being merely the accidental discoverer of this kind of electricity, because he had been engaged for _eleven_ years in electro-physiological experiments, using frogs' legs as electroscopes. It was whilst experimenting on animal irritability, Galvani noticed the important fact that when the nerve of a dead frog, recently killed, was touched with a steel needle, and the muscle with a silver one, no convulsions of the limb were produced until the two different metals were brought in contact, and he explained the cause of these singular after-death contortions by supposing that the nerves and muscles of all animals were in opposite states of electricity, and that these nervous contractions were caused by the annihilation, for the time, of this condition, by the interposition of a good conductor between them.

This theory of Galvani had several opponents, one of whom, the [Page 194] celebrated Volta, succeeded in pointing out its fallacy; he maintained that the electrical excitement was due entirely to the metals, and that the muscular contractions were caused by the electricity thus developed pa.s.sing along the nerves and muscles of the dead animal.

To Volta we are indebted for the first voltaic battery, and the distinguished philosopher may truly be said to have laid the foundation of this now _commercially_ valuable branch of science.

_First Experiment._

If a plate of clean bright zinc is placed in a vessel containing some dilute sulphuric acid, energetic action occurs from the oxidation of the metal, and its union as an oxide with the acid, and the escape of a mult.i.tude of bubbles of hydrogen gas. After the action has proceeded some time, the zinc may be removed, and if a little quicksilver is now rubbed over the surface with a woollen rag tied on the end of a stick, it unites with the metal, and the surface of the zinc a.s.sumes a brilliant silvery appearance, and is said to be amalgamated. In that condition it is no longer acted upon by dilute sulphuric acid, and for the sake of economy this is the only form in which zinc should be employed in the construction of voltaic batteries or single circles. If a clean plate of copper, with a wire attached, is now placed in the dilute acid opposite to and not touching the amalgamated zinc plate, which may also be furnished with a conducting wire, no bubbles of hydrogen escape until the wires from the two metals are brought in contact, and then, singular to relate, the hydrogen escapes from the copper plate, whilst the oxygen is rapidly absorbed by the zinc, and a current of electricity will now be found to pa.s.s from the zinc through the fluid to the copper, and back again through the wire to the starting point, and if the wires are disconnected, the chemical action ceases, and no more electricity is produced. (Fig. 179.)

[Ill.u.s.tration: Fig. 179. A single voltaic circle, consisting of a zinc and copper plate (marked Z and C) in dilute acid. The arrows show the direction of the current.]

The pa.s.sage of the current of electricity is not discoverable by the electroscope, because it is adapted only to indicate electricity of high tension or intensity, such as that produced from the electrical machine, which will pa.s.s rapidly through a certain thickness of air, and cause pith b.a.l.l.s to stand out and repel each other; such effects are not producible by a single voltaic circle, or even an ordinary voltaic battery, although one comprising some hundreds of alternations would produce [Page 195] an effect on a delicate electrometer; hence voltaic electricity is said to be of low intensity, and this property makes it much more useful to mankind, because it has no desire to leave a metallic path prepared for it, and does not seize the first opportunity, like the electricity from the electrical machine, to run away to the earth through the best and shortest conductor offered for it. If electricity had only been producible by friction, we should never have heard of electrotyping, and the other useful applications of electrical force of low intensity.