Part 4

A body is said to be in its _natural state_, when it is in the same state, with respect to electricity, as the mass of the earth.

When a body has more of the electric fluid than its natural quantity, it is said to be _electrified positively_, when less, _negatively_; but neither of these cases can occur in a conductor, unless the communication between it and the earth be cut off by the _intervention of an electric or non-conductor_. When this happens, the conductor is said to be _insulated_.

It may not be amiss here to mention, that the terms _electric_ or an _electric per se_, and _non-electric_, were at first made use of from an erroneous idea that only those called electrics, contained the electric matter _in their substance_, which was capable of being excited by friction, and communicated by them to those called non-electrics, and supposed to be destitute of it: for glass and other electrics, being rubbed, discovered signs of having it, by snapping on the approach of a finger or other conductor, and by attracting and repelling light bodies; while other substances could not be made to produce any such effect. It has however since been proved by experiments, that _both_ electrics and non-electrics contain this matter in their substance; but that non-electrics cannot be excited, owing to the fluid diffusing itself through them as soon as collected. These terms are therefore improper, and as the only difference is that some bodies will conduct electricity and others will not, the terms _non-conductor_ and _conductor_ are those which might generally be used with the most propriety in speaking on this subject; though, in conformity with custom, we shall often use _non-conductor_ and _electric_ as synonymous.

CHAP. II. _Electric substances; with some of the phenomena attending their excitation._

Those substances by which electrical phenomena may be produced, form the subject which next demands our attention; but these are so numerous that it would be vain to attempt to specify them all. Perhaps it may be doubted, whether every material substance, with the exception only of metals, water, and charcoal, may not be considered as an electric.

Some however exhibit particular phenomena more obviously than others; and hence a number of catalogues have been formed, for shewing the effects which arise when electrics are excited with different rubbers. The specification which we esteem the most complete, was formed by the ingenious Mr. Cavallo, and we shall give it in his own words.

“In the following table (says he) may be seen what electricity will be excited in different bodies, when rubbed with different substances. Smooth glass, for instance, will be found by this table to acquire a positive electricity, when rubbed with any substance hitherto tried, except the back of a cat: (by which I mean the skin of a cat while on the animal alive:) rough glass, (viz. glass, the polish of which has been destroyed by emery or otherwise) will be found to acquire the positive electricity, when rubbed with dry oiled silk, sulphur &c. and the negative when rubbed with woolen cloth, the hand &c. and so of the rest.”

_Electrics._ │ _Qualities._ │ _Rubbers._ │ │Every substance with “The back of a cat │Positive │ which it has hitherto │ │ been tried. ───────────────────────┼───────────────────────┼─────────────────────── │ │Every substance Smooth Glass │Positive │ hitherto tried, │ │ except the back of a │ │ cat. ───────────────────────┼───────────────────────┼─────────────────────── Rough Glass │Positive │Dry oiled silk, │ │ sulphur, metals. │ │Woolen cloth, quills, „ │Negative │ wood, paper, sealing │ │ wax, white wax, the │ │ human hand. ───────────────────────┼───────────────────────┼─────────────────────── Tourmaline │Positive │Amber, or air blown │ │ upon it. „ │Negative │Diamond and the human │ │ hand. ───────────────────────┼───────────────────────┼─────────────────────── White silk │Positive │Black silk, metals and │ │ black cloth. „ │Negative │Paper, hand, hare’s & │ │ weasel’s skin. ───────────────────────┼───────────────────────┼─────────────────────── Black silk │Positive │Sealing wax. │ │Hare’s, weasel’s and „ │Negative │ ferret’s skin, │ │ load-stone, brass, │ │ iron, silver, hand. ───────────────────────┼───────────────────────┼─────────────────────── │ │Metals, silk, Weasel’s skin │Positive │ load-stone, leather, │ │ hand, paper, baked │ │ wood. „ │Negative │Other fine furs. ───────────────────────┼───────────────────────┼─────────────────────── Sealing wax │Positive │Metals. │ │Hare’s, ferret’s and „ │Negative │ weasel’s skin, hand, │ │ leather, woolen │ │ cloth, paper. ───────────────────────┼───────────────────────┼─────────────────────── Baked wood │Positive │Silk. „ │Negative │Flannel.”

From the above table it appears, that the powers of electric substances vary prodigiously from one another; and that, according to the different rubbers made use of, we may sometimes produce one phenomenon and sometimes another. Hence we have a foundation for classing electric substances according to the various powers they occasionally exhibit; which may be done in the following manner.

First. Those which exhibit a _strong_ and _permanent_ attractive and repulsive power, of which the most remarkable is silk.

Second. Those which exhibit the electric light, attraction, repulsion, and all the other phenomena of electricity in a very _vigorous_, though _not durable_ manner; of these glass is eminently preferable to all others.

Third. Those which exhibit electric appearances _for any length of time_, and which communicate to conducting bodies, _the greatest electric power_.—Of these, the substances called _negative electrics_, such as sealing-wax, resinous substances, and resinous compounds, are the best.

Fourth. Those which readily exhibit electrical phenomena by _heating_ and _cooling_.—Of these, the tourmaline[9] is the principal.

The best method of disturbing the electric fluid, that is of making it pass from one body to another, is friction. This may be done either by rubbing one electric with another, or with a conductor; but the electricity is generally stronger in the latter case. Other methods for causing electrics to shew electric appearances, are, melting, or pouring a melted electric on another substance, heating and cooling, evaporating or effervescing.

CHAP. III. _Of Electrics and Conductors._

All bodies in nature are, with reference to this subject, divided into two classes, _electrics_ and _conductors_.

It has been fully demonstrated by experiment, that no substance which is a conductor can be excited so as to exhibit electrical phenomena: and in the same manner it has been found, that no substance which can be excited, is a conductor. But as we have already hinted, there is, strictly speaking, no substance which is a _perfect_ conductor or non-conductor; because, on the one hand, the electric fluid meets with some resistance in its passage through the best conductors; and on the other, it is in part transmitted through, or passes over the surface of, most if not all electrics.

The two following lists contain as complete an enumeration of electrics and conductors as the present state of knowledge, in regard to electricity, permits us to make.

The substances are disposed in the order of their perfection; that is, the best conductors and the best electrics are placed at the head of their respective lists, and those of an inferior kind follow, somewhat in the manner of a scale graduated downward. Perfect exactness however is not to be here expected, because the subject forbids it, and some of the specified articles are of classes of substances among which there may be a sensible difference.

_Conductors or non-electrics._

Gold,

Silver,

Copper,

Platina,

Brass,

Iron,

Tin,

Mercury,

Lead,

Semi-metals.

Metallic ores.—Of which those are the best which contain the greatest number of metallic parts and are nearest to a reguline state.

Charcoal, either of animal or vegetable substances—

Animal fluids,

Acids,

Saline substances,

Hot water,

Cold water,

Salt water,

All other liquids except oils,

Red hot glass,

Melted rosin,

Flame and the effluvia of flaming bodies,[10]

Ice and snow—but not below the temperature of 13° Fahrenheit.

Earthy and stony substances, of which the hardest are the worst.

Glass filled with boiling water,

Vapour or steam of boiling water,

Smoke.

All compounds which contain the above substances in different proportions, are conductors in different degrees.

_Non-conductors or electrics._

Glass and all vitrifications; even those of metals.

All gems, of which the most transparent are generally the best.

All resinous substances and resinous compounds,

Amber,

Sulphur,

Baked wood—if not suffered to imbibe moisture.

All bituminous substances,

Wax,

Silk,

Cotton.

All dry animal excrescences; as feathers, hair, wool, horn, &c.

Paper,

White sugar and sugar candy,

Atmospheric air and other gasses,

Oils,

Dry and complete metallic oxyds,

The ashes of animal and vegetable substances,

All hard stones; of which the hardest are the best,

Powders not metallic.

Ice at and below the temperature of 13° of Fahrenheit’s thermometer. According to Mr. Walsh’s and Mr. Morgan’s experiments, the Torricellian vacuum ought to be placed at the head of this list; but the singular nature of a vacuum, though a non-conductor, will hardly entitle it to the name of an electric.

CHAP. IV. _Of the electrical machine._

Having now explained the terms made use of in the study of electricity, and noted some of the phenomena of different electric substances, and the difference between electrics and conductors; we shall proceed to describe the _electrical machine_ made use of for shewing experiments, and for exhibiting other electric phenomena to the best advantage.

The principal parts of the machine are, _the electric_, _the rubber_, the _moving engine_, and the _prime conductor_. We shall take notice of each of these parts separately and then describe the whole machine together.

Formerly different kinds of electrics were used; at present smooth glass is preferred before all others, as most convenient, and because it will, by itself, answer the purposes of several others. For when the machine has an insulated rubber, which is easily prepared, the operator may produce positive or negative electricity[11] at pleasure, without changing the electric.

With respect to the forms of the glass, those commonly used are globes, cylinders and plates. The most convenient size for a globe is from ten to twelve inches in diameter. It should have two necks, centrally opposite, which must be cemented[12] to strong caps, in order to adapt them to a proper frame. Cylinders are also made with two necks. Their common size is from six to seven inches in diameter, and from ten to twelve inches in length; the glass generally used is the best flint.

It has long been questioned whether a coating[13] of some electric substance, has any effect in increasing the power of an electric; but now it seems pretty well determined, that if it does not increase the power of a good one, it at least considerably improves a bad one.

The next thing to be considered is _the rubber_ which is to excite the electric. This, as it is now made, consists of a cushion of buckskin, stuffed with hair or flannel, and fastened to a piece of wood well rounded at the edges; to this is glued a flap of Persian black silk, which goes over nearly one half of the cylinder or globe. The rubber should be supported by a small iron or brass spring, placed inside of it, as is represented edgewise by R, figure 2, in the frontispiece. This acts in a much more uniform and parallel manner than if it were placed under the cylinder. It suits any inequalities that may be on the surface of the glass, and by means of a screw may be made to press against the cylinder as occasion requires. It should likewise be insulated in the most perfect manner by glass, or by baked wood well varnished. But when experiments are to be made which do not require or admit of insulation, a communication must be made between the rubber and the earth, by a chain or conductor.

To increase the effect of the rubber several substances have been used with success, particularly whiting and pulverised chalk. But the best of all is an amalgam of zinc and mercury.[14] This amalgam is to be used by first applying a moderate quantity to the cushion; and afterwards by spreading it on a separate piece of leather, and applying it occasionally to the under part of the cylinder while turning. In this method of using it, only a small quantity of amalgam is consumed, while the glass is very strongly excited; and by degrees the whole rubber contiguous to the cylinder is covered with amalgam, in the form of a concave cake. It is with such a rubber that the cylinder is most powerfully excited.

An ingenious friend has favoured us with the following explanation of the manner in which electrics are excited, which to us is more satisfactory than any other we have seen. “In order that electricity may be accumulated in greater quantity in one body than in the surrounding ones, it must be set in motion. This may be effected by the _rubbing of electrics_; the _juxta-position_ of non-electrics of different conducting powers; and by the _chemical action_ of many, if not all bodies on each other. The rubber will act on the first principle, and the more perfect the contact between it and the electric the greater will be the effect. The chalk, whiting, amalgam &c. while they will, if properly prepared, make the contact more perfect, will also be of service on the second principle; and the amalgam will besides be of use on the third. Mercury and zinc may be exposed separately to the air without any alteration; but when combined they readily unite with the oxygen of the atmosphere; especially when the surface of contact is frequently renewed, and the temperature increased by friction.

“The glass, acquiring a different state of electricity from the rubber, will, as each portion passes from under it, carry away and impart to the prime conductor the excess which it has obtained; and this the more certainly if the dissipation of the electricity be prevented, or the accumulation increased, by a piece of silk connected with the rubber.—The chain making the communication between the rubber and the adjoining non-electrics will enable this process to go on; and perhaps may also assist on the second principle.”

With respect to the engine which is to give motion to the electric, it has been customary, simply to turn the globe or cylinder with a winch; but this will not produce the greatest power of which the glass is capable. To effect this it should be made to turn six or seven times in a second, which is more than can conveniently be done with the winch only; and therefore multiplying wheels are used with advantage.

The prime or first conductor is an insulated non-electric substance, furnished with a number of points on the end towards the electric, in order to collect the electricity from it. It is usually made cylindrical, but whatever be its form it should always be perfectly free from points or sharp edges, except the points toward the electric already mentioned; and if holes are made in it, which on many accounts are very convenient, they should be well rounded and perfectly smooth.—The larger this conductor is, if not disproportionate to the cylinder or globe, the stronger and more dense will be the electric spark, which will proceed from it when touched by a blunt conductor. There must however always be a certain proportion between the cylinder or globe and the prime conductor, for if the former be small and the latter large, the electricity will not be collected fast enough, to preserve an accumulation of it in the prime conductor, because a portion is always taken off by the air, in proportion to the surface presented to it by the conductor.

We shall now give a short connected explanation of the whole machine, a draft of which is exhibited in the frontispiece. AB and CD are two pillars of baked wood well varnished, perpendicularly raised from the top of the table EFGH—these serve to support the cylinder I, by the axles of the caps KK; from one of these proceeds the long axle L, which passes through a hole in the pillar CD, having the pulley M, fixed on its square end. N is a multiplying wheel, around which the band or strap O passes, and likewise around the pulley M.—The wheel N should be made moveable with respect to the pulley M, to accommodate the stretching of the band, or else the pulley should have a number of grooves of different radii in its circumference.

The rubber R, is fastened to a pillar of glass, or baked wood P. The pressure of the rubber may be augmented at pleasure, by means of a sliding board and tightening screw.

The prime conductor is represented by Q. It is insulated by the glass pillars SS, which support it. T represents the points which collect the electricity from the cylinder.

Cylinders and globes made for electrical machines are not always to be procured. Their place however, may be very well supplied by the large show bottles of the apothecaries. When these are used, one of the caps, instead of being concave (to receive the neck of the cylinder) must be made convex—so as to fit the hollow in the bottom of the bottle.—It is to be fastened with the cement used in the other machine.

The most powerful electrical machine ever constructed, was at Teyler’s museum at Haarlem. It had, instead of the cylinder or globe as in the common machines, two circular plates of glass, which were made to turn upon the same horizontal axis. These plates were excited by eight rubbers, which acted on their surfaces. In this machine the prime conductor had branches which collected the electricity from between the plates.

It is not necessary however in this form of the machine to have two plates, the second being added only to increase the power. The plate must be firmly fastened by its centre to an axis—so as to turn vertically between two uprights of baked wood, as in the construction of the cylindric machines; but in this case the uprights must be so close together, as barely to leave room for a rubber on each side of the plate. The rubbers may be made of the same form with that in the cylindric machine—except that they must have a projection at the back, to fit a niche cut in the uprights which support the plate. The power of the machine will be increased by having four rubbers; two above and two below the axis of the plate. The prime conductor is placed opposite one of the ends of the axis, and is divided at the end towards the electric into two branches or arms, which extend horizontally to the circumference of the plate, each of which is furnished with points to collect the electricity.

As plates are not always to be procured, a good substitute may be found in a thick pane of glass or a piece of an old looking-glass. Mark with a diamond or file a circle on the glass, of the size you intend for your plate. Then putting the plate into warm water, after some time cut the glass with a diamond in tangents. The more numerous the cuts, the nearer the plate will be to a circle. A hole may be made in the centre for the axis, by scratching with a diamond, and grinding with a rod of iron (held between the hands) and emery.

CHAP. V. _Of communicated electricity._

Having described the electrical machine, we are now to consider some of the phenomena attending its operation. When the prime conductor receives electricity from the cylinder, it is said to be _electrified by communication_, and it then acts in every respect like the cylinder itself, except that the latter, when touched by a conductor communicating with the earth, gives a considerable number of sparks before it is discharged; whereas the conductor discharges itself by a single spark.

The cause of this difference is that the cylinder, being an electric, cannot convey the electricity of all its surface to that part, to which the conducting substance is applied; but the fluid accumulated in the whole conductor, passing easily through its substance, is transmitted at once to the point from which the discharge is made. Hence it appears that the electricity discharged from an electrified conductor is more powerful than that discharged from an electric—the conductor acquiring a large quantity of electricity from an electric, by receiving it gradually, spark after spark, and afterwards, when touched, discharging it all at once.

The velocity of electricity is almost beyond conception. It is, notwithstanding, in a small degree relative to the quantity put in motion, and to the goodness of the conductor by which it is transmitted. A large quantity of electricity passes through a good conductor with such rapidity, that there is no perceptible difference in the time which it takes to go one foot, or one thousand feet. A small quantity however has been found to take a time barely perceptible, in passing through a long and imperfect conductor. Experiments relative to this point will be related hereafter.

CHAP. VI. _Of the electric spark._

If a piece of metal be presented to an over-charged prime conductor, the fluid passes with violence from the one to the other; an _electric spark_, having the appearance of fire, is seen flashing between them, and a snapping noise, like the cracking of a whip, is heard. If this piece of metal be insulated, the prime conductor will be only partially discharged, that is, the redundant electricity will be divided between it and the piece of metal, nearly in proportion to their surfaces. This electric spark has not only the appearance of fire, but, when large, will actually set fire to a variety of easily inflammable substances; such as cotton sprinkled with rosin, spirits of wine &c. This power of exciting flame is not commonly believed to arise from any culinary heat in the electric spark, because if the spark be small it will not excite flame in substances the most inflammable. It acts probably by friction on the same principle as the rubbing of sticks against each other produces fire.

The electric spark, taken upon any part of a living animal, causes an unpleasant sensation, which is more or less pungent and disagreeable, as the spark is stronger or weaker, and the part more or less delicate.

There is a slight difference between the appearance of a spark taken from a body positively electrified, and that from one negatively electrified. The former, if not very long, appears straight and sharp; the latter is generally ramified, or appears in a zig-zag line.

The noise which attends the spark, is caused by the sudden agitation into which the air is thrown, by its passage through it.

CHAP. VII. _Of the influence of pointed bodies on electricity, and some phenomena attending their operation._

If an uninsulated conductor, which is broad, round and polished at the end, be presented to the prime conductor, a short and dense spark, accompanied with some noise, will be perceived; if the conductor be less broad, the spark will be longer, less dense, and attended with less noise; if the breadth be still more diminished, so that the conductor may come under the denomination of a point, the electric matter will pass to it, from the prime conductor, and through a greater space, with a hissing noise, and in a continual stream; a still greater sharpness will enable the electricity to pass over a yet more extended space, but unaccompanied by noise, and only a small light will be seen upon the point. The same result will arise if points of different acuteness be affixed to the prime conductor, instead of the uninsulated one: but if both be pointed, the electricity will be more readily discharged.