Page images
PDF
EPUB

Electrical

ness.

If the communication between the opposite surfaces of the Leyden phial be made by the hands, a painful jarring sensation is felt at the joints of the fingers, the elbows shoulders, and chest, commonly called the electrical shock. Metallic wires, with balls at their ends, bent or jointed and fixed to a glass handle, are generally used to transfer the electric charge, and these instruments are called dischargers.

115. In all cases of electrical accumulation, the surfaces intended to retain it must be free from asperities, and points: a pointed wire held near the prime conductor instantly gains an opposite state, and rapidly discharges it; if it be affixed to the conductor, a similar effect is ob

wind or cold served; and upon holding the hand near the point, a peculiar coldness is perceived, which has been called the electrical aura, and which depends upon the rapid recession of the electrified air. If the Leyden jar be discharged by a pointed wire, the electricities quietly annihilate each other, and no explosion can be produced.

trometer.

116. To ascertain the relative charge which the jar has received, we employ the quadrant electrometer, contrived by Henly. It consists of a rounded stem of metal, A to the side of which is Menly's elec- attached an ivory semi-circle, B; to the centre is affixed a pin, upon which a very thin piece of cane or ivory, about 4 inches long, with a pith ball at its lower extremity, turns freely, traversing the semicircle as an index. The lower half of the semi-circle is divided into 90°. When not electrified, its index hangs parallel to the stem at 0°, but when electrified, the ball recedes and carries the index over the graduated circle to a greater or less extent, in proportion to the intensity of the electricity.

[graphic]

posite electri

117. The annihilation of positive by negative electricity, and vice Experiment to shew the anni-versa, may be well shown by the following experiment. Attach Henhilation of op-ly's quadrant electrometer to the knob of a Leyden jar, and give it a cities. certain charge from the positive conductor: then transfer the jar to the negative conductor, and whilst receiving a negative charge, the electrometer will fall, indicating the loss of all electrical accumulation; it then will again rise as the jar becomes negatively charged, and may again be discharged by transferring it to the positive conductor.

Leyden jars.

118. If one Leyden jar be insulated with its internal surface conConnection of nected with the positive conductor, another jar may be charged from its exterior coating; and if this second jar be insulated, a third may be charged from its exterior coating, and so on for any number of jars, provided always that the exterior coating of the last jar be connected with the ground. In this case, a polar arrangement, similar to that of the conductors just described, (112) will have been formed, glass being the medium of induction instead of air.

[merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small]

Let cr be the positive conductor of the electrical machine, and a b ⚫ three insulated Leyden phials, the outer coating of c being connected with the ground; it is then obvious, that there will be the same polar state as in the conductors just noticed; that the insides of a, b, and c, will be positive, and the outsides negative; and that, consequently, removing the jars from each other, they will all be similarly charged, and that if the three inner surfaces ppp and the outer surfaces nnn be united, the whole may be discharged as one jar.

on

119. Upon this principle a jar may be charged by the transfer of its inherent electricity from one surface to the other, by insulating it and connecting its interior coating with the positive conductor, and its exterior with the negative;-thus the electricity received by the former is withdrawn from the latter, and the jar becomes charged. This experiment well illustrates the non conducting power of glass.

of the Leyden

120. The use of the metallic coatings of the Leyden phial is equal- Use of the mely to distribute the electricities over the opposite surfaces, for if the tallic coatings coatings be made moveable the jar remains charged when they are re-jar. moved. In discharging the jar, too, the annihilation is rendered simultaneous by the conducting coating suffering the transfer of the opposite electricities from every part of the glass surfaces at the same instant.

121. There are some other electrical instruments (the operation of which is referable to the phænomena of induction, such as the electrophorus, and the condenser.

The electrophorus consists of two metallic plates, a a, with an intervening plate of resinous matter, b, for which equal parts of shelllac, resin, and Venice turpentine, are generally used, the mixture being carefully melted in a pipkin, and poured, whilst liquid, into a wooden or metal hoop, of a proper size, placed upon a polished surface of glass or marble, from which it easily separates when cold; it should be about half an inch thick, and the smooth surface being uppermost the lower side should be covered with

tin foil, or attached to any other metallic plate; a polished brass plate, with a glass handle c attached to it, is then placed upon the upper surface of the resinous plate, and of rather smaller diameter. The resin is then to be excited with a piece of dry fur, and the instrument will be found to exhibit the

following phænomena :

a

b

Electrophorus

a

Upon raising the brass plate by its insulating handle, it will be found very feebly electrical; replace it, touch it with the finger and again lift it off by its handle, and it will give a spark of positive electricity. This process may very often be repeated without fresh excitation, Mode of using which circumstance, as well as the nature of the electrical charge, it. shows that the electricity of the moveable brass plate is not directly derived from the resin, but that it depends upon induction: this is more obvious by considering the upper plate, not as in contact with but merely very near the resinous disc, which from the minute irregularities upon its surface, is really the case; the negative electricity, therefore, of the excited resinous plate is communicated from a few

G

[ocr errors][merged small][merged small]

TRICITY.

we, upon its first application, and then e of a conductor opposed to, but not end consequently in due condition to nen, when occasionally uninsulated by

Enomenon of induction may be shown, ted by any other; namely, the increasAne conductor under the influence of in⚫nen placed upon the resin, may be rehe lower surface next the resin being poung negative. Upon touching the upper santly acquires electricity, loses its po2. giving, upon removal, a positive spark to antity of electricity received from the finivalent to that given out, is shown by the *ce the metallic upon the resinous plate, and ne knob of a Leyden phial; then touch the cap ne knob of the phial, and it will give a certain neme'eaves; raise the plate and present the knob of wil pass; and upon applying the jar a second time ne leaves will entirely collapse, showing the exneormer negative, by the latter positive charge. pitorus is placed upon an insulating stand, its lowward in an opposite electrical state to the upper one,

it resembles the coatings of a Leyden jar. us may often be used for the same purposes as the and in the laboratory it furnishes a very convewhat more expensive piece of apparatus.

lated surface is opposed to another which is not affected by it by induction, the electricity commuter suffers a singular increase of tension or intensity aduction by removing the opposed uninsulated conperty is strikingly exhibited in the following experi

brass plate, 3 or 4, inches in diameter, and drop upice three small spots of sealing-wax; place it upon a erung the cap of the gold leaf electrometer, from which ted about a twentieth of an inch by the three small

nga wax.

Connect the upper plate with the ground by give a very feeble electrical charge to the electrometer, perceptibly to diverge its leaves; then suddenly remove by which the induction will be broken, and the tenveverwity suddenly increased, so as to cause a very consi

e of the leaves.

eamployed in this experihave been called condensers. mes placed perpendiDe uninsulated plate a is da wire and joint, so as to as close as possible to the ste, without touching; communication with the meer, and having received its themoveable uninsulated plate

[ocr errors][merged small]

back, as in c, and the intensity of the electricity displayed.

Sometimes the condenser is directly attached to the electrometer, as shown in the annexed cut.-a, the insulated plate; 6, the moveable plate in communication with the ground.

Various attempts have been made so to combine electrical condensers, as to multiply their effect, and render very slight electrical changes susceptible of measurement and examination; of this nature is the Electrical Multiplier, contrived by Mr. Cavallo, (Complete Treatise on Electricity, Vol. iii. p. 99;) the Doublers of Mr. Bennet and Mr. Nicholson,

[ocr errors]

(Phil. Trans. lxxvii. and lxxviii.) and Mr. Wilson's Double Multiplier, (NICHOLSON'S Journal, ix;) but the complexity of these instruments, as Mr. Singer has remarked, renders their results equivocal, and often has a tendency to produce the electrical states, independent of the intended source, or to change that originally communicated.

Different arrangement of the conden

sers.

Quantity & intric power.

123. Electricians generally employ the term quantity to indicate the absolute quantity of electric power in any body, and the term intensi-tensity of elesty to signify its power of passing through a certain stratum of air or other ill-conducting medium.

If we suppose a charged Leyden phial to furnish a spark, when discharged, of one inch in length, we should find that another uncharged Illustration. Leyden phial, the inner and outer coating of which were communicated with those of the former, would upon the same quantity of electricity being thrown in, reduce the length of the spark to half an inch; here, the quantity of electricity remaining the same, its intensity is diminished by one-half, by its distribution over the larger surface.

124. It is obvious, that the extension of surface alluded to in the last paragraph, will be attended with a greater superficial exposure to the unelectrified air; and hence it might be expected, that a similar diminution of intensity would result from the vicinity of the electrified surface to the ground, or to any other body of sufficient magnitude in its ordinary state. That this is the case, may be shown by diverging the leaves of the gold-leaf electrometer, and in that state approaching the instrument with an uninsulated plate, which, when within half an inch of the electrometer plate, will cause the leaves to collapse; but on removing the uninsulated plate, they will again diverge, in consequence of the electricity regaining its former intensity. The same fact is shown by the condensing electrometer.

Power of the

its

125. The power of the Leyden jar is proportioned to its surface, but a very large jar is inconvenient and difficult to procure; the same Leyden jar in end is attained by arranging several jars, so that by a communication communica proportion to existing between all their interior coatings, their exterior being also united, they may be charged and discharged as one jar. Such a combination is called an electrical Battery, and is useful for exhibiting the Electrical effect of accumulated electricity.

Battery.

The discharge of the battery is attended by a considerable report, Its discharge and if it be passed through small animals it instantly kills them; if attended with through fine metallic wires, they are ignited, melted, and burned; and prove fatal to gunpowder, cotton sprinkled with powdered resin, and a variety of animal life. sther combustibles, may be inflamed by the same means.

report can

produces combustion, &c.

Other sources

126. There are many other sources of electricity than those just of electricity. noticed. When glass is rubbed by mercury, it becomes electrified, and this is the cause of the luminous appearance observed when a barometer is agitated in a dark room, in which case flashes of light are seen to traverse the empty part of the tube. Even the friction of air upon glass is attended by electrical excitation: for Mr. Wilson found that by blowing upon a dry plate of glass with a pair of bellows, it acquired positive electricity. Whenever bodies change their forms, their electrical states are also altered. Thus the conversion of water into vapour, and the congelation of melted resins and sulphur, are processes in which electricity is also rendered sensible.

Of two metals properly con

127. When an insulated plate of zinc is brought into contact with one of copper or silver, it is found, after removal, to be positively electrical, and the silver or copper is left in the opposite state.

The most oxidizable metal is always positive, in relation to the least nected the oxidizable metal, which is negative, and the more opposite the metals most oxidiza in these respects, the greater the electrical excitation; and if the ways positive. metals be placed in the following order, each will become positive by

ble one is al

Metallic arrangement for electric excitation.

the contact of that which precedes it, and negative by the contact of that which follows it; and the greatest effect will result from the contact of the most distant metals.

[blocks in formation]

nerves.

If the nerve of a recently killed frog be attached to a silver probe, Electricity conducted by and a piece of zinc be brought into the contact of the muscular parts of the animal, violent convulsions are produced every time the metals thus connected are made to touch each other; exactly the same effect is produced by an electric spark, or the discharge of a very small -Ley den phial.

If a piece of zinc be placed upon the tongue, and a piece of silver under it, a peculiar sensation will be perceived every time the two metals are made to touch.

Chemical ac-
tion accompa- served
nying that of

128. In these cases the chemical properties of the metals are obto be affected. If a silver and a zinc wire be put into a wine glass electricity. full of dilute sulphuric acid, the zinc wire only will evolve gas; but upon bringing the two wires in contact with each other, the silver will also copiously produce air bubbles.

Electric co

lumn.

129. If a number of alternations be made of copper or silver leaf, zinc leaf, and thin paper, the electricity excited by the contact of the metals will be rendered evident to the common electrometer.-a represents a glass tube, in which are regularly arranged a number of alternating plates of silver, zinc, and thin paper, forming DE Luc's electrical column. The metallic cap в is in contact with the silver plate, and c with the zinc plate, at the respective extremities of the pile. Upon examining

« PreviousContinue »