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my fingers I can lead the electric fluid to any part of the glass that I please. This is a beautiful experiment when the room is darkened, and is strictly conformable to the doctrine of the Leyden jar, already illustrated. The opposite figure will give some idea of the bending of the fluid towards the fingers.
By varying the density of the air in long glass tubes, we find that its resistance to electric transmission increases with its density; and consequently becomes a better conductor in proportion to its rarity: the probability therefore is, that vacuous space offers no resistance whatever, and thus becomes the best of all conductors.
Since we have shown, in a former lecture, that the conducting metals constituting the coatings of a Leyden jar, retain but a small fraction of the charge, what are their uses ? is but a natural question. They are indispensable both in the charging and discharging process. In the former process, one of the coatings receives the Auid at a mere point only, but by its conducting character it is enabled to distribute every spark over the glass surface which it covers; whilst the opposite coating allows of the departure of the fluid from the other surface. In the discharging process they perform the reverse functions, and allow of a sudden discharge from the electro-positive to the electro-negative surfaces. Independently of these appendages, the charge could never be equally distributed, nor could a discharge be sudden and complete.
The doctrine of electric atmospheres is a subject of great interest, and is interwoven with the display of every electrical phenomena. It is a subject which requires much force of reasoning for its clear and satisfactory demonstration, and extensive series of experiments for its complete illustration. We must, however, on this occasion, content ourselves with a brief illustration of this beautiful doctrine.
It appears that Otto Guericke, Burgomaster of Magdeburg, about the year 1670, was the first philosopher who noticed electric atmospheres, and their effects on bodies immersed in them: but the late Lord Stanhope, about 1778, seems to have studied the doctrine to a much greater extent. It is supposed by this nobleman that all bodies in an electric condition electrize the air around them to a considerable extent, and this electrized air is the atmosphere in question. Stanhope's two grand propositions in this doctrine are as follows.
“If a body be positive, and if it be surrounded by air, that electrified body will deposit upon all the particles of that air which shall come successively into contact with it a proportional pait of its superabundant electricity, by which means the air surrounding that body will become positively electrified : that is to say, it will form around that positive body an electrical atmosphere which will likewise be positive.
“If on the contrary, the body be negatire, each particle of air that shall come into contact with it will deposit thereon a certain part of its natural share of electricity, by which means the circumambient air will become negative : that is to say, it will form a negative atmosphere around the body which is negatively electrified.”
Beccaria, the famous Italian electrician, who perhaps studied the doctrine of electric atmospheres with greater care than any other philosopher, instituted a most beautiful experiment, by means of which an electric atmosphere is rendered perfectly visible. I will endeavour to repeat this grand doctrinal experiment by means of the apparatus represented by the figure. The apparatus consists of a glass receiver with a brass cap and an air-tight sliding wire; and a transfer plate of an air pump. The sliding wire is furnished with a ball at each end; and another short wire which rises from the pump plate is also surmounted by a brass ball. Having attenuated the air within the receiver, I remove it from the pump, screw on to the lower end of the pipe its wooden foot, and place the whole on an insulating stand. I now connect the upper wire with the prime conductor and uninsulate the transfer plate with its ascending stem and ball. The machine being in good order is now to be brought into play. No sparks are allowed to play between the two balls in the receiver, but their polarization is perfect and complete: and the accumulated Anid on the lower side of the upper ball, is distinctly seen as a luminous electric atinosphere, covering about half of the ball. This phenomenon is represented in the figure by the dotted atmosphere round the lower half of the ball.
I will now invert the order of arrangement; by insulating the lower hall and connecting it with the prime conductor, and the upper ball of the apparatus I touch with my finger. Under these circumstances the luminous atmosphere appears on the upper side of the lower ball, and none on the upper ball; and by reversing the arrangements a few times we discover that the ball alone, which is connected with the prime conductor, displays the luminous electric atmosphere.
If now, whilst the lower ball is in connection with the prime conductor, and the machine in action, I press down the sliding wire gradually, the luminous atmosphere on The lower ball expands upwards, gradually forms into a round-topped cone, and at last discharges itself in a dense spark to the upper ball.*
• This last variation of the experiment, I beliere is quite novel. I have also shown that the luminous electro-sphere can be produced independently of attenuated air. See Annals of Electricity, vol. ii, p. 413.
What a sund of intelligence is opened to our view by the display of these beautiful phenomena! An electric atmosphere is here exposed to our view, and no longer rests in the imagination alone; and as this phenomenon appears at the positive ball alone, it is one of those principal supports of the doctrine of a single electric fuid. It proves, also, that the electric fluid is self luminous ; and the last phenomena exhibited by the experiment show that polarization precedes discharge.
We have no experiment, that I am aware of, that would favour the theoretical views of Lord Stanhope on this subject, unless under circumstances in which the charge of a body could be thrown off into the atmosphere: or, according to his second proposition, where the electric Auid could enter the negative body. When the bodies are rounded and well polished, they neither receive por deliver the fluid easily from, and to, the atmospheric air : and the polarization of such bodies must be very high before a discharge could be accomplished from one to the other through a thick plate of dense air, and especially when the vicinal surfaces are but little convex. Hence it is, that in the electro-polarizations already shown in an early lecture, no discharge took place from the positive disturbing body to the vicinal negative surface of the polarized body, except in those cases where pointed wires were employed and an uniform current transmitted.
The luminous electro-sphere of Beccaria, affords no idea of the electric matter being thrown into the surrounding air; but, on the contrary, would lead to the belief that the accumulated electric fuid repels the air with its contained electric particles towards the opposite ball, and not being able to enter its smooth surface renders its vicipal side negative by a secondary polarization : the polarization of the intermediate air being the primary. This view is supported by the fact that when the polarized body has points or sharp edges at its remote side from the polarizing body, that its own electric fluid can be driven out of it by the repulsive action on the opposite side: but no fluid enters from the neighbouring air to make up the deficiency; consequently we find the body negative when the disturbing positive body is withdrawn.
Beccaria, the illustrious Italian philosopher who discovered the luminous electrosphere shown in the last experiment, also devised another experiment, by means of which I shall be enabled to con. vince you of the resistance which the electric fluid meets with on its approach to smooth convex or flat metallic surfaces.
The apparatus for illustrating this interesting fact, is that used in the last experiment, with the addition of a Leyden jar. The air in the receiver being attenuated as before, and the jar charged to a low degree of intensity, I discharge it through the receiver, and you will observe a narrow cylinder of light between the two balls, which spreads over the upper surface of the lower ball for a perceptible time before it disappears. The jar is next charged to a little higher intensity than before, and when discharged, the cylinder of light is of greater dimensions thau by the first discharge ; and a much greater
portion of the lower ball is covered with the electric light than before.
To enhance the beauty of the experiment, I will next use three of the battery jars. When these are charged pretty high, I transmit their contents through the receiver, and now, instead of a partial covering of the lower ball, the whole of the surface and that of its stem of support, are completely enveloped in a luminous electrical cloud of some duration, which seems to find more difficulty in entering the polished surface of the ball, than flowing over it through the attenuated circumambient medium to the asperous surface of the pump plate which it enters, and thus disappears. The apparatus with the enveloping cloud on the lower hall, is represented by the opposite figure.
If, instead of a Leyden jar, we were to connect the sliding wire with the prime conductor, the stream of electric fluid between the two balls, and the luminous cloud on the top of the lower ball, might be continued for any length of time we pleased; but the light is but faint, and can only be seen by close observers.
Another experiment established by Beccaria, shows the direction of the fluid through the attenuated air, and forms a beautiful cascade. On the lower plate is placed a hollow hemisphere of glass, as represented by the accompanying figure, and the ball is removed from the lower end of the sliding rod, which is again connected with the prime conductor. On bringing the machine into action, a stream of light flows from the lower extremity of the sliding wire and falls on the top of the hemisphere, which it partially illuminates; and in a short time the stream trickles over one side of the hemisphere to the pump plate as shown in the figure. The direction of the electric stream is decisively shown and well defined, which gives a pecular interest to the phenomenon. The cascade, however, does not continue constantly on the same side of the hemisphere, but removes from place to place, which gives it a more lively and pleasing appearance.
In the preceding experiments the air within the receiver was not bigbly attenuated, nor the machine in full action; but if the attenuation be carried on till the air pump ceases to act on the remaining air, and the machine be brought into full play, a stream of purple electric light falls upon the surface of the lower ball, which it does not enter but breaks upon it, and runs over it and its stem to the pump plate in a beautiful cascade.
The direction of the electric discharge is demonstrated by several other experiments, one or two of which I will now proceed with. I place two sticks of sealing-wax close together, laterally, on the table of the universal discharger, so as to form a channel at the juncture of their rounded edges : on this channel I place a cork ball of about an inch in diameter. When the balls of the sliding wires are re
moved, I direct the points towards the ball, each being about three inches from it and pointing towards its centre. One of the sliding wires is connected with the table, and the other with the prime conductor. On turning the machine gently the ballrolls along the groove from the positive to the negative wire.
I now remove the sealing-wax and the cork ball, and place on the table of the universal discharger a lighted candle, the flame of which is about the height of the sliding wires when placed horizontally. The points of these wires are directed towards each other, having the flame of the candle directly between them. The machine is put into motion, and the flame yields to the positive aura and bends towards the negative wire, now in connection with the rubbers of the machine. If the flame of the candle be blown out, the smoke from the wick still bends in the same direction.
Both the prime and the negative conductors being insulated, I place the apparatus represented by the opposite figure in connection with them by means of wires, one to each of the insulated horizontal wires. Each of these wires carries a metallic hemispherical cup, in which is placed a small piece of phosphorus, and between is a burning taper. I now put the machine into motion, and in a short time you will observe the phosphorus in the negative cup inflame, but the other piece does not.
These are some of the experimental data which have been brought forward in favour of the doctrine of one electric fluid only. There are several other phenomena which tend to give support to that doctrine, but having selected those wbich appear most satisfactory, it would be needless to dwell longer on this part of our subject. It may be necessary, however, to observe that, although an electric current proceeded from the point of the positive wire when operating on the cork ball, flame, smoke, &c., there can be no doubt of the existence of a current of air also, which added to the mechanical action.
When the back part of the hand is presented to the point which throws out the aura, a gentle cool blast is experienced; and uninsulated bodies, although attracted by presenting them to the side of the wire, are absolutely driven away from the point of it. If, for instance, I suspend a light pith ball by a moistened hempen thread held between my finger and thumb, and present it to any part of the prime conductor, or to the side of the pointed wire fixed into its remote end, the ball is forcibly drawn into close contact with the metal, where it will remain as long as the machine keeps in action; but if I present the ball to the projecting point of the wire it is driven off, and will pot come near to it.
The pith ball, in this case, may represent a particle of air, which being first attracted to the side of the wire, would travel towards the point, in consequence of the electric force being gradually stronger in that direction ; but when it arrived at the point itself, it would be thrown off by a repulsive force, and multitudes of particles of air following its example, would produce a current of air from the point.