Experimental Researches in Electricity Part 45

1497. To counteract this interfering influence, I made the interval _n_ = 0.79 and interval _o_ = 0.58 of an inch. Then, when the b.a.l.l.s A and B were _inductric positive_, the discharge was about equal at both intervals.

When, on the other hand, the b.a.l.l.s A and B were inductric _negative_, there was discharge, still at both, but most at _n_, as if the small ball _negative_ could discharge a little easier than the same ball _positive_.

1498. The small b.a.l.l.s and terminations used in these and similar experiments may very correctly be compared, in their action, to the same b.a.l.l.s and ends when electrified in free air at a much greater distance from conductors, than they were in those cases from each other. In the first place, the discharge, even when as a spark, is, according to my view, determined, and, so to speak, begins at a spot on the surface of the small ball (1374.), occurring when the intensity there has risen up to a certain maximum degree (1370.); this determination of discharge at a particular spot first, being easily traced from the spark into the brush, by increasing the distance, so as, at last, even to render the time evident which is necessary for the production of the effect (1436. 1438.). In the next place, the large b.a.l.l.s which I have used might be replaced by larger b.a.l.l.s at a still greater distance, and so, by successive degrees, may be considered as pa.s.sing into the sides of the rooms; these being under general circ.u.mstances the inducteous bodies, whilst the small ball rendered either positive or negative is the inductric body.

1499. But, as has long been recognised, the small ball is only a blunt end, and, electrically speaking, a point only a small ball; so that when a point or blunt end is throwing out its brushes into the air, it is acting exactly as the small b.a.l.l.s have acted in the experiments already described, and by virtue of the same properties and relations.

1500. It may very properly be said with respect to the experiments, that the large negative ball is as essential to the discharge as the small positive ball, and also that the large negative ball shows as much superiority over the large positive ball (which is inefficient in causing a spark from its opposed small negative ball) as the small positive ball does over the small negative ball; and probably when we understand the real cause of the difference, and refer it rather to the condition of the particles of the dielectric than to the sizes of the conducting b.a.l.l.s, we may find much importance in such an observation. But for the present, and whilst engaged in investigating the point, we may admit, what is the fact, that the forces are of higher intensity at the surfaces of the smaller b.a.l.l.s than at those of the larger (1372. 1374.); that the former, therefore, determine the discharge, by first rising up to that exalted condition which is necessary for it; and that, whether brought to this condition by induction towards the walls of a room or the large b.a.l.l.s I have used, these may fairly be compared one with the other in their influence and actions.

1501. The conclusions I arrive at are: first, that when two equal small conducting surfaces equally placed in air are electrified, one positively and the other negatively, that which is negative can discharge to the air at a tension a little lower than that required for the positive ball: second, that when discharge does take place, much more pa.s.ses at each time from the positive than from the negative surface (1491.). The last conclusion is very abundantly proved by the optical a.n.a.lysis of the positive and negative brushes already described (1468.), the latter set of discharges being found to recur five or six times oftener than the former[A].

[A] A very excellent mode of examining the relation of small positive and negative surfaces would be by the use of drops of gum water, solutions, or other liquids. See onwards (1581. 1593.).

1502. If, now, a small ball be made to give brushes or brushy sparks by a powerful machine, we can, in some measure, understand and relate the difference perceived when it is rendered positive or negative. It is known to give when positive a much larger and more powerful spark than when negative, and with greater facility (1482.): in fact, the spark, although it takes away so much more electricity at once, commences at a tension higher only in a small degree, if at all. On the other hand, if rendered negative, though discharge may commence at a lower degree, it continues but for a very short period, very little electricity pa.s.sing away each time.

These circ.u.mstances are directly related; for the extent to which the positive spark can reach, and the size and extent of the positive brush, are consequences of the capability which exists of much electricity pa.s.sing off at one discharge from the positive surface (1468. 1501.).

1503. But to refer these effects only to the form and size of the conductor, would, according to my notion of induction, be a very imperfect mode of viewing the whole question (1523. 1600.). I apprehend that the effects are due altogether to the mode in which the particles of the interposed dielectric polarize, and I have already given some experimental indications of the differences presented by different electrics in this respect (1475. 1476.). The modes of polarization, as I shall have occasion hereafter to show, may be very diverse in different dielectrics. With respect to common air, what seems to be the consequence of a superiority in the positive force at the surface of the small ball, may be due to the more exalted condition of the negative polarity of the particles of air, or of the nitrogen in it (the negative part being, perhaps, more compressed, whilst the positive part is more diffuse, or _vice versa_ (1687. &c.)); for such a condition could determine certain effects at the positive ball which would not take place to the same degree at the negative ball, just as well as if the positive ball had possessed some special and independent power of its own.

1504. The opinion, that the effects are more likely to be dependent upon the dielectric than the ball, is supported by the character of the two discharges. If a small positive ball be throwing off brushes with ramifications ten inches long, how can the ball affect that part of a ramification which is five inches from it? Yet the portion beyond that place has the same character as that preceding it, and no doubt has that character impressed by the same general principle and law. Looking upon the action of the contiguous particles of a dielectric as fully proved, I see, in such a ramification, a propagation of discharge from particle to particle, each doing for the one next it what was done for it by the preceding particle, and what was done for the first particle by the charged metal against which it was situated.

1505. With respect to the general condition and relations of the positive and negative brushes in dense or rare air, or in other media and gases, if they are produced at different times and places they are of course independent of each other. But when they are produced from opposed ends or b.a.l.l.s at the same time, in the same vessel of gas (1470. 1477.), they are frequently related; and circ.u.mstances may be so arranged that they shall be isochronous, occurring in equal numbers in equal times; or shall occur in multiples, i.e. with two or three negatives to one positive; or shall alternate, or be quite irregular. All these variations I have witnessed; and when it is considered that the air in the vessel, and also the gla.s.s of the vessel, can take a momentary charge, it is easy to comprehend their general nature and cause.

1506. Similar experiments to those in air (1485. 1493.) were made in different gases, the results of which I will describe as briefly as possible. The apparatus is represented fig. 131, consisting of a bell-gla.s.s eleven inches in diameter at the widest part, and ten and a half inches high up to the bottom of the neck. The b.a.l.l.s are lettered, as in fig. 130, and are in the same relation to each other; but A and B were on separate sliding wires, which, however, were generally joined by a cross wire, _w_, above, and that connected with the bra.s.s conductor, which received its positive or negative charge from the machine. The rods of A and B were graduated at the part moving through the stuffing-box, so that the application of a diagonal scale applied there, told what was the distance between these b.a.l.l.s and those beneath them. As to the position of the b.a.l.l.s in the jar, and their relation to each other, C and D were three and a quarter inches apart, their height above the pump plate five inches, and the distance between any of the b.a.l.l.s and the gla.s.s of the jar one inch and three quarters at least, and generally more. The b.a.l.l.s A and D were two inches in diameter, as before (1493.); the b.a.l.l.s B and C only 0.15 of an inch in diameter.

Another apparatus was occasionally used in connection with that just described, being an open discharger (fig. 132.), by which a comparison of the discharge in air and that in gases could be obtained. The b.a.l.l.s E and F, each 0.6 of an inch in diameter, were connected with sliding rods and other b.a.l.l.s, and were insulated. When used for comparison, the bra.s.s conductor was a.s.sociated at the same time with the b.a.l.l.s A and B of figure 131 and ball E of this apparatus (fig. 132.); whilst the b.a.l.l.s C, D and F were connected with the discharging train.

1507. I will first tabulate the results as to the _restraining power_ of the gases over discharge. The b.a.l.l.s A and C (fig. 131.) were thrown out of action by distance, and the effects at B and D, or the interval _n_ in the gas, compared with those at the interval _p_ in the air, between E and F (fig. 132.). The Table sufficiently explains itself. It will be understood that all discharge was in the air, when the interval there was less than that expressed in the first or third columns of figures; and all the discharge in the gas, when the interval in air was greater than that in the second or fourth column of figures. At intermediate distances the discharge was occasionally at both places, i.e. sometimes in the air, sometimes in the gas.

_____________________________________________________________________ | | | | | Interval _p_ in parts of an inch | |_________________|___________________________________________________| | | | | | | When the small ball B | When the small ball B | | Constant inter- | was inductric and | was inductric and | | val _n_ between | _positive_ the | _negative_ the | | B and D = 1 | discharge was all | discharge was all | | inch | at _p_ in at _n_ in | at _p_ in at _n_ in | | | air before the gas | air before the gas | | | after | after | |_________________|_________________________|_________________________| | | _p_ = | _p_ = | _p_ = | _p_ = | |In Air | 0.10 | 0.50 | 0.28 | 0.33 | |In Nitrogen | 0.30 | 0.65 | 0.31 | 0.40 | |In Oxygen | 0.33 | 0.52 | 0.27 | 0.30 | |In Hydrogen | 0.20 | 0.10 | 0.22 | 0.24 | |In Coal Gas | 0.20 | 0.90 | 0.20 | 0.27 | |In Carbonic Acid | 0.61 | 1.30 | 0.30 | 0.15 | |_________________|____________|____________|____________|____________|

1508. These results are the same generally, as far as they go, as those of the like nature in the last series (1388.), and confirm the conclusion that different gases restrain discharge in very different proportions. They are probably not so good as the former ones, for the gla.s.s jar not being varnished, acted irregularly, sometimes taking a certain degree of charge as a non-conductor, and at other times acting as a conductor in the conveyance and derangement of that charge. Another cause of difference in the ratios is, no doubt, the relative sizes of the discharge b.a.l.l.s in air; in the former case they were of very different size, here they were alike.

1509. In future experiments intended to have the character of accuracy, the influence of these circ.u.mstances ought to be ascertained, and, above all things, the gases themselves ought to be contained in vessels of metal, and not of gla.s.s.

1510. The next set of results are those obtained when the intervals _n_ and _o_ (fig. 131.) were made equal to each other, and relate to the greater facility of discharge at the small ball, when rendered positive or negative (1493.).

1511. In _air_, with the intervals = 0.4 of an inch, A and B being inductric and positive, discharge was nearly equal at _n_ and _o_; when A and B were inductric and negative, the discharge was mostly at _n_ by negative brush. When the intervals were = 0.8 of an inch, with A and B inductric positively, all discharge was at _n_ by positive brush; with A and B inductric negatively, all the discharge was at _n_ by a negative brush. It is doubtful, therefore, from these results, whether the negative ball has any greater facility than the positive.

1512. _Nitrogen._--Intervals _n_ and _o_ = 0.4 of an inch: A, B inductric positive, discharge at both intervals, most at _n_, by positive sparks; A, B inductric negative, discharge equal at _n_ and _o_. The intervals made = 0.8 of an inch: A, B inductric positive, discharge all at _n_ by positive brush; A, B inductric negative, discharge most at _o_ by positive brush. In this gas, therefore, though the difference is not decisive, it would seem that the positive small ball caused the most ready discharge.

1513. _Oxygen._--Intervals _n_ and _o_ = 0.4 of an inch: A, B inductric positive, discharge nearly equal; inductric negative, discharge mostly at _n_ by negative brush. Made the intervals = 0.8 of an inch: A, B inductric positive, discharge both at _n_ and _o_; inductric negative, discharge all at _o_ by negative brush. So here the negative small ball seems to give the most ready discharge.

1514. _Hydrogen._--Intervals _n_ and _o_ = 0.4 of an inch: A, B inductric positive, discharge nearly equal: inductric negative, discharge mostly at _o_. Intervals = 0.8 of an inch: A and B inductric positive, discharge mostly at _n_, as positive brush; inductric negative, discharge mostly at _o_, as positive brush. Here the positive discharge seems most facile.

1515. _Coal gas._--_n_ and _o_ = 0.4 of an inch: A, B inductric positive, discharge nearly all at _o_ by negative spark: A, B inductric negative, discharge nearly all at _n_ by negative spark. Intervals = 0.8 of an inch, and A, B inductric positive, discharge mostly at _o_ by negative brush: A, B inductric negative, discharge all at _n_ by negative brush. Here the negative discharge most facile.

1516. _Carbonic acid gas._--_n_ and _o_ = 0.1 of an inch: A, B inductric positive, discharge nearly all at _o_, or negative: A, B inductric negative, discharge nearly all at _n_, or negative. Intervals = 0.8 of an inch: A, B inductric positive, discharge mostly at _o_, or negative. A, B inductric negative, discharge all at _n_, or negative. In this case the negative had a decided advantage in facility of discharge.

1517. Thus, if we may trust this form of experiment, the negative small ball has a decided advantage in facilitating disruptive discharge over the positive small ball in some gases, as in carbonic acid gas and coal gas (1399.), whilst in others that conclusion seems more doubtful; and in others, again, there seems a probability that the positive small ball may be superior. All these results were obtained at very nearly the same pressure of the atmosphere.

1518. I made some experiments in these gases whilst in the air jar (fig.

131.), as to the change from spark to brush, a.n.a.logous to those in the open air already described (1486. 1487.). I will give, in a Table, the results as to when brush began to appear mingled with the spark; but the after results were so varied, and the nature of the discharge in different gases so different, that to insert the results obtained without further investigation, would be of little use. At intervals less than those expressed the discharge was always by spark.

_______________________________________________________________________ | | | | | | Discharge between | Discharge between | | | b.a.l.l.s B and D. | b.a.l.l.s A and C. | | |___________________________|___________________________| | | | | | | | | Small ball | Small ball | Large ball | Large ball | | | B inductric | B inductric | A inductric | A inductric | | | _pos_. | _neg_. | _pos_. | _neg_. | |_______________|_____________|_____________|_____________|_____________| | | | | | | | Air | 0.55 | 0.30 | 0.40 | 0.75 | | Nitrogen | 0.30 | 0.40 | 0.52 | 0.41 | | Oxygen | 0.70 | 0.30 | 0.45 | 0.82 | | Hydrogen | 0.20 | 0.10 | | | | Coal gas | 0.13 | 0.30 | 0.30 | 0.44 | | Carbonic acid | 0.82 | 0.43 | 1.60 | {above 1.80;| | | | | | had not | | | | | | s.p.a.ce.) | |_______________|_____________|_____________|_____________|_____________|

1519. It is to be understood that sparks occurred at much higher intervals than these; the table only expresses that distance beneath which all discharge was as spark. Some curious relations of the different gases to discharge are already discernible, but it would be useless to consider them until ill.u.s.trated by further experiments.

1520. I ought not to omit noticing here, that Professor Belli of Milan has published a very valuable set of experiments on the relative dissipation of positive and negative electricity in the air[A]; he finds the latter far more ready, in this respect, than the former.

[A] Bibliotheque Universelle, 1836, September, p. 152.

1521. I made some experiments of a similar kind, but with sustained high charges; the results were less striking than those of Signore Belli, and I did not consider them as satisfactory. I may be allowed to mention, in connexion with the subject, an interfering effect which embarra.s.sed me for a long time. When I threw positive electricity from a given point into the air, a certain intensity was indicated by an electrometer on the conductor connected with the point, but as the operation continued this intensity rose several degrees; then making the conductor negative with the same point attached to it, and all other things remaining the same, a certain degree of tension was observed in the first instance, which also gradually rose as the operation proceeded. Returning the conductor to the positive state, the tension was at first low, but rose as before; and so also when again made negative.

1522. This result appeared to indicate that the point which had been giving off one electricity, was, by that, more fitted for a short time to give off the other. But on closer examination I found the whole depended upon the inductive reaction of that air, which being charged by the point, and gradually increasing in quant.i.ty before it, as the positive or negative issue was continued, diverted and removed a part of the inductive action of the surrounding wall, and thus apparently affected the powers of the point, whilst really it was the dielectric itself that was causing the change of tension.

1523. The results connected with the different conditions of positive and negative discharge will have a far greater influence on the philosophy of electrical science than we at present imagine, especially if, as I believe, they depend on the peculiarity and degree of polarized condition which the molecules of the dielectrics concerned acquire (1503. 1600.). Thus, for instance, the relation of our atmosphere and the earth within it, to the occurrence of spark or brush, must be especial and not accidental (1464.).

It would not else consist with other meteorological phenomena, also of course dependent on the special properties of the air, and which being themselves in harmony the most perfect with the functions of animal and vegetable life, are yet restricted in their actions, not by loose regulations, but by laws the most precise.

1524. Even in the pa.s.sage through air of the voltaic current we see the peculiarities of positive and negative discharge at the two charcoal points; and if these discharges are made to take place simultaneously to mercury, the distinction is still more remarkable, both as to the sound and the quant.i.ty of vapour produced.

1525. It seems very possible that the remarkable difference recently observed and described by my friend Professor Daniell[A], namely, that when a zinc and a copper ball, the same in size, were placed respectively in copper and zinc spheres, also the same in size, and excited by electrolytes or dielectrics of the same strength and nature, the zinc ball far surpa.s.sed the zinc sphere in action, may also be connected with these phenomena; for it is not difficult to conceive how the polarity of the particles shall be affected by the circ.u.mstance of the positive surface, namely the zinc, being the larger or the smaller of the two inclosing the electrolyte. It is even possible, that with different electrolytes or dielectrics the ratio may be considerably varied, or in some cases even inverted.

[A] Philosophical Transactions, 1838, p. 47.

_Glow discharge._

1526. That form of disruptive discharge which appears as a _glow_ (1359.

1405.), is very peculiar and beautiful: it seems to depend on a quick and almost continuous charging of the air close to, and in contact with, the conductor.

1527. _Diminution of the charging surface_ will produce it. Thus, when a rod 0.3 of an inch in diameter, with a rounded termination, was rendered positive in free air, it gave fine brushes from the extremity, but occasionally these disappeared, and a quiet phosph.o.r.escent continuous glow took their place, covering the whole of the end of the wire, and extending a very small distance from the metal into the air. With a rod 0.2 of an inch in diameter the glow was more readily produced. With still smaller rods, and also with blunt conical points, it occurred still more readily; and with a fine point I could not obtain the brush in free air, but only this glow. The positive glow and the positive star are, in fact, the same.

1528. _Increase of power in the machine_ tends to produce the glow; for rounded terminations which will give only brushes when the machine is in weak action, will readily give the glow when it is in good order.

1529. _Rarefaction of the air_ wonderfully favours the glow phenomena. A bra.s.s ball, two and a half inches in diameter, being made positively inductric in an air-pump receiver, became covered with glow over an area of two inches in diameter, when the pressure was reduced to 4.4 inches of mercury. By a little adjustment the ball could be covered all over with this light. Using a bra.s.s ball 1.25 inches in diameter, and making it inducteously positive by an inductric negative point, the phenomena, at high degrees of rarefaction, were exceedingly beautiful. The glow came over the positive ball, and gradually increased in brightness, until it was at last very luminous; and it also stood up like a low flame, half an inch or more in height. On touching the sides of the gla.s.s jar this lambent flame was affected, a.s.sumed a ring form, like a crown on the top of the ball, appeared flexible, and revolved with a comparatively slow motion, i.e.

about four or five times in a second. This ring-shape and revolution are beautifully connected with the mechanical currents (1576.) taking place within the receiver. These glows in rarefied air are often highly exalted in beauty by a spark discharge at the conductor (1551. _Note_.).

1530. To obtain a _negative glow_ in air at common pressures is difficult.