Part 41 (2/2)

1381. We have now arrived at the important question, how will the inductive tension requisite for insulation and disruptive discharge be sustained in gases, which, having the same physical state and also the _same pressure_ and the _same temperature_ as _air_, differ from it in specific gravity, in chemical qualities, and it may be in peculiar relations, which not being as yet recognized, are purely electrical (1361.)?

1382. Into this question I can enter now only as far as is essential for the present argument, namely, that insulation and inductive tension do not depend merely upon the charged conductors employed, but also, and essentially, upon the interposed dielectric, in consequence of the molecular action of its particles (1292.).

1383. A gla.s.s vessel _a_ (fig. 127.)[A] was ground at the top and bottom so as to be closed by two ground bra.s.s plates, _b_ and _c_; _b_ carried a stuffing-box, with a sliding rod _d_ terminated by a bra.s.s ball _s_ below, and a ring above. The lower plate was connected with a foot, stop-c.o.c.k, and socket, _e_, _f_ and _g_; and also with a bra.s.s ball _l_, which by means of a stem attached to it and entering the socket _g_, could be fixed at various heights. The metallic parts of this apparatus were not varnished, but the gla.s.s was well-covered with a coat of sh.e.l.l-lac previously dissolved in alcohol. On exhausting the vessel at the air-pump it could be filled with any other gas than air, and, in such cases, the gas so pa.s.sed in was dried whilst entering by fused chloride of calcium.

[A] The drawing is to a scale of 1/6.

1384. The other part of the apparatus consisted of two insulating pillars, _h_ and _i_, to which were fixed two bra.s.s b.a.l.l.s, and through these pa.s.sed two sliding rods, _k_ and _m_, terminated at each end by bra.s.s b.a.l.l.s; _n_ is the end of an insulated conductor, which could be rendered either positive or negative from an electrical machine; _o_ and _p_ are wires connecting it with the two parts previously described, and _q_ is a wire which, connecting the two opposite sides of the collateral arrangements, also communicates with a good discharging train _r_ (292.).

1385. It is evident that the discharge from the machine electricity may pa.s.s either between _s_ and _l_, or S and L. The regulation adopted in the first experiments was to keep _s_ and _l_ with their distance _unchanged_, but to introduce first one gas and then another into the vessel _a_, and then balance the discharge at the one place against that at the other; for by making the interval at _a_ sufficiently small, all the discharge would pa.s.s there, or making it sufficiently large it would all occur at the interval _v_ in the receiver. On principle it seemed evident, that in this way the varying interval _u_ might be taken as a measure, or rather indication of the resistance to discharge through the gas at the constant interval _v_. The following are the constant dimensions.

Ball _s_ 0.93 of an inch.

Ball S 0.96 of an inch.

Ball _l_ 2.02 of an inch.

Ball L 0.62 of an inch.

Interval _v_ 0.62 of an inch.

1386. On proceeding to experiment it was found that when air or any gas was in the receiver _a_, the interval _u_ was not a fixed one; it might be altered through a certain range of distance, and yet sparks pa.s.s either there or at _v_ in the receiver. The extremes were therefore noted, i.e.

the greatest distance short of that at which the discharge _always_ took place at _v_ in the gas, and the least distance short of that at which it _always_ took place at _u_ in the air. Thus, with air in the receiver, the extremes at _u_ were 0.56 and 0.79 of an inch, the range of 0.23 between these distances including intervals at which sparks pa.s.sed occasionally either at one place or the other.

1387. The small b.a.l.l.s _s_ and S could be rendered either positive or negative from the machine, and as gases were expected and were found to differ from each other in relation to this change (1399.), the results obtained under these differences of charge were also noted.

1388. The following is a Table of results; the gas named is that in the vessel _a_. The smallest, greatest, and mean interval at _u_ in air is expressed in parts of an inch, the interval _v_ being constantly 0.62 of an inch.

Smallest. Greatest. Mean.

_ | Air, _s_ and S, pos. 0.60 0.79 0.695 |_Air, _s_ and S, neg. 0.59 0.68 0.635 _ | Oxygen, _s_ and S, pos. 0.41 0.60 0.505 |_Oxygen, _s_ and S, neg. 0.50 0.52 0.510 _ | Nitrogen, _s_ and S, pos. 0.55 0.68 0.615 |_Nitrogen, _s_ and S, neg. 0.59 0.70 0.645 _ | Hydrogen, _s_ and S, pos. 0.30 0.44 0.370 |_Hydrogen, _s_ and S, neg. 0.25 0.30 0.275 _ | Carbonic acid, _s_ and S, pos. 0.56 0.72 0.640 |_Carbonic acid, _s_ and S, neg. 0.58 0.60 0.590 _ | Olefiant gas, _s_ and S, pos. 0.64 0.86 0.750 |_Olefiant gas, _s_ and S, neg. 0.69 0.77 0.730 _ | Coal gas, _s_ and S, pos. 0.37 0.61 0.490 |_Coal gas, _s_ and S, neg. 0.47 0.58 0.525 _ | Muriatic acid gas, _s_ and S, pos. 0.89 1.32 1.105 |_Muriatic acid gas, _s_ and S, neg. 0.67 0.75 0.710

1389. The above results were all obtained at one time. On other occasions other experiments were made, which gave generally the same results as to order, though not as to numbers. Thus:

Hydrogen, _s_ and S, pos. 0.23 0.57 0.400 Carbonic acid, _s_ and S, pos. 0.51 1.05 0.780 Olefiant gas, _s_ and S, pos. 0.66 1.27 0.965

I did not notice the difference of the barometer on the days of experiment[A].

[A] Similar experiments in different gases are described at 1507.

1508.--_Dec. 1838._

1390. One would have expected only two distances, one for each interval, for which the discharge might happen either at one or the other; and that the least alteration of either would immediately cause one to predominate constantly over the other. But that under common circ.u.mstances is not the case. With air in the receiver, the variation amounted to 0.2 of an inch nearly on the smaller interval of 0.6, and with muriatic acid gas, the variation was above 0.4 on the smaller interval of 0.9. Why is it that when a fixed interval (the one in the receiver) will pa.s.s a spark that cannot go across 0.6 of air at one time, it will immediately after, and apparently under exactly similar circ.u.mstances, not pa.s.s a spark that can go across 0.8 of air?

1391. It is probable that part of this variation will be traced to particles of dust in the air drawn into and about the circuit (1568.). I believe also that part depends upon a variable charged condition of the surface of the gla.s.s vessel _a_. That the whole of the effect is not traceable to the influence of circ.u.mstances in the vessel _a_, may be deduced from the fact, that when sparks occur between b.a.l.l.s in free air they frequently are not straight, and often pa.s.s otherwise than by the shortest distance. These variations in air itself, and at different parts of the very same b.a.l.l.s, show the presence and influence of circ.u.mstances which are calculated to produce effects of the kind now under consideration.

1392. When a spark had pa.s.sed at either interval, then, generally, more tended to appear at the _same_ interval, as if a preparation had been made for the pa.s.sing of the latter sparks. So also on continuing to work the machine quickly the sparks generally followed at the same place. This effect is probably due in part to the warmth of the air heated by the preceding spark, in part to dust, and I suspect in part, to something unperceived as yet in the circ.u.mstances of discharge.

1393. A very remarkable difference, which is _constant_ in its direction, occurs when the electricity communicated to the b.a.l.l.s _s_ and S is changed from positive to negative, or in the contrary direction. It is that the range of variation is always greater when the small bulls are positive than when they are negative. This is exhibited in the following Table, drawn from the former experiments.

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