Excitation by the Yellow Lines.

We have measured the wave-lengths of the lines in the resonance spectrum excited by the yellow lines of the CooperHewitt arc, and the quartz arc operating at 115 volts. The values given in the following table are on the International Scale and reduced to vacuum. They were determined from plates made with the plane grating and Cooke lens of 1 metre focus, and can be considered correct only to about 0·1 AU. We have, however, made some measurements of the doublets photographed with the 3-metre lens, which are correct probably to 0.02 AU., and as the same irregularities were found in the spacing, we have not thought it worth while to measure the complete spectrum to the highest degree of

accuracy.

The wave-lengths are given in the following table. The lines or doublets excited by the Cooper-Hewitt lamp are marked thus*.

The series excited by the Cooper-Hewitt lamp in the case of the 5769 line differs from that excited by the green line in a number of respects.

In the first place, at the point of excitation we have only the R.R. line with no companion. At orders 1, 2, 3, 6, and 9 we have doublets, 4 and 8 are missing, and at 5, 7, and 10 we have single lines.

Excitation by Hg 5769-6 (5771.2 Reduced to Vacuum).

The 1/ difference, in the case of the components of the doublets, is not constant, as in the previous case, but varies from 143 to 126.

As to the spacing of the doublets along the spectrum, we find that in this case the 1/λ difference is very nearly constant, as is shown by the following table :

:

The variation is irregular, and it is obvious that the series is of a different type from the one excited by the green line, a portion of which at least was well represented by a formula.

In the case of the excitation by the 5790-7 line, we obtained different values of A in the case of the Cooper-Hewitt lamp, consequently these values only are given in the table. It is probable that in the case of the quartz lamp at 115 volts reversal of the line causes the disappearance of the doublets excited by the lamp running at a lower temperature :— Excitation by Cooper-Hewitt 5790·7 (5792·3 Reduced

to Vacuum.)

298}

69

Seventh Order.

157997}

Eighth Order.

156152

086.) 66

In the case of the series excited by this line the doublets are present in all orders, but the 1/λ difference between their components varies in a very irregular manner from a minimum value of 34 to a maximum value of 168.

The spacing of the series along the spectrum is more regular, however, the 1/λ differences being as follows (the last significant figure is omitted):

6325.8

158083

29.2

86

6404.0
06.7

It does not appear to be worth while at this stage of the investigation to give the wave-lengths of the lines in the more complicated groups excited by the quartz arc operating at various voltages, as the simpler series excited by the Cooper-Hewitt lamp does not appear at the present time to conform to any law. The cause of this may appear when the relation of this spectrum to the band spectrum developed when the iodine is in helium has been studied. This will require exposures of many days, however.

XXIX. On the Pressure Effect in Corona Discharge. By A. M. TYNDALL, D.Sc., and Miss N. S. SEARLE, B.Sc.* NEVERAL papers have been published within the past few

years on what has been termed "the ionization pressure in corona discharge." When a glow or "corona discharge starts between a cylinder and an axial wire in a closed tube at atmospheric pressure, a sudden rise in pressure is observed. Farwell† and Kunz‡ have contended that this pressure effect is quite distinct from that produced by the heating effect of the discharge, and have suggested that it is due to the increase in the number of gas particles resulting from ionization in the tube.

66

Kunz also has deduced the following formula connecting the rise in pressure "pi-Po" with the current "i," potential difference "e," and the volume of the tube "vo":

In support of this formula Warner § has shown experimentally in a number of gases that, if the potential difference is constant, "i" is proportional to "Pi-Po' Arnold, however, has suggested that the pressure effect can be accounted for by thermal considerations alone, and has pointed out that the small magnitude of the current excluded the possibility of any appreciable contribution to pressure by ionization. Recently Warner has replied to this criticism. He has described a number of experiments on the pressure

* Communicated by the Authors.
+ Farwell, Proc. A. M. I. E. E. vol. xxxiii. p.
Kunz, Phys. Rev. vol. viii. p. 28 (1916).
Warner, Phys. Rev. vol. viii. p. 285 (1916).
Arnold, Phys. Rev. vol. ix. p. 93 (1917).
Warner, Phys. Rev. vol. x. p. 483 (1917).

1717 (1914).

effect (1) due to discharge, and (2) due to a heating current passed through the wire, and has re-affirmed the view that the two effects are quite distinct.

Now in the past there has been general agreement among physicists that in an ordinary glow discharge the fraction of inolecules which are ionized is a negligible quantity. It seems therefore desirable to settle conclusively what is the origin of the pressure rise.

The object of the present paper is to point out several fallacies in the arguments of the advocates of the ionization theory, and to present a quantitative verification of the statement that this sudden rise of pressure accompanying the start of corona discharge is purely a thermal effect.

It must first be pointed out that the formula given above, upon which the advocates of the ionization theory base much of their work, is necessarily incorrect. This may readily be seen by applying the test of dimensions to the terms of the equation. Before the dimensions of the two terms can be equated, the left-hand term must be multiplied by a time "t." The equation then takes the form it would have if the pressure effect was due entirely to heat generated in a vessel of constant volume, from which the radiation losses were always a constant fraction of the heat supplied. If reference be made to the proof of this formula in Kunz's paper, it will be seen that in steps 1 and 4 a time factor has been erroneously omitted.

But this does not dispose of a further argument which has been brought forward namely, that the pressure effect at the start of the discharge is far too sudden to be accounted for by heat generated during discharge, and that the rise in pressure due to the latter is only appreciable after the discharge has been passing for some time.

Thus Warner has published a number of curves showing that the "corona pressure" reached its full value within 3 seconds of the start of discharge, whereas in the corresponding effect produced by heating the wire this did not occur until about 15 seconds after the heating current was switched on. Moreover, the rise in pressure produced by the dissipation of a given amount of energy in the wire was much greater than that accompanying the same dissipation of energy in the glow discharge.

But the argument that the pressure effects in the two cases must therefore be different in origin entirely breaks