THE RELATIVE INTENSITIES OF METAL AND GAS SPECTRA FROM ELECTRICALLY CONDUCTING GASES."

By P. G. NUTTING.

From an arc may be obtained a nearly pure spectrum of the electrodes, the spectrum of the surrounding atmosphere under certain conditions scarcely appearing at all. On the other hand, the light from a discharge tube may give a pure spectrum of the atmosphere about the electrodes, but no spectrum of the electrodes themselves. Various kinds of arcs, sparks, and discharge tubes under various conditions may exhibit mixed spectra in all intermediate proportions.

Arc, spark, and low-pressure discharge are but particular cases of a general case in which a gas forms part of an electric circuit otherwise metallic. It is the purpose of this paper to discuss the many conditions that may govern the preponderance of the spectrum of the electrodes over the spectrum of the intervening gas, select the predominant influences and then if possible to map out the conditions necessary for the production of a given pure spectrum with lines of a given character. To obtain coordinate data over the wide range necessary to warrant general conclusions, the spectra of twenty-one different metals were photographed each under a uniform schedule of eightyone different conditions.

The relative intensity of the spectrum of the electrodes depends primarily, of course, on the relative proportion of its vapor present in the part of the arc, spark, or discharge tube observed. We are

This paper is the third of a series of four on the general subject of the preponderance of one spectrum over another in a mixture of two spectra. The first of this series dealt with the conditions governing the relative intensities of the spectra of two mixed elementary gases under low-pressure discharge. (The Spectra of Mixed, Gases, Bulletin No. 1, pp. 77-81, November, 1904.) The second dealt with the relative intensities of the two different spectra exhibited by the electro-negative elementary gases. (On Secondary Spectra, Bulletin No. 1, pp. 83-93, November, 1904.) The fourth paper of the series is to be on the spectra of alloys.

concerned first with the conditions governing the amount of electrode metal vaporized and then with the laws governing spectral preponderance in mixed gases, namely; (1) increasing the relative amount of a gas present in a mixture of conducting gases increases the relative intensity of its spectrum; (2) other things being equal, in a steady discharge the spectrum of that component of a mixture having greater atomic weight will be brighter;" (3) in a disruptive or forced oscillatory discharge the atomic weight effect is less.

When the internal oscillations are free (instead of being forced) as in the arrangement used by Ramsey, Lilienfeld, and others to obtain the spectrum of argon in that of ordinary air, perhaps a fourth law would be necessary. Konen (Beibl., 1904, p. 1000) has cited the case of mixtures containing oxygen as apparent exceptions to the atomicweight law (2) above. But oxygen, it must be remembered, has only a very faint (visible) anode glow. Hence to make a fair test of the law we must either take the original faintness of the anode glow into account, or else use the cathode-glow spectrum for comparisons. I have always done the latter. In explanation of the exceptional absence of luminosity in the oxygen anode column, I would suggest that there may be the usual amount of radiation in this case, but that it is chiefly in the infra-red region. The anode column is known to be a region of frequent ionization and recombination, hence we should expect it would be a region of great chemical changes as well. In oxygen, ozone would be rapidly formed and decomposed; hence the radiation from the anode column would consist largely of the strong infra-red ozone bands recently studied by Angstrom.

The metallic or nonmetallic character of the atmosphere surrounding a spark appears to make no difference whatever with the behavior of the spectrum of the electrodes. A spark between aluminum electrodes in an atmosphere of mercury vapor shows identically the same changes on the introduction of capacity, inductance, spark in series, etc., that it exhibits in hydrogen. On the other hand, the spectrum of an iron spark preserves its invariant character as well in mercury vapor as in air, oxygen, or hydrogen.

The vaporization of the electrodes with which we are concerned in the arc, spark and vacuum tube is by no means a mere temperature effect. The amount of vaporization of an electrode appears to bear no simple relation either to vapor pressure or temperature. It does,

a The Spectra of Mixed Gases, Astrophys. J. 19, p. 107; Mar., 1904. Thomson: Conduction of Electricity Through Gases, p. 486, ¶ 269.

© Arkiv. för "Matematik," Ast. och Fysik 1, pp. 347-353; 1904. Beiblätter, 28, p. 1142; Nov., 1904.

however, vary enormously with the electrical conditions in the conducting gas, and for this reason a brief discussion of the phenomena of gas conduction is here given.

ELECTRICAL CONDUCTION IN THE ARC, SPARK, AND DISCHARGE TUBE.

Our first problem is to find a relation—a sort of generalized Ohm's law-between the current passing through a gas, the drop in potential and certain constants relating to the gas and the metal used as electrodes. This relation involves the solution of a differential equation containing an unknown function. This function I have constructed, but it gives a nonintegrable form to the differential equation. However, the particular case of steady current admits of a graphical solution (due to W. Kaufmann") and covers a wide variety of the phenomena with which we are concerned.

Consider a circuit containing an e. m. f. E, an ohmic resistance R, inductance L, capacity C, and a column of gas through which the drop of potential is expressible by the (experimentally determined) function e(i) of the current. Equating energy supplied to energy taken up by the circuit,

The final term, ie(i), represents the energy used up in the discharge in whatever manner the drop of potential e varies with the current. As an illustration, consider the energy used by a column of nitrogen 10 cm long and at 1 mm pressure, with platinum electrodes carrying 1 milliampere of current. The anode fall of potential is about 20 volts, the fall through the gas about 600 volts (60 volts per cm), and the cathode fall 260 volts. Hence the total energy used will be about 0.9 watt, of which about a third is used up at the cathode, two-thirds in the gas, and a negligible amount at the anode. If the column were shortened to 5 cm the energy used at the electrodes would be the same, while in the gas but half the previous amount of energy would be used up, and the total fall of potential would be 580 instead of 880 volts. In an ordinary illuminating arc carrying a current of 10 amperes, of the total drop in potential of approximately 50 volts, about half is at the electrodes, so that the luminescent gas receives about half the energy supplied. In the spark we may be reasonably certain that the energy losses are distributed in a manner intermediate between a Ann. d. Phys. 2, pp. 158–178; 1900.

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that of the arc and the discharge tube mentioned above. In every case it appears to require a greater expenditure of energy to force a (positive) current from a gas into a metal than from a metal into a gas.

Now, the loss of energy at the electrodes is not within the electrode itself, nor is it in the adjacent gas outside; it is localized right at the surface separating the electrode from the conducting gas, and represents energy used in forcing the current from the metal to the gas or vice versa. Apparently all the electrode loss of electrical energy reappears as heat energy, about half being given to the gas and half to the electrode. In the spark with capacity, probably a large part of this energy is used up directly in vaporizing the surface layer of the metal, the discharge being so sudden that there is no time for the heat to be communicated to the adjacent metal or gas. In the arc a considerable part of this surface heat is conducted back and radiated from the solid electrode.

The electrical energy equation above can only be solved when the function e(i) is known. This function may be constructed from the properties of its derivative in the following manner: since e() has in general a single maximum and a single minimum, but no real roots, write for the derivative de=a (e-e) (e-e) di, where e, and e, are the ordinates of the maximum and minimum. By integration

a function having the general properties of the characteristic curve for a gas, but which does not give a solvable form when substituted in the energy equation.

When the current is steady the energy equation becomes

E=iR+e(i),

from which the value of the current may be obtained (Kaufmann, l. c.) graphically, it being the abscissa of the point of intersection of the line E-R with e(i). (The abscissa of the point of intersection of y=P1(x) and y=9,(x) is a root of the equation P1(x)—P,(x)=0.) Since in a conducting gas but little energy is stored in comparison with what is dissipated, the e(i) in the last equation is the same as that in the general equation above.

Some typical characteristic curves, e(), are shown in fig. 1. The curve marked g shows the drop of potential as a function of the current in its general form. It is to be noted that: (1) the current starts with a brush discharge, during which the drop increases with increasing current; (2) there is a maximum drop at which the gas

breaks down; (3) after the gas breaks down for a time the drop decreases as the current increases; (4) there is a minimum drop at which the drop is independent of the current, (5) followed by a region in which drop and current increase together. This form of curve may be realized in a gas at moderate pressures with the electrodes at a moderate distance apart."

The curve a, fig. 1, is characteristic of an ordinary arc. The maximum is very high (say 20,000 volts), while the minimum is low, only about 50 volts. The downward sloping part of the curve is the portion usually observed, using but little series resistance and low e. m. f. To start an arc, like any other form of gas conductor, one must either (1)

raise the e. m. f. above the maximum drop or (2) depress the maximum drop below the e. m. f.; the latter may be done by bringing the electrodes into contact, starting ionization by other means or lowering the pressure on the intervening gas.

The curve b is characteristic of a brush discharge at high pressures between electrodes wide apart or pointed. The electromotive forces concerned are very large, while the currents are small. c is typical of very small distances and high pressures. It was obtained by Guthe and Trowbridge, using steel balls nearly in contact (107 cm apart) as a coherer. The maximum drop is only about one-fourth volt, while

a Studied more in detail, this curve shows steps and other irregularities indicating saturation, ionization by impact, discharge not covering electrodes, etc., but which need not be considered here.

Phys. Rev., 11, p. 29; 1900.