Aston 1922/Chapter 3

Chapter III - Positive Rays

14. Nature of Positive Rays

Positive rays were discovered by Goldstein in 1886 in electrical discharge at low pressure. In some experiments with a perforated cathode he noticed streamers of light behind the perforations. This luminosity, he assumed, was due to rays of some sort which travelled in the opposite direction to the cathode rays and so passed through the apertures in the cathode, these he called "canalstrahlen."^[1] Subsequently Wien showed that they could be deflected by a magnetic field.^[2] They have been very fully investigated in this country by Sir J, J. Thomson,^[3] who called them Positive Rays on account of the fact that they normally carry a charge of positive electricity.

The conditions for the development of the rays are, briefly, ionisation at low pressure in a strong electric field. lonisation, which may be due to collisions or radiation, means in its simplest case the detachment of one electron from a neutral atom. The two resulting fragments carry charges of electricity of equal quantity but of opposite sign. The negatively charged one is the electron, the atomic unit of negative electricity itself,^[4] and is the same whatever the atom ionised. It is extremely light and therefore in the strong electric field rapidly attains a high velocity and becomes a cathode ray. The remain ing fragment is clearly dependent on the nature of the atom ionised. It is immensely more massive than the electron, for the mass of the lightest atom, that of hydrogen, is about 1845 times that of the electron, and so will attain a much lower velocity under the action of the electric field. However, if the field is strong and the pressure so low that it does not colhde with other atoms too frequently it will ultimately attain a high speed in a direction opposite to that of the detached electron, and become a "positive ray." The simplest form of positive ray is therefore an atom of matter carrying a positive charge and endowed, as a result of faUing through a high potential, with sufficient energy to make its presence detectable. Positive rays can be formed from molecules as well as atoms, so that it will at once be seen that any measurement of their mass will give us direct information as to the masses of atoms of elements and molecules of compounds, and that this information will refer to the atoms or molecules individually, not, as in chemistry, to the mean of an immense aggregate. It is on this account that the accurate analysis of positive rays is of such importance.

In order to investigate and analyse them it is necessary to obtain intense beams of the rays. This can be done in several ways. The one most generally available is by the use of the discharge in gases at low pressure.

15. Mechanism of the electric discharge in gases at low pressure

It is a somewhat striking anomaly that while the working of the very recently invented "Coolidge" X-ray bulb can be simply described and explained, this is far from being the case with the much older ordinary "gas" tube. Notwithstanding the immense amount of research work done on the discharge at low pressure its most obvious phenomena are well nigh entirely lacking explanation. Modern measurements and other data have merely destroyed the older theories, without, as yet, giving others to replace them.

For the purposes of describing positive rays it is not necessary to consider such puzzles as the "striated discharge" or other phenomena connected with the anode end of the tube, but some ideas as to what is going on near the cathode will be a considerable help in our interpretation of the results of positive ray analysis, and vice versa.

16. The Crookes Dark Space

The comparatively dimly lit space in front of the cathode, terminating at the bright "negative glow" was first observed by Crookes. Its length is roughly inversely proportional to the pressure of the gas in the tube. Its boundary the edge of the negative glow is remarkably sharp in most gases, quite amazingly so in pure oxygen. If large plane cathodes are used so that the effect of the glass walls up to now a complete mystery is minimised very accurate and consistent measurements can be obtained. Such measurements have been made under a great variety of conditions by the writer.^[5] The distribution of electric force in the dark space has also been determined for large plane electrodes^[6] but no theory yet put forward can account for the numerical relations obtained in these investigations, nor for others obtained later with perforated electrodes.^[7]

One can, however, be fairly certain that ionisation is going on at aU points throughout the dark space, and that it reaches a very high intensity in the negative glow. This ionisation is probably caused for the most part by electrons liberated from the surface of the cathode (Cathode Rays). These, when they reach a speed sufficient to ionise by collision, fiberate more free electrons which, in their turn, become ionising agents, so that the intensity of ionisation from this cause will tend to increase as we move away from the cathode. The fiberation of the original electrons from the surface of the cathode is generally regarded as due to the impact of positive ions (Positive Rays) generated in the negative glow and the dark space, but this idea, for which there is a fair amount of definite evidence, is now called in question by some recent experiments of Ratner.^[8]

In addition to cathode ray ionisation the positive rays traveling towards the cathode themselves are capable of ionising the gas, and radiation may also play an important part in the same process. The surface of the cathode will therefore be under a continuous hail of positively charged particles. Their masses may be expected to vary from that of the Lightest atom to that of the heaviest molecule capable of existence in the discharge tube, and their energies from an indefinitely small value to a maximum expressed by the product of the charge they carry x the total potential applied to the electrodes. The latter is practically the same as the fall of potential across the dark space. If the cathode be pierced the rays pass through the aperture and form a stream heterogeneous both in mass and velocity which can be subjected to examination and analysis.

17. Methods of detecting positive rays

The glow caused by the passage of the rays through rarefied gas led to their original discovery but is not made use of in accurate work. For visual effects the rays are best detected by a screen made of powdered willemite, which glows a faint green when bom arded by them. When permanent effects are required this screen is replaced by a photographic plate. The sensitivity of the plate to positive rays bears no particular relation to it s sensitivity to light, and so far the best results have been obtained from comparatively slow "process" plates of the type known as "Half-Tone." The real relative intensities of rays of different mass cannot be compared by screens or photographic plates, except in the possible case of isotopes of the same element; they can only be determined reliably by collecting the rays in a Faraday cylinder and measuring their total electric charge.

18. Sir J. J. Thomson's "Parabola" method of analysis

The method by which Sir J. J. Thomson made such a complete investigation into the properties of positive rays, and which still remains pre-eminent in respect to the variety of information it supplies, consists essentially in allowing the rays to pass through a very narrow tube and then analysing the fine beam so produced by electric and magnetic fields.

The construction of one of the types of apparatus used is indicated in Fig. 3. The discharge by which the rays are made takes place in a large flask A similar to an ordinary X-ray bulb of about 1½ litres capacity. The cathode B is placed in the neck of the bulb. Its face is made of aluminium, and so shaped that it presents to the bulb a hemispherical front provided in the centre with a funnel-shaped depression. This hole through which the rays pass is continued as an extremely fine-bore tube, usually of brass, about 7 cms. long, mounted in a thick iron tube forming the continuation of the cathode as indicated. The finer the bore of this tube the more accurate are the results obtained, and tubes have been made with success as narrow as one-tenth of a millimetre, but as the intensity of the beam of rays falls off with the inverse fourth power of the diameter a practical Limit is soon reached. The cathode is kept cool during the discharge by means of the water-jacket C.

The anode is an aluminium rod D, which is generally placed for convenience in a side tube. In order to ensure a supply of the gas under examination a steady stream is allowed to leak in through an exceedingly fine glass capillary tube E, and after circulating through the apparatus is pumped off at F by a Gaede rotating mercury pump. By varying the speed of the pump and the pressure in the gas-holder communicating with E, the pressure in the discharge tube may be varied at will and maintained at any desired value for considerable lengths of time. The pressure is usually adjusted so that the discharge potential is 30,000 to 50,000 volts. During the discharge all the condition s necessary for the production of positive rays are present in A. Under the influence of the enormous potentials they attain high speeds as they fly towards the cathode, and those falling axially pass right through the fine tube, emerging as a narrow beam.

This beam is subjected to analysis by causing it to pass between the pieces of soft iron P, P' which are placed between the poles M, M' of a powerful electromagnet, P and P' constitute the pole pieces of the magnet, but are electrically insulated from it by thin sheets of mica N, N', and so can be raised to any desired potential difference by means of the leads shown in the diagram. The rays then enter the highly exhausted "camera" G, and finally impinge upon the fluorescent screen or photographic plate H. In order that the stray magnetic field may not interfere with the main discharge in A, shields of soft iron, I, I' are interposed between the magnet and the bulb.

If there is no field between the plates P, P' the beam of rays will strike the screen at a point in fine with the fine tube called the undeflected spot. If an electric field of strength X is now applied between the plates a particle of mass m, charge e, moving with velocity v, will be deflected in the plane of the paper and will no longer strike the screen at the undeflected spot, but at a distance x from it. Simple dynamics show that if the angle of deflection is small $x = k (\frac{X e}{m v^{2}})$ In the same way, if the electric field is removed and a magnetic field of strength H applied between P and P' the particle will be deflected at right angles to the plane of the paper and strike the screen at a distance y from the undeflected spot where $y = k^{'} (\frac{H e}{m v})$ , k and k' being constants depending solely on the dimensions and form of the apparatus used. If now, with the undeflected spot as origin, we take axes of co-ordinates OX, OY along the fines of electric and magnetic deflection, when both fields are applied simultaneously the particle will strike the screen at the point (x, y) where y/x is a measure of its velocity and y²/x is a measure of m/e its ratio of mass to charge.

Now e can only exist as the electronic charge 4.77 x 10^-10 C.G.S. or a simple multiple of it. Thus if we have a beam of positive rays of constant mass, but moving with velocities varying over a considerable range, y²/x will be constant and the locus of their impact with the screen will be a parabola pp' (Fig. 4). When other rays having a larger mass m but the same charge are introduced into the beam, they will appear as another parabola qq having a smaller magnetic displacement. If any straight line p, q, n be drawn parallel to the magnetic axis OY cutting the two parabolas and the electric axis OX in p, q, n it will be seen at once that m'/m = pn²/qn². That is to say, the masses of two or more particles can be compared directly by merely measuring lengths the ratio of which is entirely independent of the form of the apparatus and the experimental conditions.

This is really the fundamental principle upon which the method is based. A photographic record is obtained on which we can identify at least one parabola as being associated with atoms or molecules of known mass; all the other parabolas can then be measured and compared with this one and their masses deduced. With electric and magnetic fields roughly known there is little difficulty in such an identification, and to make quite sure the absolute value of m/e for the hydrogen atom was determined and found to agree with the values obtained by other methods. In actual practice, since OX is an imaginary line and has no existence on the photograph, in order that the measurements may be made with greater convenience and accuracy the magnetic field is reversed during the second half of the exposure, when in the case we are considering two new parabolas will appear at rr, ss, due to m and m respectively ; the masses can now be compared by the equation m'/m = pr²/qs²: p, q, r, s being any straight line cutting the curves approximately parallel to the magnetic axis. The measurement of these lengths is independent of zero determination, and if the curves are sharp can be carried out with considerable accuracy.

Some of the photographic results obtained by this method of analysis are shown in Plate I. The fact that the streaks are definite sharp parabolas, and not mere blurs, was the first experimental proof that the atoms of the same element had very approximately the same mass.

It has been shown that the electrical displacement is in inverse proportion to the energy of the particle. Since this energy is simply dependent on and proportional to the electrical potential through which the charged particle fell before it reached the cathode and not upon its mass, the distribution of intensity along the parabolas will be somewhat similar. There will also be a definite maximum energy corresponding to the whole drop of potential across the discharge tube, with a corresponding minimum displacement on the plate; so that all normal parabolas will end fairly sharply at points p, q, etc., equidistant from the magnetic axis OY. As the ionisation is a maximum in the negative glow the parabolas are brightest at or near these points. The extension of the curves in the other direction indicates the formation of ions at points in the discharge nearer the cathode which will so have fallen through a smaller potential.

19. Secondary Rays

As the pressure in the camera, though as low as possible, is never entirely negligible, the particles may make collisions, and so gain and lose electrons, while passing through the deflecting fields. This results in what Sir J. J. Thomson calls "secondary rays,"^[9] which may be of a great many types. Some appear on the plate as general fog, others as straight beams seeming to radiate from the undeflected spot, these will easily be recognised on the photographs produced in Plate I. Secondary rays can produce parabolas which are very much like the genuine ones caused by particles which have retained their charge through both fields, and which may easily be mistaken for them unless special precautions are taken.

20. Negatively Charged Rays

As there is intense ionisation in the fine tube the charged particles may easily collide with and capture electrons in passing through it. A singly charged particle capturing a single electron will, of course, proceed as a neutral ray, and being unafiEected by the fields will strike the screen at the central spot. If, however, it makes a second collision and capture it will become a negatively charged ray. Rays of this kind will suffer deflection in both fields in the opposite direction to the normal ones, and will therefore give rise to parabolas of a similar nature but situaated in the opposite quadrants, as indicated by the dotted lines in the figure. Such negative parabolas are always less intense than the corresponding normal ones, and are usually associated with the atoms of electronegative elements such as carbon, oxygen, chlorine, etc.

The negative parabolas of H, C and 0 can be seen in the photographs. Plate I (1) and (2).

21. Rays with Multiple Charges

If during ionisation more than one electron is split off, the resulting positive ray will have a double or multiple charge. Taking the case of a doubly charged particle it may give rise to two distinct effects. In the first place, if it retains its double charge while passing through the analysing fields its behaviour will be quite indis tinguishable from that of a normal ray of haff its mass. Thus the effective mass of the doubly charged oxygen atom, written^[10] O⁺⁺, will be 8. Parabolas due to C⁺⁺ and O⁺⁺ can be seen in Plate I (2). In the second place, the particle may retain its double charge through the whole potential fall of the discharge but capture an electron in the fine tube. It will then constitute a ray of normal ratio of mass to charge but with double the normal energy, so that the normal end of the parabolas will be extended towards the axis OY to a point half-way between that axis and the line pq. Such extensions will be seen on the bright parabolas due to carbon and oxygen in the photographs reproduced in Plate I.

Most elements are capable of losing two electrons, some, such as krypton, three or more, while mercury can lose no less than eight at a time. The results of the multiple charge on atoms of mercury is beautifully illustrated in Plate I (3). The parabola a corresponding to normal single charge will be seen extended almost to the origin itself, while above a series of parabolas of diminishing intensity β, γ, etc., indicate the atoms which have retained two, three or more charges.

22. Dempster's method of positive ray analysis

It is clear from the considerations on page 27 that if the positive particles all fell through the same potential and so possessed the same energy, a magnetic field alone would suffice to perform their analysis with regard to mass. A method of analysis based on this idea has been devised by Dempster at the Ryer son Physical Laboratory, Chicago.^[11]

The method is essentially identical with that used by Classen in his determination of e/m for electrons.^[12] The charged particles from some source fall through a definite potential difference. A narrow bundle is separated out by a slit and is bent into a semicircle by a strong magnetic field; the rays then pass through a second slit and fall on a plate connected to an electrometer. The potential difference P, magnetic field H, and radius of curvature r determine the ratio of the charge to the mass of the particle by the formula $\frac{e}{m} = \frac{2 P}{H^{2} r^{2}}$ .

The apparatus consisted of a glass tube G, Fig. 5, where the positive particles fell through a definite potential difference, and the analysing chamber A, in which a strong magnetic field was produced between two semicircular iron plates 2.8 cm. thick and 13 cm. in diameter. The iron plates were soldered into half of a heavy brass tube B, so as to leave a passage or slot 4 mm. wide between the plates. A brass plate C closed this slot except for three openings into which short brass tubes were soldered. The glass tube G fitted into the first opening and a tube for exhausting into the second. The electrometer connection passed to a receiving plate through an ebonite plug E which formed a ground conical joint with the third brass tube. The two openings for the rays had adjustable slits S₁, S₁, and a screen D was introduced into the analysing chamber to prevent reflected rays getting into the second slit. The whole was placed between the poles of a powerful electromagnet.

The accelerating potential P was applied by means of a large battery and was from 500 to 1750 volts or thereabouts. The experimental procedure consisted in maintaining a constant magnetic field and plotting the ionic current, measured by the electrometer, against the potential. The peaks on the curve corresponded to definite values of m/e, measured by the potential, and their heights to the relative quantities of the particles present in the beam.

The method is limited in its application by the fact that the ions must be generated with a velocity negligible compared with that produced by the accelerating potential. The first results were obtained from ions produced by heating salts on platinum strips, or by bombarding them with electrons. It was shown that the ions given off from heated aluminium phosphate consisted for the most part of sodium and potassium atoms, and that these had masses 23 and 39 respectively. The resolution possible with the first apparatus was claimed to be about 1 in 100. Dempster's recent successful application of this method to the analysis of magnesium and lithium will be described in a later chapter.^[13]

References

↑ Goldstein, Berl. Ber., 39, 691, 1886.
↑ Wien, Verh. d. Phys. Gesell, 17, 1898.
↑ J. J. Thomson, Rays of Positive Electricity and their Application to Chemical Analyses, Longmans, Green, 1913.[1]
↑ R. A. Millikan, The Electron, University Chicago Press, 1918.
↑ Aston, Proc. Roy. Soc. 79A, 80, 1907[2]; Aston and Watson, ibid. 86A, 168, 1912 ; Aston, ibid. 87A, 428, 437, 1912.
↑ Aston, Proc. Roy. Soc. 84A, 526, 1911.
↑ Aston, Proc. Roy. Soc. 96A, 2200, 1919.[3]
↑ Ratner, Phil Mag. 40, 795, 1920.
↑ J. J. Thomson, Rays of Positive Electricity, p. 32.
↑ In the normal singly-charged ray the plus sign is omitted for convenience.
↑ 1 Dempster, Phys. Rev. 11, 316, 1918.
↑ Classen, Jahrb. d. Hamburg Wiss. AnsL, Beiheft, 1907.
↑ V. p. 80.