Rays of Positive Electricity and Their Application to Chemical Analyses/Effect at Very Low Pressures
When the pressure Is reduced to as low a value as is possible the appearance of the luminosity on the screen entirely changes. At these low pressures It Is exceedingly difficult to get the discharge to pass through tubes of moderate size when the cathodes are made of aluminium or any of the metals ordinarily used for this purpose, and there Is great danger of sparks passing through the glass and breaking the tube. This can be avoided to a great extent by facing the cathode with a thin layer of calcium, or smearing the face of the cathode with the liquid alloy of sodium and potassium. This reduces considerably the difficulty of getting the discharge to pass and diminishes the risk of perforating the tube. The appearance at these low pressures when hydrogen or air is In the tube Is shown In Fig. 9. It will be noticed that the straight bands of phosphorescence have almost disappeared and that most of phosphorescent light Is concentrated Into two parabolic curves which are connected with the undeflected spot by straight faintly luminous lines. The value of e/m for one parabola is 104, that for the other 5 x 103 so that they are due to the atom and molecule of hydrogen respectively. At these low pressures the luminosity In the negative direction disappears. But both at the low and higher pressure there is, even when the magnetic and electric fields are in action, an appreciable amount of luminosity at the position occupied by the undeflected spot.
There is considerable advantage in using very large glass vessels for the discharge tubes when studying positive rays; with large vessels the pressure can be made very small before the tube offers great resistance to the passage of the discharge through It The increase in the difficulty of getting the discharge to pass comes in at the pressure when the dark space round the cathode reaches the walls of the tube. When the tube is big the walls are far away from the cathode and the pressure has to be exceedingly low before the dark space reaches the sides of the tube. We can work with much lower pressures with these large tubes and therefore reduce the obstruction which the positive rays meet with in their passage from the cathode to the screen. Using vessels of about 2 litres capacity I have observed1 on a willemite screen the parabolas corresponding to carbon, oxygen, neon, and mercury vapour as well as those corresponding to the atom and molecule of hydrogen and the atom of helium. The photographic plate is, however, for most purposes a much more convenient detector than a willemite screen. It is more sensitive, it gives a permanent record, and measurements can be made with much greater accuracy on the plate than they can on the screen. Before entering into the discussion of the theory of the positive rays it is desirable to describe the results obtained with the photographic method, as well as the experimental details by which these results have been procured.
The apparatus now in use at the Cavendish Laboratory is represented in Fig. 10. The discharge takes place in a large glass flask A: a volume of from one to two litres is a convenient size for this purpose. The cathode C is placed In the neck of the flask. The position of the front of the cathode has a very considerable influence on the brightness of the positive rays and ought to be carefully attended to. The best position seems to be when the front of the cathode is flush with the prolong tion of the wider portion of the flask. The shape of the cathode is represented in section in Fig. 11: the face of the cathode is made of aluminium, the other portion is soft iron. A hole is bored right through the cathode to admit the fine tube through which the positive rays are to pass. Care should be taken to bore this hole so that its axis is the axis of symmetry of the cathode. The tube through which the positive rays pass is fastened into the cathode in the way shown in Fig. 11.
The bore of this tube will vary with the object of the experiment If very accurate measurements are required, the diameter of the tube must be reduced to 1 mm. or less. With these very fine tubes, however, very long exposures (1½ to 2 hours) are necessary. The length of the tube is about 7 cm. The tubes are prepared by drawing out very fine bore copper tubing until the bore is reduced to the desired size. The tube is straightened by rolling it between two plane surfaces, and great care must be taken to get the tube accurately straight, as the most frequent cause of dimness in the positive rays is the crookedness of the tube. After long use the end of the tube nearest the discharge tube gets pulverized by the impact of the positive rays, and the metallic dust sometimes silts up the tube and prevents the rays getting through. The cathode is fastened In the glass vessel by a little sealing-wax, and a similar joint unites it to the ebonite box, UV. To keep the joints cool and prevent any vapour coming from the wax, the joints are surrounded by a water jacket J through which a stream of cold water circulates.
The electric field is produced between the faces of L and M which are pieces of soft Iron with plane faces. These are fitted Into the ebonite box UV so that their faces are parallel: the distance between the faces should be small compared with their lengths. In many of the experiments described subsequently the length of the faces was 3 cm. and their distance apart 1.5 mm. Their faces are connected with the terminals of a battery of small storage cells: In this way any required differ- ence of potential can be maintained between them.
These pieces of soft Iron practically form the poles of an electromagnetic, for the poles of the electromagnet P and Q are made of soft Iron of the same cross section as L,M ; they fit into Indentations In the outside of the ebonite box and are only separated from the pieces L,M, by the thin flat pieces of ebonite which form the walls of the box. This arrangement makes the magnetic field as nearly coterminous as possible with the electric, which Is desirable in several of the experiments. A conical glass vessel F 40 cm. long is fastened by wax to the ebonite box while the other end is fixed to the apparatus which contains the photographic plate. One form of this, designed by Mr. Aston, Is represented in Fig. 12. The photographic plate Is suspended by a silk thread wound round a tap T which fits into a ground glass joint; by turning the tap the thread can be rolled or unrolled and the plate lifted up or let down. The plate slides in a vertical box B made of thin metal; this Is light tight except at the openings A which are placed so that the positive rays can pass through them. The openings are on both sides of the box and about 5 cm. in diameter. When the silk thread is wound up the strip DEFG of photographic plate in the box Is above the opening A, so that there Is a free way for the rays to pass through A and fall on a willemite screen behind It This screen is not used for purposes of measurement, but only to see before taking the photograph that the tube Is giving an adequate supply of positive rays. The box Is sufficiently large to hold a film long enough for two or more photographs; If It Is wished to take two photographs, the plate Is lowered until the bottom half comes opposite to the opening A, a photograph Is taken In this position, the plate Is then let down still further until the top half of the plate comes opposite to the opening, then a second photograph Is taken. This plan Is convenient because the deflections of the different kinds of positive rays differ so much that it is difficult to measure them accurately when they are all on one plate, For example the magnetic deflection of the hydrogen atoms Is about fourteen times that of the mercury one, thus If the deflection of the hydrogen atom is within the limits of the plate, that of the mercury atom would be too small to measure accurately. When we can take two photographs, however, without opening the tube, we may take one with a small magnetic fleld to get the deflection of the hydrogen atom, and the second with a much larger one to get the deflection of the mercury one.
Two tubes containing coconut charcoal are fused to this part of the apparatus; by immersing these in liquid air the pressure can be made exceedingly small. As the only communication between this part of the apparatus and that through which the discharge passes is through the long and very narrow tube in the cathode, it is possible to have the pressure on the camera side of the apparatus very much less than the pressure on the side through which the discharge is passing.
A Gaede pump worked by a motor is connected with the discharge tube, and keeps the pressure in this part of the apparatus at a suitable value. When the rays in some particular gas are under examination a constant stream of this gas is kept flowing through the discharge tube. The gas is stored in the vessel A, Fig. 13, over a column of mercury: this vessel is connected with the discharge tube by the system shown in Fig. 13, where EC is an exceedingly fine capillary tube. When the tap T is turned the gas has to pass through this capillary: it does so exceedingly slowly. The rate can be adjusted by raising or lowering a mercury reservoir connected with A, this is held in such a position that when the Gaede pump is in action the pressure in the discharge tube is such as to give well developed positive rays. To screen off the magnetic field due to the electromagnet, thick iron plates V,W, Fig. 10, are placed round the neck of the tube.
The curves on the photographic plates made by the positive particles are measured by the apparatus represented in Fig. 14. The photographic plate is clamped in a holder A, and the position of any point on it is determined by moving the carrier C until the tip of the needle NN comes just over the point in question. The carrier C has two movements, one parallel to the base BB, and the other, by means of the screw S, at right angles to this direction, the position of the point is read off on the two verniers. The plate is placed In the holder so that the direction of the magnetic deflection Is parallel and that of the electrostatic deflection, at right angles to BB.
1 J. J. Thomson, " PML Mag.," VI, xx, p. 752, 1910.