During the discharge all the conditions necessary for the pro duction of positive rays are present in A. Under the influence of the high potentials they attain high speeds as they fly towards the cathode, and those falling axially pass right through the narrow tube emerging as a fine circular beam. This beam is sub jected 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 by means of the leads shown. 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 inter posed 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 line 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 distance x from it. If the angle of deflection is small x=k In the same way if this electric field is now 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' (He/mv), k and k' being constants depending solely on the form of the apparatus. If now, with the undeflected spot as origin, we take axes of co-ordinates OX, OY along the lines 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 is a measure of e/m, its ratio of mass to charge.
Now e can only exist as the electronic charge 4.77X 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 con siderable range, will be constant and the locus of their impact with the screen will be a parabola pp' (fig. 2). When other rays having a larger mass in' 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 In' 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 inde pendent of the form of the apparatus and the experimental conditions.
The principle of the method is, there fore, to obtain a photographic record upon which at least one parabola can be identi fied with particles of known mass; all the other parabolas can then he measured against this one and their masses deduced. In prac tice, 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 S , due to m and In' respectively; the masses can now be compared by the equation where p,q,r,s is any straight line cutting the curves approximately parallel to the magnetic axis. The measurement of these lengths is independent of zero determinations, and if the curves are sharp, can be carried out with considerable accuracy. It has been shown that the electrical displacement is in inverse proportion to the energy of the particle. This energy has a maxi mum limit determined by the potential of the discharge so that normal parabolas will end fairly sharply at points p,q, etc., equi distant from the magnetic axis This is clearly shown in the photographs reproduced in Plate, figs. r and 2.
Negatively Charged Rays.—As there is intense ionization 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 unaffected 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 situated in the opposite quadrants, as indicated by the fine lines in fig. 2. Such negative parabolas are shown in the photographs. They are always less intense than the corre sponding normal ones, and are usually associated with electro negative atoms.