Page:EB1911 - Volume 23.djvu/730

Rh Rontgen rays. The nature of this effect may be illustrated by fig. r. Suppose that AB is a stream of cathode rays striking against a solid obstacle B and

giving rise to Rontgen rays, let

these rays impinge on a small

body P, P under these conditions

will emit secondary rays

in all directions. Barkla (Phil.

Trans., 1905, A, 204, p. 467;

Proc. Roy. Soc. 77, p. 247)

found that the intensity of

the secondary rays, tested by

the ionization they produced

in air, was less intense in the

plane ABP than in a plane through PB at right angles to this plane, the distances from P being the same in the two cases; the difference in the intensities amounting to about 15%. Haga (Arm. d. Phys. 28, p. 439), who tried a similar experiment but used a photographic method to measure the intensity of the secondary rays, could not detect any difference of intensity in the two planes, but experiments by Bassler (Ann. der Phys. 28, p. 808) and Vegard (Proc. Roy. Soc. 83, p. 37Q) have confirmed Barkla's original observations. C P

A B

FIG. 1.

The “ polarization " is much more marked if instead of exciting the secondary radiation in P by the Rontgen rays from a discharge tube we do so by means of secondary rays. If, for example, in the case illustrated by fig. I we allow a beam of Rontgen rays to fall u on B instead of the cathode ra s the difference between the P Y ..

intensities in the plane ABP and in the plane at right angles to it are very much increased. It is only the scattered secondary radiation which shows this “ polarization "; the characteristic secondary radiation emitted by the body at P is quite unpolarized. The existence of this effect has a very important bearing on the nature of Rontgen rays. Whether Rontgen rays are or are not a form of li ht, i.e. are some form of electromagnetic disturbance propagate cf through the aether, is a question on which opinion is not unanimous. They resemble light in their rectilinear propagation; they affect a photographic plate and, Brandes and Dorn have shown, they produce an effect, though a small one, on the retina, giving rise to a very faint illumination of the whole field of view. They resemble light in not being defected by either electric or magnetic forces, while the characteristic secondary radiation may be compared with the phosphorescence produced by ultra-violet light, and the cathodic secondary rays with the photo-electric effect. The absence of refraction is not an argument against the rays being a kind of light, for all theories of refraction make this property depend upon the relation between the natural time of vibration T of the refracting substance and the period t of the light vibrations, the refraction vanishing when t/T is very small. Thus there would be no refraction for light of a very small period, and this would also be true if instead of regular periodic undulations we had a pulse of electromagnetic disturbance, provided the time taken by the light to travel over the thickness of the pulse is small compared with the periods of vibration of the molecules of the refracting substance. Experiments on the diffraction of Rontgen rays are ver difficult, for, in addition to the difficulties caused by the smallness ofythe wavelength or the thinness of the pulse, the secondary radiation produced when the rays strike against a photographic plate or pass through air might give rise to what might easily be mistaken for diffraction effects. Rontgen has never succeeded in observing effects which prove the existence of diffraction. Fomm (Wied. Ann. 59, p. 50) observed in the photograph of a narrow slit light and dark bands which looked like diffraction bands; but observation with slits of different sizes showed that they were not of this nature, and Haga and Wind (Wied. Ann. 68, p. 884) have explained them as contrast effects. These observers, however, noticed with a very narrow wedge»shaped slit a broadening of the image of the narrow part which they are satisfied could not be explained by the causes. 'Valter and Pohl (Ann. der Phys. 29, p. 331) could not observe any diffraction effects, though their arrangement would have enabled them to do so if the wave-length had not been smaller than I-5X I0-gcm. Sir George Stokes (Proc. Manchester Lit. and Phil. Soc., 1898) put forward the view that the disturbances which constitute the rays are not regular periodic undulations but very thin pulses. Thomson (Phil. lllag. 45, p. 172) has shown that when charged particles are suddenly stopped, pulses of very intense electric and magnetic disturbances are started. As the cathode rays consist of negatively electrified particles, the impact of these on a solid would give rise to these intense pulses. The electromagnetic theory therefore shows that effects resembling light, inasmuch as they are electromagnetic disturbances propagated through the aether, must be produced when the cathode rays strike against an obstacle. Since under these circumstances Rontgen rays are produced, it seems natural, unless direct evidence to the contrary is obtained, to connect the Rontgen rays with these pulses. This view explains very simply the "polarization " of the rays; for, suppose the cathode particle moving from A to B were stopped at its first impact with the plate B (fig. I), the electric force transmitted along BP would be in the plane ABP at right angles to BP. /Vhen this electric force reached the body at P it would accelerate any electrified particles in that body, the acceleration being parallel to AB. Each of these accelerated particles would start electric waves. The theory of such waves shows that their intensity vanishes along a line through the particle parallel to the direction of acceleration, while it is a maximum at right angles to this line; thus the intensity of the rays along a horizontal line through P would vanish, while it would be a maximum in the plane at right angles to this line. In this case there would be complete polarization. In reality the cathode particle is not stopped at its first encounter, but makes many collisions, changing its direction between each; and these collisions will send out electric disturbances which when they fall on P are able to excite waves which send some energy along PC. The polarization will therefore be only partial and will be of the kind found by Barkla.

The velocity with which the waves 'travel has not yet been definitely set tled. Marx (Ann. der Phys. 20, p. 677) by an ingenious but elaborate method came to the conclusion that they travelled with the velocity of light; his interpretation of his experiments has, however, been criticized by Franck and Pohl (Verh. d. D. Physik Ges. Io, p. 489).,

Another view of the nature of Rontgen rays has been advocated by Bragg (PML Mag- I4, p. 429); he re ards them as neutral electric doublets consisting of a negative anti a positive charge of electricity which are usually held together by the attraction between them, but which may be knocked asunder when the rays strike against matter and turned into cathodic rays. On this view when the rays pass' through a gas only a few of the molecules of the gas are struck by the rays and so we can easily understand why so few of the molecules are ionized. On the ordinary view of an electric wave all the molecules would be affected by the wave when it passed through a gas, and to explain the small fraction ionized we must either suppose that systems sensitive to the Rontgen rays are at any time present only in a very small fraction of the molecule or else that the front of an electric or light wave is not continuous but that the energy is concentrated in patches which only occupy a fraction of the wave front.

Apparatus for producing Rontgen Rays.-The tube now used most frequently for producing Rontgen rays is of the kind introduced by Porter and known as a focus tube (fig. 2). The cathode is a portion of a hollow sphere,

and the cathode rays come,

to a point on or near a ~

metal plate A, called the - ' *

anti-cathode, connected 4,

with the anode; this plate ', /W ~ — '— ~ ilshthe sollirce og the éaysi a ~7;.:: Q    fl is oug t to e ma e o .|. 'T —,,  ., , "

a very unfusible metal '/

such as platinum or, still '- f~.. 1/

better, tantalum, and kept FIG 2

cool by a water-cooling

arrangement. The anti-cathode is generally set at an angle of 45° to the rays; it is probable that the action of the tube would be improved by putting the anti-cathode at right angles to the cathode rays. The walls of the tube get strongly electrified. This electrification afiects the working of the tube, and the production of rays can often be improved by having an earth-connected piece of tin-foil on the outside of the bulb, and moving it about until the best position is attained. To produce the discharge an induction coil is generally employed with a mercury interrupter. Excellent results have been obtained by using an electrostatic induction machine to produce the current, the emission of rays is more uniform than when an induction coil is used. The rays are emitted pretty uniformly in all directions until the plane of the anti-cathode is approached; in the neighbourhood of this plane there is a rapid falling off in the intensity of the rays. After long use the glass of the bulb often becomes distinctly purple. This is believed to be due to the presence of manganese compounds in the glass. (J. J. T.)

ROOD (O.E. rod, a stick, another form of “ rod, ” O.E. rodd, possibly cognate with Lat. rudis, a staff), properly a rod or pole, and so used as the name of a surface measure of land. The rood varies locally but is generally taken as = 40 square rods, poles or perches; 4 roods=r acre. The term was, however, particularly applied, in- O.E., to a gallows or cross, especially to the Holy Cross on which Christ was crucified, the sense in which the word survives. A crucifix, often accompanied by figures of St John and the Virgin Mary, was usually placed in churches above the screen, hence known as “rood screen ”