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VII. On a New Secondary Radiation of Positive Rays. By M. WOLFKE, Dr. phil., Lecturer at the Federal Technical High School and at the University of Zurich, Switzerland*.

P to now, two secondary radiations of positive rays

have been known the slow electron rays † and the very soft X-rays recently discovered by J. J. Thomson ‡, which are probably caused by a retardation of the ions when encountering a rigid body.

Some years ago Chadwick § and Russell | showed that a-rays are capable of exciting the y-radiation of heavy

elements.

So far, however, it has not been known whether positive rays were capable of producing a similar effect. Now this question is of very vital interest, for if it can be proved that positive rays are able to excite characteristic rays, it then becomes possible to obtain a better insight into the connexion between the process of excitation and the chemical nature and charge of the exciting particles. And such an investigation might throw new light on the questions relating to the mechanism of excitation and emission of X-spectra.

These considerations have prompted me to investigate the question as to whether positive rays are capable of exciting the characteristic X-radiation.

The experimental method was based upon similar principles to that employed by Chadwick¶ in his investigation of the excitation of the characteristic y-radiation of gold by a-rays.

Through a channel of circular section 10 mm. wide a pencil of positive rays was let fall upon a circular opening provided in a brass box. This opening is divided into two halves, and each of these is covered up by a double foil made up of one foil of a heavy metal, say tin or lead, and of a second foil of a light metal, say aluminium, laid over the first one. These foils are so placed that over one half of the opening the heavy-metal foil is on the outside with the aluminium foil turned towards the inside of the box, while the second similar double foil is so arranged over the other half of the

* Communicated by the Author.

J. J. Thomson, Proc. of Cambr. Phil. Soc. xiii. p. 212 (1905); Ch. Füchtbauer, Phys. Z. S. vii. p. 153 (1906); L. W. Austin, Phys. Rev, xxii. p. 312 (1906).

J. J. Thomson, Phil. Mag. [6] xxviii. p. 620 (1914).

$ J. Chadwick, Phil. Mag. [6] xxiv. p. 594 (1912); xxv. p. 193 (1913). J. Chadwick & A. S. Russell, Proc. Roy. Soc. A. lxxxviii. p. 217 (1913); A. S. Russell & J. Chadwick, Phil. Mag. [6] xxvii. p. 112 (1914).

J. Chadwick, Phil. Mag. [6] xxv. p. 193 (1913).

opening as to have its aluminium side outwards. Behind these foils the photographic plate is placed.

Thus on one half of the opening the positive rays fall upon a heavy-metal surface, and on the other half upon an aluminium surface. The characteristic radiation of the heavy metal being more intense and harder than that of aluminium, it reaches the photographic plate with an intensity not perceptibly diminished. On the other hand, the characteristic radiation of aluminium is weak and soft and is absorbed to a large extent by the layer of heavy metal through which it has to pass. Therefore, if it is true that the characteristic radiation of the heavy metal is excited by positive rays, then the impression produced on the photographic plate must be stronger beneath that half of the opening where the positive rays fall upon heavy metal, and less strong beneath the other half, where they encounter an aluminium foil. The higher the intensity of excitation of the characteristic rays, the more pronounced will be this difference in the strength of the impression obtained.

Such secondary cathode rays as might have been produced by the positive rays and the X-rays in the channel itself were deflected behind the channel, and prevented from entering the opening in the box by means of a field of sufficient power created between channel and box.

In order to eliminate the effect that might have been produced by any irregularity of thickness in the two foils, every test made was checked by a second exposure with the same foils as in the first case, but turned about so as to have their relative position reversed.

In order to avoid too strong heating of the foils by these powerful positive rays, the exposures were intermittent so that short exposures alternated with longer breaks.

Two heavy metals, tin and lead, were treated. The foils used were 016 mm. thick in the case of tin and 028 mm. in the case of lead; the thickness of the aluminium foil was ⚫007 mm.

The experiments have shown that when acted upon by positive rays, either of these metals emitted a penetrating radiation of fair intensity which is probably its characteristic radiation.

When tin was tested, all photographs without exception showed a very marked contrast-that is, where the positive rays fell upon the tin surface, the darkening of the plate was strongly pronounced, while the other half of the circular imprint showed but a faint darkening. The photograph obtained is seen in the annexed figure, and this result was in accordance with anticipation.

Several similar exposures were made with an induction-coil and others with an influence-machine. The duration of exposure was varied between 2.5 and 22 minutes, the potential between 25 and 40 mm. spark-gap, and the pressure between 0007 and 0037 mm. mercury.

The annexed photograph was obtained with an influencemachine and an exposure of 22 minutes. The spark-gap was 25-30 mm. and the pressure 0037 mm. mercury.

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When photographs were taken with lead foil, with potentials ranging between 25 mm. and 40 mm. spark-gap, the darkening of the plate was only faint, and no difference was visible in intensity of the impression on either half of the image. As soon, however, as the spark-gap was increased to 45 mm. a very distinct contrast became visible in the darkening of the two halves.

The intensity of the photograph was greatest on that half where the positive rays fell upon the lead surface, and there only on that part where the intensity of the positive rays would be a maximum.

From this it would seem that the energy required for the excitation of characteristic X-rays has a lower limit, just as Duane, Hunt, Hull, Webster and others have observed in the case of cathode rays in a Coolidge tube.

For cathode rays the relation e>hv has been established, where e is the minimum energy of the electron necessary for exciting the K-series, and the maximum frequency of the line KB, that corresponds to this series. Supposing this relation to hold also for the excitation of characteristic rays by positive particles, then to excite the KB, line of tin (A=432.10-8 cm.) the voltage required would have to exceed 57 KV. In the described experiments, however, the

maximum width of spark-gap used was 45 mm., the spheres being 31 mm. in diameter; so that according to tests made by C. Müller* the voltage applied could not have exceeded 50 KV. The K-series could not then have been excited, and it is probable that the wave-length excited belonged to the L-series of either metal.

I propose to investigate further the wave-lengths and other properties of these radiations. A report of such studies will be published shortly.

Summary of Results.

1. For the first time the excitation of a penetrating radiation by positive rays was observed. This effect was retained on photographic plates in the case of tin and lead, and it is surmised that it is the characteristic X-radiation of these elements.

2. A lower limit was found to exist for the voltage necessary

for excitation.

3. Einstein's quantum condition leads to the supposition that the new effect that has been observed is excitation of the L-characteristic rays of either element.

The Physical Laboratory, Technical High School of Zurich. August 1917.

VIII. Variably-Coupled Vibrations: II. Unequal Masses or Periods. By EDWIN H. BARTON, D.Sc., F.R.S., Professor of Physics, and H. MARY BROWNING, B.Sc., Lecturer and Demonstrator in Physics, University College, Nottingham↑.

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* C. Müller, Ann. d. Phys. [4] xxviii. p. 585 (1910).
† Communicated by the Authors.

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63

69

72

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V. EXPERIMENTAL RESULTS..
Masses 20: 1.
Logarithmic Decrements.
Masses 5: 1.
Length 3: 4.

VI. SUMMARY

I. INTRODUCTION.

IN

N a recent paper* two types of coupled pendulums were experimented with, their lengths and the masses of their bobs being in each case equal. The present paper, the second of the series, deals with the double-cord pendulum only, but in cases where either the masses of the bobs are unequal or else the lengths of their suspensions are unequal.

These mechanical cases may be regarded as somewhat analogous to the electrical cases of inductively-coupled circuits. with unequal inductances or unequal periods respectively.

With unequal masses and equal lengths it is noticeable that with small couplings a great increase in the amplitude of vibration of the small bob entailed very little loss in that of the large bob. Indeed, for masses as 20:1 we almost realised the case of forced vibrations.

The funnel of the light bob was here of cardboard and so had an appreciable damping. This rendered it necessary to make corresponding modifications in the theory.

With unequal lengths and equal masses the response showed a great diminution for small couplings, whereas for larger couplings the mistuning seemed without appreciable effect.

The paper includes twenty-seven photographic reproductions of double sand traces obtained simultaneously one from each bob of the coupled pendulum.

d2y

P dt2

II. THEORY FOR UNEQUAL MASSES.

Equations of Motion and Coupling.--Throughout the work described in the present paper the double-cord pendulum was used. This was shown in figs. 1, 2, and 4 of the first paper. The equations of motion and coupling were given as (27)-(29) and may now be rewritten here as follows:

P+Q+BQ

B

PQ

2,

+ (1+8) (P+Q) P {y = (1+8) · (P+Q) 7.

78

dz2 Q + dt2

and

P+BP+Q

(1 + B) (P + Q) Q & 2

=

B

PQ g

Y

(1+ß) ' (P+Q) 7

{

y2=

B2PQ
(P+Q+ßQ)(P+ßP+Q) '

* Phil. Mag. (6) vol. xxxiv. no. 202, pp. 246–270 (Oct. 1917).

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