where a is the radius of the body, but it increases rapidly as the speed of light is approached. It is very important to settle whether the mass of the electron is due partly to mechanical and partly to electrical mass, or whether it can be explained by virtue of electricity in motion independently of the usual conception of mass.
Slightly different formulae expressing the variation of mass with speed have been developed by J. J. Thomson, Heaviside, and Searle. To interpret his results Kaufmann used a formula developed by M.
Abraham[128].
where
The experimental method employed to determine e/m and u is similar to the method of crossed spectra. Some strongly active radium was placed at the bottom of a brass box. The rays from this passed between two brass plates insulated and about 1·2 mm. apart. These rays fell on a platinum diaphragm, containing a small tube about 0·2 mm. in diameter, which allowed a narrow bundle of rays to pass. The rays then struck a photographic plate enveloped in a thin layer of aluminium.
In the experiments the diaphragm was about 2 cms. from the active material and at the same distance from the photographic plate. When the whole apparatus was placed in a vacuum, a P.D. of from 2000 to 5000 volts could be applied between the plates without a spark. The rays were deflected in their passage through the electric field, and produced what may be termed an electric spectrum on the plate.
Fig. 28.
If a magnetic field is superimposed parallel to the electric field by means of an electromagnet, a magnetic spectrum is obtained perpendicular to the electric spectrum. The combination of the two spectra gives rise to a curved line on the plate. The double trace obtained on the photographic plate with reversal of the magnetic field is shown in Fig. 28. Disregarding some small corrections, it can readily be shown that if y and z are the electric and magnetic deviations respectively,
From these two equations, combined with (1), we obtain
where κ, κ1, κ2 are constants.
Equation (5) gives the curve that should be obtained on the plate according to the electromagnetic theory. This is compared by trial with the actual curve obtained on the plate.
In this way Kaufmann[129] found that the value of e/m decreased with the speed, showing that, assuming the charge constant, the mass of the electron increased with the speed.
The following numbers give some of the preliminary results obtained by this method.
| Velocity of electron | e/m |
|---|---|
| 2·36 × 1010 cms. per sec. | 1·31 × 107 |
| 2·48 „ „ | 1·17 × 107 |
| 2·59 „ „ | 0·97 × 107 |
| 2·72 „ „ | 0·77 × 107 |
| 2·85 „ „ | 0·63 × 107 |
For the cathode rays S. Simon[130] obtained a value for e/m of 1·86 × 107 for an average speed of about 7 × 109 cms. per second.
In a later paper[131] with some very active radium, more satisfactory photographs were obtained, which allowed of accurate measurement. The given equation of the curve was found to agree satisfactorily with experiment.
The table given below, deduced from the results given by Kaufmann, shows the agreement between the theoretical and experimental values, u being the velocity of the electron and V that of light.
The average percentage error between the observed and calculated value is thus not much more than one per cent. It is remarkable how nearly the velocity of the electron has to approach the velocity of light before the value of m/m₀ becomes large. This is shown in the following table which gives the calculated values of m/m₀ for different velocities of the electron.
| Value of u/V | Observed value of m/m₀ | Percentage difference from theoretical values |
|---|---|---|
| Small | 1 | |
| ·732 | 1·34 | -1·5 % |
| ·752 | 1·37 | -0·9 „ |
| ·777 | 1·42 | -0·6 „ |
| ·801 | 1·47 | +0·5 „ |
| ·830 | 1·545 | +0·5 „ |
| ·860 | 1·65 | 0 „ |
| ·883 | 1·73 | +2·8 „ |
| ·933 | 2·05 | -7·8 „ ? |
| ·949 | 2·145 | -1·2 „ |
| ·963 | 2·42 | +0·4 „ |
| Value of u/V | small | ·1 | ·5 | ·9 | ·99 | ·999 | ·9999 | ·999999 |
| Calculated value m/m₀ | 1·00 | 1·015 | 1·12 | 1·81 | 3·28 | 4·96 | 6·68 | 10·1 |
Thus for velocities varying from 0 to ⅒ the velocity of light, the mass of the electron is practically constant. The increase of mass becomes appreciable at about half the velocity of light, and increases steadily as the velocity of light is approached. Theoretically the mass becomes infinite at the velocity of light, but even when the velocity of the electron only differs from that of light by one part in a million, its mass is only 10 times the value for slow speeds.
The above results are therefore in agreement with the view that the mass of the electron is altogether electrical in origin and can be explained purely by electricity in motion. The value of e/m₀, for slow speeds, deduced from the results was 1·84 × 107, which is in very close agreement with the value obtained by Simon for the cathode rays, viz. 1·86 × 107.
If the electricity carried by the electron is supposed to be distributed uniformly over a sphere of radius a, for speeds slow compared with the velocity of light, the apparent mass
Therefore
Taking the value of e as 1·13 × 10-20, a is 1·4 × 10-13 cms.
Thus the diameter of an electron is minute compared with the diameter of an atom.
83. Distribution of velocity amongst the β particles. Some interesting experiments have been recently made by Paschen[132] to determine the relative number of β particles which are expelled from radium at the different speeds. The experimental arrangement is shown in Fig. 29.
Fig. 29.
A small thin silvered glass tube b, containing 15 mgrs. of radium bromide, was placed in the axis of a number of lead vanes arranged round a cylinder of diameter 2 cms. and length 2·2 cms. When no magnetic field was acting, the β particles from the radium passed through the openings and were absorbed in an outer concentric cylinder aa of lead of inner diameter 3·7 cms. and of thickness 5·5 mms. This outer cylinder was rigidly connected to the inner cylinder cc by quartz rods ii, which also served to insulate it. The cylinder c and the radium were connected with earth. A gold-leaf electroscope E was attached to a, and the whole apparatus was enclosed in a glass vessel which was exhausted to a low vacuum by means of a mercury pump. The glass vessel was placed in the uniform field of a large electromagnet, so that the axis of the lead cylinder was parallel to the lines of force.
The outer cylinder gains a negative charge on account of the particles which are absorbed in it. This negative charge, which is indicated by the movement of the gold-leaf, tends to be dissipated by the small ionization produced in the residual gas by the passage of the β rays. This action of the gas can be eliminated by observing the rate of movement of the gold leaf when charged alternately to an initial positive and negative potential. The mean of the two rates is proportional to the number of β particles which give up their charge to the lead cylinder. This is evidently the case, since, when the charge is positive, the ionization of the gas assists the rate of movement of the gold-leaf, and, when negative, diminishes it to an equal extent.
When a magnetic field is applied, each of the particles describes a curved path, whose radius of curvature depends on the velocity of the particle. For weak fields, only the particles of smallest velocity will be deflected sufficiently not to strike the outer cylinder, but, as the field is raised, the number will increase until finally all the β particles fail to reach the outer cylinder. The decrease of the charge communicated to the outer cylinder with the increase of the strength of the magnetic field is shown graphically in Fig. 30, Curve I.
The ordinates represent in arbitrary units the charge communicated to the lead cylinder per second, and thus serve as a measure of the number of β particles which reach the cylinder. Knowing the dimensions of the apparatus, and assuming the value e/m found by Kaufmann, the velocity of the particles which just fail to reach the lead cylinder can be deduced from any strength of the magnetic field. Curve II, Fig. 30 is the first differential of Curve I, and the ordinates represent the relative number of β particles which are projected at each velocity.
Fig. 30.
From the data given by Kaufmann (see section 82) Paschen deduced that the group of rays examined by the former, which had velocities lying between 2·12 × 1010 and 2·90 × 1010 cms. per second, corresponded to the group of rays between the points A and B, that is, to the group of rays which were completely deflected from the lead cylinder between the magnetic fields of strengths of 1875 and 4931 C.G.S. units. Since radium gives off β particles which require a field of strength over 7000 units to deflect them, Paschen concluded that β particles are expelled from radium with still greater velocities than the highest recorded by Kaufmann.
Paschen considered that the small charge observed in still higher fields was mainly due to the γ rays. The effect is small and is probably not due to an actual charge carried by the γ rays but to a secondary effect produced by them. This question will be discussed in more detail in section 112.
There is a group of low velocity β particles emitted by radium (see Fig. 30) which have about the same speed as the electrons set free in a vacuum tube. In consequence of their small velocity, these probably produce a large proportion of the ionization due to the β rays at short distances from the radium, for it will be shown (section 103) that the ionization produced by an electron per unit length of path steadily decreases with increase of its velocity above a small limiting value. This observation is confirmed by experiments on the absorption of the β rays in passing through matter.
In Paschen’s experiments, the glass tube containing the radium was ·5 mms. thick, so that a considerable proportion of the low velocity β particles must have been stopped by it. This is borne out by some later experiments of Seitz which will be described in section 85.
84. Absorption of the β rays by matter. The β particles produce ions in their passage through the gas and their energy of motion is consequently diminished. A similar action takes place also when the β rays pass through solid and liquid media, and the mechanism of absorption is probably similar in all cases. Some of the particles in their passage through matter are completely stopped, while others have their velocity reduced. In addition, there is a considerable scattering or diffuse reflection of the rays in traversing matter. The amount of this scattering depends upon the density of the substance and also upon the angle of incidence of the rays. This scattering of the rays will be discussed later in section 111.
There are two general methods of determining the absorption of the β rays. In the first method, the variation of the ionization current is observed in a testing vessel when the active matter is covered by screens differing in material and thickness. This ionization in the vessel depends upon two quantities, viz. the number of β particles which pass through the matter and also upon the number of ions produced by them per unit path. In the absence of any definite information in regard to the variation of ionization by the electron with its velocity, no very definite conclusions can be drawn from such experiments.
The advent of pure radium-bromide has made it possible to determine the actual number of electrons which are absorbed in their passage through a definite thickness of matter, by measuring the negative charge carried by the issuing rays. Experiments of this character have been made by Seitz and will be considered later.
These two methods of determining the absorption of β rays are quite distinct in principle, and it is not to be expected that the values of the coefficients of absorption obtained in the two cases should be the same. The whole question of the absorption of electrons by matter is very complicated, and the difficulty is still further increased by the complexity of the β rays emitted by the radio-active substances. Many of the results obtained by different methods, while pointing to the same general conclusion, are quantitatively in wide disagreement. Before any definite advance can be made to a better understanding of the mechanism of absorption, it will be necessary to determine the variation of the ionization with the speed of the electron over a very wide range. Some work has already been done in this direction but not between sufficiently wide limits.
Ionization method.
We shall first consider the results obtained on the absorption of β rays by measuring the variation of the ionization current, when screens of different thickness are placed over the active substance. When the active matter is covered with aluminium foil of thickness ·1 mm., the current in a testing vessel such as is shown in Fig. 17, is due almost entirely to the β rays. If a uranium compound is used, it is found that the saturation current decreases with the thickness of matter traversed nearly according to an exponential law. Taking the saturation current as a measure of the intensity of the rays, the intensity I after passing through a thickness d of matter is given by
where λ is the constant of absorption of the rays and I₀ is the initial intensity. For uranium rays, the current is reduced to half its value after passing through about ·5 mm. of aluminium.
If a compound of thorium or radium is examined in the same way, it is found that the current does not decrease regularly according to the above equation. Results of this kind for radium rays have been given by Meyer and Schweidler[133]. The amount of absorption of the rays by a certain thickness of matter decreases with the thickness traversed. This is exactly opposite to what is observed for the α rays. This variation in the absorption is due to the fact that the β rays are made up of rays which vary greatly in penetrating power. The rays from uranium are fairly homogeneous in character, i.e. they consist of rays projected with about the same velocity. The rays from radium and thorium are complex, i.e. they consist of rays projected with a wide range of velocity and consequently with a wide range of penetrating power. The electrical examination of the deviable rays thus leads to the same results as their examination by the photographic method.
Results on the absorption of cathode rays have been given by Lenard[134], who has shown that the absorption of cathode rays is nearly proportional to the density of the absorbing matter, and is independent of its chemical state. If the deviable rays from active bodies are similar to cathode rays, a similar law of absorption is to be expected. Strutt[135], working with radium rays, has determined the law of absorption, and has found it roughly proportional to the density of matter over a range of densities varying from 0·041 for sulphur dioxide to 21·5 for platinum. In the case of mica and cardboard, the values of λ divided by the density were 3·94 and 3·84 respectively, while the value for platinum was 7·34. In order to deduce the absorption coefficient, he assumed that the radiation fell off according to an exponential law with the distance traversed. As the rays from radium are complex, we have seen that this is only approximately the case.
Since the β rays from uranium are fairly homogeneous, and are at the same time penetrating in character, they are more suitable for such a determination than the complex rays of radium. I have in consequence made some experiments with uranium rays to determine the dependence of absorption on the density. The results obtained are given in the following table, where λ is the coefficient of absorption.
| Substance | λ | Density | λ/Density |
|---|---|---|---|
| Glass | 14·0 | 2·45 | 5·7 |
| Mica | 14·2 | 2·78 | 5·1 |
| Ebonite | 6·5 | 1·14 | 5·7 |
| Wood | 2·16 | ·40 | 5·4 |
| Cardboard | 3·7 | ·70 | 5·3 |
| Iron | 44 | 7·8 | 5·6 |
| Aluminium | 14·0 | 2·60 | 5·4 |
| Copper | 60 | 8·6 | 7·0 |
| Silver | 75 | 10·5 | 7·1 |
| Lead | 122 | 11·5 | 10·8 |
| Tin | 96 | 7·3 | 13·2 |
It will be observed that the value of the absorption constant divided by the density is very nearly the same for such different substances as glass, mica, ebonite, wood, iron and aluminium. The divergences from the law are great, however, for the other metals examined, viz. copper, silver, lead and tin. In tin the value of λ divided by the density is 2·5 times its value for iron and aluminium. These differences show that a law for the absorption of the β rays depending only on the density does not hold for all substances. With an exception in the case of tin, the value of λ divided by the density for the metals increases in the same order as their atomic weights.
The absorption of the β rays by matter decreases very rapidly with increase of speed. For example, the absorption of cathode rays in Lenard’s experiment (loc. cit.) is about 500 times as great as for the uranium β rays. The velocity of the β rays of uranium was found by Becquerel to be about 1·6 × 1010 cms. per sec. The velocity of the cathode rays used in Lenard’s experiment was certainly not less than ⅒ of this, so that, for a decrease of speed of less than 10 times, the absorption has increased over 500 times.
85. Number of electrons stopped by matter. An account will now be given of the experiments made by Seitz[136], to determine the relative number of electrons which are stopped in their passage through different thicknesses of matter. The experimental arrangement is shown in Fig. 31.
Fig. 31.
The radium was placed outside a glass vessel containing an insulated brass plate P, the connection of which with a wire leading to the electrometer could be made or broken by a simple electromagnetic device. The β rays from the radium R, after passing through openings in a brass plate A, covered with thin aluminium foil, were absorbed in the plate P. The glass vessel was exhausted, and the charge communicated to P by the β rays was measured by an electrometer.
In a good vacuum, the magnitude of the current observed is a measure of the number of β particles absorbed by the upper plate[137]. The following table shows the results obtained when different thicknesses of tin foil were placed over the radium. The second table gives the ratio I/I₀ where I₀ is the rate of discharge observed before the absorbing screen is introduced. The mean value of the absorption constant λ was deduced from the equation
where d is the thickness of matter traversed.
The values included in the brackets have not the same accuracy as the others. There is thus a wide difference in penetrating power of the β particles emitted from radium, and some of them are very readily absorbed.
When a lead screen 3 mms. thick was placed over the radium—a thickness sufficient to absorb all the readily deflectable β rays—a small negative charge was still given to the plate, corresponding to ·29 per cent. of the maximum. This is a very much smaller value than was observed by Paschen (see Fig. 30).
| Thickness of Tin in mms. | I/I₀ | λ |
|---|---|---|
| 0·00834 | ·869 | 175 |
| 0·0166 | ·802 | 132·5 |
| 0·0421 | ·653 | 101·5 |
| 0·0818 | ·466 | 93·5 |
| 0·124 | ·359 | 82·5 |
| 0·166 | ·289 | 74·9 |
| 0·205 | ·230 | 71·5 |
| 0·270 | ·170 | 65·4 |
| 0·518 | ·065 } | 53} |
| 0·789 | ·031 } | 44} |
| 1·585 | ·0059} | 32} |
| 2·16 | ·0043} | 25} |
This difference may, in part, be due to the fact that, in Paschen’s experiments, a large proportion of the slow velocity electrons were absorbed in the glass tube of ·5 mm. thickness containing the radium.
Seitz also determined the relative thickness, compared with tin, of different substances which reduced the negative charge communicated to P by a definite amount. A few of the numbers are given below, and expressed in terms of tin as unity.
| Substance | Thickness Tin = 1 |
|---|---|
| Lead | ·745 |
| Gold | ·83 |
| Platinum | ·84 |
| Silver | 1 |
| Steel | 1·29 |
| Aluminium | 1·56 |
| Water | 1·66 |
| Paraffin | 1·69 |
The thickness required to stop a given proportion of the β rays thus decreases with the density, but not nearly so fast as the density increases. These results are difficult to reconcile with the density-law of absorption found by Lenard from the cathode rays, or with the results of the ionization method already considered. A further experimental examination of the whole question is very much to be desired.
86. Variation of the amount of radiation with the thickness of the layer of radiating material. The radiations are sent out equally from all portions of the active mass, but the ionization of the gas which is measured is due only to the radiations which escape into the air. The depth from which the radiations can reach the surface depends on the absorption of the radiation by the active matter itself.
Let λ be the absorption constant of the homogeneous radiation by the active material. It can readily be shown that the intensity I of the rays issuing from a layer of active matter, of thickness d, is given by
where I₀ is the intensity at the surface due to a very thick layer.
This equation has been confirmed experimentally by observing the current due to the β rays for different thicknesses of uranium oxide. In this case I = (½)I₀ for a thickness of oxide corresponding to ·11 gr. per sq. cm. This gives a value of λ divided by density of 6·3. This is a value slightly greater than that observed for the absorption of the same rays in aluminium. Such a result shows clearly that the substance which gives rise to the β rays does not absorb them to a much greater extent than does ordinary matter of the same density.
The value of λ will vary, not only for the different active substances, but also for the different compounds of the same substance.
PART III.
The α Rays.
87. The α rays. The magnetic deviation of the β rays was discovered towards the end of 1899, at a comparatively early stage in the history of radio-activity, but three years elapsed before the true character of the α rays was disclosed. It was natural that great prominence should have been given in the early stages of the subject to the β rays, on account of their great penetrating power and marked action in causing phosphorescence in many substances. The α rays were, in comparison, very little studied, and their importance was not generally recognized. It will, however, be shown that the α rays play a far more important part in radio-active processes than the β rays, and that the greater portion of the energy emitted in the form of ionizing radiations is due to them.
88. The nature of the α rays. The nature of the α rays was difficult to determine, for a magnetic field sufficient to cause considerable deviation of the β rays produced no appreciable effect on the α rays. It was suggested by several observers that they were, in reality, secondary rays set up by the β or cathode rays in the active matter from which they were produced. Such a view, however, failed to explain the radio-activity of polonium, which gave out α rays only. Later work also showed that the matter, which gave rise to the β rays from uranium, could be chemically separated from the uranium, while the intensity of the α rays was unaffected. These and other results show that the α and β rays are produced quite independently of one another. The view that they are an easily absorbed type of Röntgen rays fails to explain a characteristic property of the α rays, viz. that the absorption of the rays in a given thickness of matter, determined by the electrical method, increases with the thickness of matter previously traversed. It does not seem probable that such an effect could be produced by a radiation like X rays, but the result is to be expected if the rays consist of projected bodies, which fail to ionize the gas when their velocity is reduced below a certain value. From observations of the relative ionization produced in gases by the α and β rays, Strutt[138] suggested in 1901 that the α rays might consist of positively charged bodies projected with great velocity. Sir William Crookes[139], in 1902, advanced the same hypothesis. From a study of the α rays of polonium Mme. Curie[140] in 1900 suggested the probability that these rays consisted of bodies, projected with great velocity, which lost their energy by passing through matter.
The writer was led independently to the same view by a mass of indirect evidence which received an explanation only on the hypothesis that the rays consisted of matter projected with great velocity. Preliminary experiments with radium of activity 1000 showed that it was very difficult to determine the magnetic deviation of the α rays. When the rays were passed through slits sufficiently narrow to enable a minute deviation of the rays to be detected, the ionizing effect of the issuing rays was too small to be measured with certainty. It was not until radium of activity 19,000 was obtained that it was possible to detect the deviation of these rays in an intense magnetic field. How small the magnetic deviation is may be judged from the fact that the α rays, projected at right angles to a magnetic field of 10,000 C.G.S. units, describe the arc of a circle of about 39 cms. radius, while under the same conditions the cathode rays produced in a vacuum tube would describe a circle of about ·01 cm. radius. It is therefore not surprising that the α rays were for some time thought to be non-deviable in a magnetic field.
89. Magnetic deviation of the α rays. The general method employed[141] to detect the magnetic deviation of the α rays was to allow the rays to pass through narrow slits and to observe whether the rate of discharge of an electroscope, due to the issuing rays, was altered by the application of a strong magnetic field. Fig. 32 shows the general arrangement of the experiment. The rays from a thin layer of radium of activity 19,000 passed upwards through a number of narrow slits G, in parallel, and then through a thin layer of aluminium foil, ·00034 cm. thick, into the testing vessel V. The ionization produced by the rays in the testing vessel was measured by the rate of movement of the leaves of a gold-leaf electroscope B. The gold-leaf system was insulated inside the vessel by a sulphur bead C, and could be charged by means of a movable wire D, which was afterwards earthed. The rate of movement of the gold-leaf was observed through small mica windows in the testing vessel by means of a microscope provided with a micrometer eye-piece.
Fig. 32.
In order to increase the ionization in the testing vessel, the rays passed through 20 to 25 slits of equal width, placed side by side. This was arranged by cutting grooves at regular intervals in side-plates into which brass plates were slipped. The width of the slit varied in different experiments between ·042 cm. and ·1 cm. The magnetic field was applied perpendicular to the plane of the paper, and parallel to the plane of the slits. The rays are thus deflected in a direction perpendicular to the plane of the slits and a very small amount of deviation is sufficient to cause the rays to impinge on the sides of the plate where they are absorbed.
The testing vessel and system of plates were waxed to a lead plate P so that the rays entered the vessel V only through the aluminium foil. It is necessary in these experiments to have a steady stream of gas passing downwards between the plates in order to prevent the diffusion of the emanation from the radium upwards into the testing vessel. The presence in the testing vessel of a small amount of this emanation, which is always given out by radium, would produce great ionization and completely mask the effect to be observed. For this purpose, a steady current of dry electrolytic hydrogen of about 2 c.c. per second was passed into the testing vessel; it then streamed through the porous aluminium foil, and passed between the plates carrying the emanation with it away from the apparatus. The use of a stream of hydrogen instead of air greatly simplifies the experiment, for it increases the ionization current due to the α rays in the testing vessel, and at the same time greatly diminishes that due to the β and γ rays. This is caused by the fact that the α rays are much more readily absorbed in air than in hydrogen, while the rate of production of ions due to the β and γ rays is much less in hydrogen than in air. The intensity of the α rays after passing between the plates is consequently greater when hydrogen is used; and since the rays pass through a sufficient distance of hydrogen in the testing vessel to be largely absorbed, the total amount of ionization produced by them is greater with hydrogen than with air.
The following is an example of an observation on the magnetic deviation:—
| Rate of discharge of electroscope in volts per minute | |
|---|---|
| (1) Without magnetic field | 8·33 |
| (2) With magnetic field | 1·72 |
| (3) Radium covered with thin layer of mica to absorb all α rays | 0·93 |
| (4) Radium covered with mica and magnetic field applied | 0·92 |
The mica plate, ·01 cm. thick, was of sufficient thickness to absorb completely all the α rays, while it allowed the β rays and γ rays to pass through without appreciable absorption. The difference between (1) and (3), 7·40 volts per minute, gives the rate of discharge due to the α rays alone; the difference between (2) and (3), 0·79 volts per minute, that due to the α rays not deviated by the magnetic field employed.
The amount of α rays not deviated by the field is thus about 11% of the total. The small difference between (3) and (4) measures the small ionization due to the β rays, for they would be completely deviated by the magnetic field; (4) comprises the effect of the γ rays together with the natural leak of the electroscope in hydrogen.
In this experiment there was a good deal of stray magnetic field acting on the rays before they reached the pole-pieces. The diminution of the rate of discharge due to the α rays was found to be proportional to the strength of field between the pole-pieces. With a more powerful magnetic field, the whole of the α rays were deviated, showing that they consisted entirely of projected charged particles.
In order to determine the direction of deviation of the rays, the rays were passed through slits one mm. in width, each of which was half covered with a brass strip. The diminution of the rate of discharge in the testing vessel for a given magnetic field in such a case depends upon the direction of the field. In this way it was found that the rays were deviated in the opposite sense to the cathode rays. Since the latter consist of negatively charged particles, the α rays must consist of positively charged particles.
These results were soon after confirmed by Becquerel[142], by the photographic method, which is very well adapted to determine the character of the path of the rays acted on by a magnetic field. The radium was placed in a linear groove cut in a small block of lead. Above this source, at a distance of about 1 centimetre, was placed a metallic screen, formed of two plates, leaving between them a narrow opening parallel to the groove. Above this was placed the photographic plate. The whole apparatus was placed in a strong magnetic field parallel to the groove. The strength of the magnetic field was sufficient to deflect the β rays completely away from the plate. When the plate was parallel to the opening, there was produced on it an impression, due to the α rays alone, which became more and more diffuse as the distance from the opening increased. This distance should not exceed 1 or 2 centimetres on account of the absorption of the rays in air. If, during the exposure, the magnetic field is reversed for equal lengths of time, on developing the plate two images of the α rays are observed which are deflected in opposite directions. This deviation, even in a strong field, is small though quite appreciable and is opposite in sense to the deviation observed for the β or cathodic rays from the same material.
M. Becquerel[143], by the same method, found that the α rays from polonium were deviated in the same direction as the α rays from radium; and thus that they also consist of projected positive bodies. In both cases, the photographic impressions were sharply marked and did not show the same diffusion which always appears in photographs of the β rays.
90. Electrostatic deviation of the α rays. If the rays are charged bodies, they should be deflected in passing through a strong electric field. This was found by the writer to be the case, but the electric deviation is still more difficult to detect than the magnetic deviation, as the intensity of the electric field must of necessity be less than that required to produce a spark in the presence of radium. The apparatus was similar to that employed for the magnetic deviation (Fig. 32) with this exception, that the brass sides which held the plates in position, were replaced by ebonite. Alternate plates were connected together and charged to a high potential by means of a battery of small accumulators. The discharge in the electroscope, due to the α rays, was found to be diminished by application of the electric field. With plates ·055 cm. apart and 4·5 cms. high, the diminution was only 7% with a P.D. of 600 volts between the slits. With a special arrangement of plates, with slits only ·01 cm. apart, the discharge was diminished about 45% with an electric field corresponding to 10,000 volts per cm.
91. Determination of the constants of the rays. If the deviation of the rays in both an electric and magnetic field is known, the values of the velocity of the rays, and the ratio e/m of the charge of the particle to its mass can be determined by the method, first used by J. J. Thomson for the cathode rays, which is described in section 50. From the equations of a moving charged body, the radius of curvature ρ of the path of the rays in a magnetic field of strength H perpendicular to the path of the rays is given by