section 226.

=188. Effect of variation of E.M.F. on amount of excited activity from thorium.= It has been shown that the excited activity is confined to the cathode in a strong electric field. In weaker fields the activity is divided between the cathode and the walls of the vessel. This was tested in an apparatus shown in Fig. 70.

A is a cylindrical vessel of 5·5 cms. diameter, B the negative electrode passing through insulating ends C, D. For a potential difference of 50 volts, most of the excited activity was deposited on the electrode B. For about 3 volts, half of the total excited activity was produced on the rod B, and half on the walls of the vessel. Whatever the voltage applied, the sum of the activities on the central rod and the walls of the cylinder was found to be a constant when a steady state was reached.

When no voltage was applied, diffusion alone was operative, and in that case about 13 per cent. of the total activity was on the rod B. The application of an electric field has thus no influence on the sum total of excited activity, but merely controls the proportion concentrated on the negative electrode.

A more detailed examination of the variation with strength of field of the amount on the negative electrode was made in a similar manner by F. Henning. He found that in a strong electric field the amount of excited activity was practically independent of the diameter of the rod B, although the diameter varied between ·59 mm. and 6·0 mms. With a small voltage, the amount on the negative electrode varied with its diameter. The curves showing the relation between the amount of excited activity and voltage are very similar in character to those obtained for the variation of the current through an ionized gas with the voltage applied.

The amount of excited activity reaches a maximum when all the active matter is removed from the gas as rapidly as it is formed. With weaker fields, a portion diffuses to the sides of the vessel, and produces excited activity on the positive electrode.

=189. Effect of pressure on distribution of excited activity.= In a strong electric field, the amount of excited activity produced on the cathode is independent of the pressure down to a pressure of about 10 mms. of mercury. In some experiments made by the writer, the emanating thorium compound was placed inside a closed cylinder about 4 cms. in diameter, through which passed an insulated central rod. The central rod was connected to the negative pole of a battery of 50 volts. When the pressure was reduced below 10 mms. of mercury, the amount of excited activity produced on the negative electrode diminished, and was a very small fraction of its original value at a pressure of ⅒ mm. Some excited activity was in this case found to be distributed over the interior surface of the cylinder. It may thus be concluded that at low pressures the excited activity appears on both anode and cathode, even in a strong electric field. The probable explanation of this effect is given in the next section.

Curie and Debierne observed that when a vessel containing an emanating radium compound was kept pumped down to a low pressure, the amount of excited activity produced on the vessel was much reduced. In this case the emanation given off by the radium was removed by the pump with the other gases continuously evolved from the radium compound. On account of the very slow decay of activity of the emanation, the amount of excited activity produced on the walls of the vessel, in the passage of the emanation through it, was only a minute fraction of the amount produced when none of the emanation given off was allowed to escape.

=190. Transmission of excited activity.= The characteristic property of excited radio-activity is that it can be confined to the cathode in a strong electric field. Since the activity is due to a deposit of radio-active matter on the electrified surface, the matter must be transported by positively charged carriers. The experiments of Fehrle showed that the carriers of excited activity travel along the lines of force in an electric field. For example, when a small negatively charged metal plate was placed in the centre of a metal vessel containing an emanating thorium compound, more excited activity was produced on the sides and corners of the plate than at the central part.

A difficulty however arises in connection with the positive charge of the carrier. According to the view developed in section 136 and later in chapters X and XI, the active matter which is deposited on bodies and gives rise to excited activity, is itself derived from the emanation. The emanations of thorium and radium emit only α rays, i.e. positively charged particles. After the expulsion of an α particle, the residue, which is supposed to constitute the primary matter of the active deposit, should retain a negative charge, and be carried to the anode in an electric field. The exact opposite however is observed to be the case. The experimental evidence does not support the view that the positively charged α particles, expelled from the emanation, are directly responsible for the phenomena of excited activity; for no excited activity is produced in a body exposed to the α rays of the emanation, provided the emanation itself does not come in contact with it.

There has been a tendency to attach undue importance to this apparent discrepancy between theory and experiment. The difficulty is not so much to offer a probable explanation of the results as to select from a number of possible causes. While there can be little doubt that the main factor in the disintegration of the atom consists in the expulsion of an α particle carrying a positive charge, a complicated series of processes probably occurs before the residue of the atom is carried to the negative electrode. The experimental evidence suggests that one or more negative electrons of slow velocity escape from the atom at the same time as the particle. This is borne out by the recent discovery that the particle expelled from radium, freed from the ordinary β rays, and also from polonium, is accompanied by a number of slowly moving and consequently easily absorbed electrons. If two negative electrons escaped at the same time as the α particle, the residue would be left with a positive charge and would be carried to the negative electrode. There is also another experimental point which is of importance in this connection. In the absence of an electric field, the carriers remain in the gas for a considerable time and undergo their transformation in situ. There is also some evidence (section 227) that, even in an electric field, the carriers of the active deposit are not swept to the electrode immediately after the break up of the emanation, but remain some time in the gas before they gain a positive charge. It must be remembered that the atoms of the active deposit do not exist as a gas and by the process of diffusion would tend to collect together to form aggregates. These aggregates would act as small metallic particles, and, if they were electro-positive in regard to the gas, would gain a positive charge from the gas.

There can be little doubt that the processes occurring between the break up of the emanation and the deposit of the residue in the cathode in an electric field are complicated, and further careful experiment is required to elucidate the sequence of the phenomena.

Whatever view is taken of the process by which these carriers obtain a positive charge, there can be little doubt that the expulsion of an α particle with great velocity from the atom of the emanation must set the residue in motion. On account of the comparatively large mass of this residue, the velocity acquired will be small compared with that of the expelled α particle, and the moving mass will rapidly be brought to rest at atmospheric pressure by collision with the gas molecules in its path. At low pressures, however, the collisions will be so few that it will not be brought to rest until it strikes the boundaries of the vessel. A strong electric field would have very little effect in controlling the motion of such a heavy mass, unless it has been initially brought to rest by collision with the gas molecules. This would explain why the active matter is not deposited on the cathode at low pressures in an electric field. Some direct evidence of a process of this character, obtained by Debierne on examination of the excited activity produced by actinium, is discussed in section 192.

=191.= The following method has been employed by the writer to determine the velocity of the positive carriers of excited activity of radium and thorium in an electric field. Suppose A and B (Fig. 71) are two parallel plates exposed to the influence of the emanation, which is uniformly distributed between them. If an alternating E.M.F. E₀ is applied between the plates, the same amount of excited activity is produced on each electrode. If, in series with the source of the alternating E.M.F., a battery of E.M.F. E₁ less than E₀ is placed, the positive carrier moves in a stronger electric field in one half alternation than in the other. A carrier consequently moves over unequal distances during the two half alternations, since the velocity of the carrier is proportional to the strength of the electric field in which it moves. The excited activity will in consequence be unequally distributed over the two electrodes. If the frequency of alternation is sufficiently great, only the positive carriers within a certain small distance of one plate can be conveyed to it, and the rest, in the course of several succeeding alternations, are carried to the other plate.

When the plate B is negatively charged, the E.M.F. between the plates is E₀ − E₁, when B is positive the E.M.F. is E₀ + E₁.

Let d = distance between the plates, T = time of a half alternation, ρ = ratio of the excited radio-activity on the plate B to the sum of the radio-activities on the plates A and B, K = velocity of the positive carriers for a potential-gradient of 1 volt per centimetre.

On the assumption that the electric field between the plates is uniform, and that the velocity of the carrier is proportional to the electric field, the velocity of the positive carrier towards B is

E₀ − E₁ ---------- K d

and, in the course of the next half alternation,

E₀ + E₁ ---------- K d

towards the plate A.

If x₁ is less than d, the greatest distances x₁, x₂ passed over by the positive carrier during two succeeding half alternations is thus given by

E₀ − E₁ x₁ = ---------- KT d

and

E₀ + E₁ x₂ = ---------- KT d

Suppose that the positive carriers are produced at a uniform rate of q per second for unit distance between the plates. The number of positive carriers which reach B during a half alternation consists of two parts:

(1) One half of those carriers which are produced within the distance x₁ of the plate B. This number is equal to

1 --- x₁ qT 2

(2) All the carriers which are left within the distance x₁ from B at the end of the previous half alternation. The number of these can readily be shown to be

1 x₁ --- x₁ ---- qT 2 x₂

The remainder of the carriers, produced between A and B during a complete alternation, will reach the other plate A in the course of succeeding alternations, provided no appreciable recombination takes place. This must obviously be the case, since the positive carriers travel further in a half alternation towards A than they return towards B during the next half alternation. The carriers thus move backwards and forwards in the changing electric field, but on the whole move towards the plate A.

The total number of positive carriers produced between the plates during a complete alternation is 2dqT. The ratio ρ of the number which reach B to the total number produced is thus given by

$$ \rho = \frac {\frac {1}{2} x1qT + \frac {1}{2} x1 \frac {x1}{x2} qT} {2dqT} = \frac {1}{4} \frac {x1}{d} \frac {x1 + x2} {x2} $$ .

Substituting the values of x₁ and x₂, we find that

$$ K = \frac {2 (E₀ + E1) d^2} {E₀ (E₀ − E1) T} \rho $$ .

In the experiments, the values of E₀, E₁, d, and T were varied, and the results obtained were in general agreement with the above equation.

The following were the results for thorium:

Plates 1·30 cms. apart.

E₀ + E₀ − Alternations ρ K E₁ E₁ per second

152 101 57 ·27 1·25

225 150 57 ·38 1·17

300 200 57 ·44 1·24

Plates 2 cms. apart.

E₀ + E₀ − Alternations ρ K E₁ E₁ per second

273 207 44 ·37 1·47

300 200 53 ·286 1·45

The average mobility K deduced from a large number of experiments was 1·3 cms. per sec. per volt per cm. for atmospheric pressure and temperature. This velocity is about the same as the velocity of the positive ion produced by Röntgen rays in air, viz. 1·37 cms. per sec. The results obtained with the radium emanation were more uncertain than those for thorium on account of the distribution of some excited activity on the positive electrode. The values of the velocities of the carriers were however found to be roughly the same for radium as for thorium.

These results show that the carriers of the active deposit travel in the gas with about the same velocity as the positive or negative ions produced by the radiations in the gas. This indicates either that the active matter becomes attached to positive ions, or that the active matter itself, acquiring in some way a positive charge, collects a cluster of neutral molecules which travel with it.

=192. Carriers of the excited activity from actinium and “emanium.”= Giesel observed that “emanium” gave off a large quantity of emanation, and that this emanation gave rise to a type of radiation which he termed the E rays. A narrow metal cylinder containing the active substance was placed with the open end downwards, about 5 cms. above the surface of a zinc sulphide screen. The screen was charged negatively to a high potential by an electric machine, and the cylinder connected with earth. A luminous spot of light was observed on the screen, which was brighter at the edge than at the centre. A conductor, connected with earth, brought near the luminous spot apparently repelled it. An insulator did not show such a marked effect. On removal of the active substance, the luminosity of the screen persisted for some time. This was probably due to the excited activity produced on the screen.

The results obtained by Giesel support the view that the carriers of excited activity of “emanium” have a positive charge. In a strong electric field the carriers travel along the lines of force to the cathode, and there cause excited activity on the screen. The movement of the luminous zone on the approach of a conductor is due to the disturbance of the electric field. Debierne found that actinium also gave off a large amount of emanation, the activity of which decayed very rapidly with the time, falling to half value in 3·9 seconds.

This emanation produces excited activity on surrounding objects, and at diminished pressure the emanation produces a uniform distribution of excited activity in the enclosure containing the emanation. The excited activity falls to half value in 41 minutes.

Debierne observed that the distribution of excited activity was altered by a strong magnetic field. The experimental arrangement is shown in Fig. 71A. The active matter was placed at M, and two plates A and B were placed symmetrically with regard to the source. On the application of a strong magnetic field normal to the plane of the paper, the excited activity was unequally distributed between the plates A and B. The results showed that the carriers of excited activity were deviated by a magnetic field in the opposite sense to the cathode rays, i.e. the carriers were positively charged. In some cases, however, the opposite effect was obtained. Debierne considers that the excited activity of actinium is due to “ions activants,” the motion of which is altered by a magnetic field. Other experiments showed that the magnetic field acted on the “ions activants” and not on the emanation.

The results of Debierne thus lead to the conclusion that the carriers of excited activity are derived from the emanation and are projected with considerable velocity. This result supports the view, advanced in