IONS AND ELECTRONS
Early Theories and Laws of Electricity.
The fundamental phenomena of electricity, which were first made the
subject of careful study about two centuries ago, are that certain
substances can be electrified by friction so that somehow they can
attract light bodies, and that the charges of electricity may be
either “positive” or “negative.” Bodies with like charges repel each
other, while those with unlike charges attract each other, and either
partially or entirely neutralize each other when they are brought
close together. Moreover, it had long ago been discovered that in some
substances electricity can move freely from place to place, while in
others there is resistance to the movement. The former bodies are now
called _conductors_ and include metals, while the latter are
called _insulators_, glass, porcelain and air being members of
this class.
In order to explain the phenomena some imagined that there were two
kinds of “electric substances” or “fluids”; and since no change in
weight could be discovered in a body when it was electrified, it was,
in general, assumed that the electric fluids were weightless. In the
normal, neutral body it was believed that these fluids were mixed in
equal quantities, thereby neutralizing each other; on this account they
were supposed to be of opposite characteristics, so one was called
positive and the other negative. According to a second theory, there
was assumed to be just one kind of electricity, which was present in a
normal amount in neutral bodies; positive electricity was caused by a
superfluity of the fluid; negative, by a deficit. In both theories it
was possible to talk of the amount of positive or negative electricity
which a body contained or with which it was “charged,” because the
supporters of the one-fluid idea understood by the terms positive and
negative a superfluity and a deficit, respectively, of the one fluid.
In both theories it was possible to talk about the _direction_
of the electric current in a conductor, since the supporters of the
two-fluid theory understood by “direction” that in which the electric
forces sent the positive electricity, or the opposite to that in which
the negative would be sent. It could not be decided whether positive
electricity went in the one direction or the negative in the other,
or whether each simultaneously moved in its own direction. Both
theories were quite arbitrary in designating the electric charge in
glass, which was rubbed with woollen cloth, as positive. On the whole,
neither theory seemed to have any essential advantage over the other;
the difference between them seemed to lie more in phraseology than in
actual fact.
That the positive and negative states of electricity could not be
taken as “symmetric” seemed, however, to follow from the so-called
discharge phenomena, in which electricity, with the emission of light,
streams out into the air from strongly charged (positive or negative)
bodies, or passes through the air between positive and negative bodies
in sparks, electric arcs or in some other way. In a discharge in air
between a metal point and a metal plate, for instance, a bush-shaped
glow is seen to extend from the point when the charge there is
positive, while only a little star appears when the charge is negative.
Naturally, we cannot discuss here the many electric phenomena and laws,
and must be satisfied with a brief description of those which are of
importance in the atomic theory.
In this latter category belongs _Coulomb’s Law_, formulated about
1785. According to this law, the repulsions or attractions between
two electrically charged bodies are directly as the product of the
charges and inversely as the square of the distance between them (as in
the case of the gravitational attraction between two neutral bodies,
according to Newton’s Law). The unit in measuring electric charges can
be taken as that amount which will repel an equal amount of electricity
of the same kind at unit distance with unit force. If we use the
scientific or “absolute” system, in which the unit of length is one
centimetre, that of time one second and that of mass one gram, then
the unit of force is _one dyne_, which is a little greater than
the earth’s attraction on a milligram weight. Let us suppose that two
small bodies with equal charges of positive (or negative) electricity
are at a distance of one centimetre from each other. If they repel each
other with a charge of one dyne, then the amount of electricity with
which each is charged is called the _absolute electrostatic unit of
electricity_. If one body has a charge three times as great and
the other has a charge four times as great, the repulsion is 3 × 4 =
12 times greater. If the distance between the bodies is increased from
one to five, the repulsion is twenty-five times as small, since 5² =
25. If the charge of one body is substituted by a negative one of same
magnitude the repulsion becomes an attraction of the same magnitude.
In the early part of the nineteenth century methods were found for
producing a steady _electric current_ in metal wires. In 1820, the
Danish physicist, H. C. Ørsted, discovered that an electric current
influences a magnet in a characteristic way, and that, conversely,
the current is affected by the forces emanating from the magnet, by
a magnetic field in other words. The French scientist, Ampère, soon
afterwards formulated exact laws for the _electromagnetic_ forces
between magnets and currents. In 1831, the English physicist, Faraday,
discovered that an electric current is _induced_ in a wire when
currents or magnets in its neighbourhood are moved or change strength.
Faraday’s views on electric and magnetic fields of force around
currents and magnets were further of fundamental importance to the
_electromagnetic wave theory_ as developed by Maxwell. The branch
of physics dealing with all these phenomena is now generally known as
_electrodynamics_.
[Illustration: FIG. 14.—Picture of electrolysis of hydrogen chloride.
_A_, anode; _K_, cathode; _H_, hydrogen atoms; _Cl_, chlorine atoms.]
Electrolysis.
Faraday also studied the chemical effects which an electric current
produces upon being conducted between two metal plates, called
_electrodes_, which are immersed in a solution of salts or acids.
The current separates the salt or acid into two parts which are carried
by the electric forces in two opposite directions. This separation is
called electrolysis. If the liquid is dilute hydrochloric acid (HCl),
the hydrogen goes with the current to the negative electrode, the
_cathode_, and takes the positive electricity with it, while the
chlorine goes against the current and takes the negative electricity to
the positive electrode, the _anode_. We must then assume with the
Swedish scientist, Arrhenius, that, under the influence of the water,
the molecules of hydrogen chloride always are separated into positive
hydrogen atoms and negative chlorine atoms, and that the electric
forces from the anode and the cathode carry these atoms respectively
with and against the current. The electrically charged wandering atoms
are called _ions_, _i.e._ wanderers. The positive electricity
taken by the hydrogen atoms to the cathode goes into the metal
conductor, while the anode must receive from the metal conductor an
equal amount of positive electricity to be given to the chlorine atoms
to neutralize them. The negative charge of a chlorine atom must then be
as large as the positive charge of a hydrogen atom. These assumptions
imply that equal numbers of the two kinds of atoms are present in the
whole quantity of atoms transferred in any period of time.
Faraday found that the quantity of hydrogen which in the above
experiment is transferred to the cathode in a given time is
proportional to the quantity of electricity transferred in the same
time. A gram of hydrogen always takes the same amount of electricity
with it. By experiment this amount of electricity can be determined,
and, since the weight in grams of the hydrogen atom is known, it is
possible to calculate the amount of one atom. In electrostatic units
it is 4·77 × 10⁻¹⁰, _i.e._, 477 billionth[1] parts. A chlorine atom
then carries with it 4·77 × 10⁻¹⁰ electrostatic units of negative
electricity. Since its atomic weight is 35·5, then 35·5 grams of
chlorine will take as much electricity as 1 gram of hydrogen. The ratio
_e_/_m_ between the charge _e_ and the mass _m_ is then 35·5 times as
great for hydrogen as for chlorine.
[1] Billion used here to mean one million million, and trillion to mean
one million billion.
We have temporarily restricted ourselves to the electrolysis of
hydrogen chloride. Let us now assume that we have chloride of zinc
(ZnCl₂), which, by electrolysis, is separated into chlorine and zinc.
Each atom of chlorine will, as before, carry 4·77 × 10⁻¹⁰ units of
negative electricity to the anode; but since zinc is divalent (cf. p.
17) and one atom of zinc is joined to two of chlorine, therefore one
atom of zinc must carry a charge of 2 × 4·77 × 10⁻¹⁰ units of positive
electricity to the cathode. An atom or a group of atoms, with valence
of three, in electrolysis carries 3 × 4·77 × 10⁻¹⁰ units, etc.
We see then, that the quantity of electricity which accompanies the
atoms in electrolysis is always 4·77 × 10⁻¹⁰ electrostatic units or an
integral multiple thereof. This suggests the thought that electricity
is atomic and that the quantity 4·77 × 10⁻¹⁰ units is the smallest
amount of electricity which can exist independently, _i.e._, the
_elementary quantum of electricity_ or the “atom of electricity.”
The atom of a monovalent element, when charged or ionized, should
have one atom of electricity; a divalent, two, etc. On the two-fluid
theory it was most reasonable to assume that there were two kinds
of atoms of electricity representing, respectively, positive and
negative electricity. In Fig. 15 there is given, in accordance with the
two-fluid theory, a rough picture of a chlorine ion and a hydrogen ion
and their union into a molecule.
[Illustration: FIG. 15.—Provisional representation (according
to the two-fluid theory) of
A, a hydrogen ion; B, a chlorine ion; and C, a molecule of hydrogen
chloride.]
The atoms of electricity seemed to differ essentially from the
usual atoms of the elements in their apparent inability to live
independently; they seemed to exist only in connection with the atoms
of the elements. They would seem much more real if they could exist
independently. That such existence really is possible, has been
discovered by the study of the motion of electricity in gases.
Vacuum Tube Phenomena.
[Illustration: FIG. 16.—Vacuum tube with cathode rays and a
shadow-producing cross.
P and N, conducting wires for the electric current; _a_, cathode; _b_,
anode and shadow-producer; _c_, _d_, the shadow.]
[Illustration: FIG. 17.—Vacuum tube, where a bundle of cathode rays are
deviated by electric forces.
_A_, anode; _K_, cathode.]
It has previously been said that air is an insulator for electricity,
a statement which is, in general, true; however, as has also been
said, electric sparks and arcs can pass through air. Moreover, it has
been discovered that exhausted air is a very good conductor, so that a
strong current can pass between two metal electrodes in a glass tube
where the air is exhausted, if the electrodes are connected to an outer
conductor by metal wires fused into the glass. In these vacuum tubes
there are produced remarkable light effects, at first inexplicable.
When the air is very much exhausted, to a hundred thousandth of the
atmospheric pressure or less, strong electric forces (large difference
of potential between the electrodes) are needed to produce an electric
discharge. Such a discharge assumes an entirely new character; in the
interior of the glass tube there is hardly any light to be seen, but
the glass wall opposite the negative electrode (the cathode) glows
with a greenish tint (fluorescence). If a small metal plate is put in
the tube between the cathode and the glass wall, a shadow is cast on
the wall, just as if light were produced by rays, emitted from the
cathode at right angles to its surface (cf. Fig. 16). The English
physicist, Crookes, was one of the first to study these cathode rays.
He assumed that they are not ether waves like the light rays, but
that they consist of particles which are hurled from the cathode
with great velocity in straight lines; they light the wall by their
collisions with it. There was soon no doubt as to the correctness of
Crookes’ theory. The cathode rays are evidently particles of negative
electricity, which by repulsions are driven from the cathode (the
negative electrode). A metal plate bombarded by the rays becomes
charged negatively. Let us suppose that we have a small bundle of
cathode rays, obtained by passing the rays from the cathode _K_
(cf. Fig. 17) through two narrow openings _S₁_ and _S_. It
can then be shown that the bundle of rays is deviated not only by
electric forces, but also by magnetic action from a magnet which is
held near the glass. In the figure there is shown a deviation of the
kind mentioned, caused by making the plates at _B_ and _C_
respectively positive and negative; since _B_ attracts the
negative particles and _C_ repels them, the light spot produced by
the bundle of rays is moved from _M_ to _M_₁. The magnetic
deviation is in agreement with Ørsted’s rules for the reciprocal
actions between currents and magnets, if we consider the bundle of
rays produced by moving electric particles as an electric current.
(Since the electric particles travelling in the direction of the rays
are negative, and since it is customary by the expression “direction
of current” to understand the direction opposite to that in which the
negative electricity moves, then, in the case of the cathode rays just
mentioned, the direction of the current must be opposite to that of the
rays.)
From measurements of the magnetic and electric deviations it is
possible to find not only the velocity of the particles, but also the
ratio _e_/_m_ between the charge _e_ of the particle and its mass _m_.
The velocity varies with the potential at the cathode, and may be very
great, 50,000 km. per second, for instance (about one-sixth the speed
of light), or more. It has been found that _e_/_m_ always has the same
value, regardless of the metal of the cathode and of the gas in the
tube; this means that the particles are not atoms of the elements, but
something quite new. It has also been found that _e_/_m_ is about two
thousand times as large as the ratio between the charge and the mass of
the hydrogen atom in electrolysis. If we now assume that _e_ is just
the elementary quantum of electricity 4·77 × 10¹⁰, which in magnitude
amounts to the charge of the hydrogen atom in electrolysis (but is
negative), then _m_ must have about ¹/₂₀₀₀ the mass of the hydrogen
atom. This assumption as to the size of _e_ has been justified by
experiments of more direct nature. The experiments with charge and mass
of electrons which have in particular been carried out by the English
physicist, J. J. Thomson, give reason then to suppose these quite new
and unknown particles to be free atoms of negative electricity; they
have been given the name of _electrons_. Gradually more information
about them has been acquired. Thus it has been possible in various ways
to determine directly the charge on the electron, independently of its
mass. Special mention must be made of the brilliant investigations
of the American, Millikan, on the motion of very small electrified
oil-drops through air under the influence of an electric force. To
Millikan is due the above-mentioned value of _e_, which is accurate to
one part in five hundred. Further, the mass of the electron has been
more exactly calculated as about ¹/₁₈₃₅ that of the hydrogen atom.
Their magnitude has also been learned; the radius of the electron is
estimated as 1·5 × 10⁻¹³ cm. or 1·5 × 10⁻⁶ μμ, an order of magnitude
one ten-thousandth that of the molecule or atom.
After the atom of negative electricity had been isolated, in the form
of cathode rays, the next suggestion was that corresponding positive
electric particles might be discharged from the anode in a vacuum tube.
By special methods success has been attained in showing and studying
rays of positive particles. In order to separate them from the negative
cathode ray particles the German scientist, Goldstein, let the positive
particles pass through canals in the cathode; they are therefore
called _canal rays_. The velocity of the particles is much less
than that of the cathode rays, and the ratio _e_/_m_ between
charge and mass is much smaller and varies according to the gas in the
tube. In experiments where the tube contains hydrogen, rays are always
found for which _e_/_m_, as in electrolysis, is about ¹/₂₀₀₀
of the ratio in the cathode rays. Therefore there can be scarcely any
doubt that these canal rays are made up of charged hydrogen atoms
or hydrogen ions. The values found with other gases indicate that
the particles are atoms (or molecules sometimes) of the elements in
question, with charges one or more times the elementary quantum of
electricity (4·77 × 10⁻¹⁰ electrostatic units). Research in this field
has also been due in particular to J. J. Thomson. From his results, as
well as from those obtained by other methods, it follows that positive
electricity, unlike negative, cannot appear of its own accord, but is
inextricably connected to the atoms of the elements.
The Nature of Electricity.
The earlier conceptions of a one or two-fluid explanation of the
phenomena of electricity appear now in a new light. We are led to think
of a neutral atom as consisting of one mass charged with positive
electricity together with as many electrons negatively charged as are
sufficient to neutralize the positive. If the atom loses one, two or
three electrons, it becomes positive with a charge of one, two or
three elementary quanta of electricity, or for the sake of simplicity
and brevity we say that the atom has one, two or three “charges.”
If, on the other hand, it takes up one, two or three extra electrons
it has one, two or three negative charges. Fig. 18 can give help
in understanding these ideas, but it must not be thought that the
electrons are arranged in the way indicated. The substances, which
appear as electropositive in electrolysis—_i.e._ hydrogen and
metals—should then be such that their atoms easily lose one or more
electrons, while the electronegative elements should, on the other
hand, easily take up extra electrons. Elements should be monovalent or
divalent according as their atoms are apt to lose or to take up one or
two electrons. From investigations with the vacuum tube it appears,
however, that the atoms of the same element can in this respect behave
in more ways than would be expected from electrolysis or chemical
valence.
[Illustration: FIG. 18.—Provisional representation (according
to the electron theory) of
A, a neutral atom; B, the same atom with two positive charges (a
divalent positive ion) and C, the same atom with two negative charges
(a divalent negative ion).]
When an electric current passes through a metal wire, it must be
assumed that the atoms of the metal remain in place, while the
electrical forces carry the electrons in a direction opposite to that
which usually is considered as the direction of the current (cf. p.
70). The motion of the electrons must not be supposed to proceed
without hindrance, but rather as the result of a complicated interplay,
by no means completely understood, whereby the electrons are freed from
and caught by the atoms and travel backwards and forwards, in such a
way that through every section of the metal wire a surplus of electrons
is steadily passing in the direction opposite to the so-called
direction of the current. The number of surplus electrons which in
every second passes through a section of the thin metal wire in an
ordinary twenty-five candle incandescent light, at 220 volts, amounts
to about one trillion (10¹⁸), or 1000 million (10⁹) in 0·000,000,001 of
a second. If the metal conducting wire ends in the cathode of a vacuum
tube, the electrons carried through the wire pass freely into the tube
as cathode rays from the cathode.
This motion of electricity agrees best with the one-fluid theory,
since the electrons, which here alone accomplish the passage of the
electricity, may be considered as the fundamental parts of electricity.
In this respect the choice of the terms positive and negative is very
unfortunate, since a body with a negative charge actually has a surplus
of electrons. Moreover, the electrons really have mass; but since
the mass of a single electron is only ¹/₁₈₃₅ that of the atom of the
lightest element, hydrogen, and since in an electrified body which can
be weighed by scale there is always but an infinitesimal number of
charged atoms, it is easy to understand that, formerly, electricity
seemed to be without weight.
In electrolysis, where the motion of electricity is accomplished by
positive and negative ions, we have a closer connection with the
two-fluid theory. In motions of electricity through air the situation
suggests both the one-fluid and the two-fluid theories, since the
passage of electricity is sometimes carried on exclusively by the
electrons, and sometimes partly by them and partly by larger positive
and negative ions, _i.e._, atoms or molecules with positive and
negative charges.
The Electron Theory.
Proceeding on the assumption that the electric and optical properties
of the elements are determined by the activity of the electric
particles, the Dutch physicist Lorentz and the English physicist
Larmor succeeded in formulating an extraordinarily comprehensive
“electron theory,” by which the electrodynamic laws for the variations
in state of the ether were adapted to the doctrine of ions and
electrons. This Lorentz theory must be recognized as one of the finest
and most significant results of nineteenth century physical research.
It was one of the most suggestive problems of this theory to account
for the emission of light waves from the atom. From the previously
described electromagnetic theory of light (cf. p. 42) it follows that
an electron oscillating in an atom will emit light waves in the ether,
and that the frequency ν of these waves will naturally be equal to
the number of oscillations of the electron in a second. If this last
quantity is designated as ω, then
ν = ω
It may then be supposed that the electrons in the undisturbed atom
are in a state of rest, comparable with that of a ball in the bottom
of a bowl. When the atom in some way is “shaken,” one or more of the
electrons in the atom begins to oscillate with a definite frequency,
just as the ball might roll back and forth in the bowl if the bowl was
shaken. This means that the atom is emitting light waves, which, for
each individual electron have a definite wave-length corresponding
to the frequency of the oscillations, and that, in the spectrum of
the emitted light, the observed spectral lines correspond to these
wave-lengths.
Strong support for this view was afforded by Zeeman’s discovery of the
influence of a magnetic field upon spectral lines. Zeeman, a Dutch
physicist, discovered, about twenty-five years ago, that when a glowing
vacuum tube is placed between the poles of a strong electromagnet,
the spectral lines in the emitted light are split so that each line
is divided into three components with very little distance between
them. It was one of the great triumphs of the electron theory that
Lorentz was able to show that such an effect was to be expected if
it was assumed that the oscillations of light were produced by small
oscillating electric particles within the atom. From the experiments
and from the known laws concerning the reciprocal actions of a magnet
and an electric current (here the moving particle), the theory enabled
Lorentz to find not only the ratio _e_/_m_ between the electric charge
of each of these particles and its mass, but also the nature of the
charge. He could conclude from Zeeman’s experiment that the charge
is negative and that the ratio _e_/_m_ is the same as that found for
the cathode rays. After this there could not well be doubt that the
electrons in the atoms were the origin of the light which gives the
lines of the spectrum. It seemed, however, quite unfeasible for the
theory to explain the details in a spectrum—to derive, for instance,
Balmer’s formula, or to show why hydrogen has these lines, copper
those, etc. These difficulties, combined with the great number of
lines in the different spectra, seemed to mean that there were many
electrons in an atom and that the structure of an atom was exceedingly
complicated.
Ionization by X-rays and Rays from Radium. Radioactivity.
As has been said, the electrons in a vacuum tube cause its wall to emit
a greenish light when they strike it. Upon meeting the glass wall or a
piece of metal (the anticathode) placed in the tube the electrons cause
also the emission of the peculiar, penetrating rays called Röntgen rays
in honour of their discoverer, or more commonly X-rays. They may be
described as ultra-violet rays with exceedingly small wave-lengths (cf.
p. 54). When, further, the electrons meet gas molecules in the tube
they break them to pieces, separating them into positive and negative
ions (_ionization_). The positive ions are the ones which appear in the
canal rays. The ions set in motion by electrical forces can break other
gas molecules to pieces, thus assisting in the ionization process.
At the same time the gas molecules and atoms are made to produce
disturbances in the ether, and thus to cause the light phenomena which
arise in a tube which is not too strongly exhausted.
The free air can be ionized in various ways; this ionization can be
detected because the air becomes more or less conducting. In fact,
electric forces will drive the positive and negative ions through the
air in opposite directions, thus giving rise to an electric current.
If the ionization process is not steadily continued, the air gradually
loses its conductivity, since the positive and negative ions recombine
into neutral atoms or molecules. Ionization can be produced by flames,
since the air rising from a flame contains ions. A strong ionization
can also be brought about by X-rays and by ultra-violet rays. In the
higher strata of the atmosphere the ultra-violet rays of the sun
exercise an ionizing influence. Most of all, however, the air is
ionized by rays from the so-called _radioactive_ substances which in
very small quantities are distributed about the world.
The characteristic radiation from these substances was discovered in
the last decade of the nineteenth century by the French physicist,
Becquerel, and afterwards studied by M. and Mme. Curie. From the
radioactive uranium mineral, pitchblende, the latter separated the many
times more strongly radioactive element radium. The proper nature of
the rays was later explained, particularly through the investigations
of the English physicists, Rutherford, Soddy and Ramsay. These rays,
which can produce heat effects, photographic effects and ionization,
are of three quite different classes, and accordingly are known as
α-rays, β-rays, and γ-rays. The last named, like the X-rays, are
ultra-violet rays, but they have often even shorter wave-lengths and
a much greater power of penetration than the usual X-rays. The β-rays
are electrons which are ejected with much greater velocity than the
cathode rays; in some cases their velocity goes up to 99·8 per cent.
that of light. The α-rays are positive atomic ions, which move with a
velocity varying according to the emitting radioactive element from
¹/₂₀ to almost ¹/₁₀ that of light. It has further been proved that
the α-particles are atoms of the element helium, which has the atomic
weight 4, and that they possess two positive charges, _i.e._, they
must take up two electrons to produce a neutral helium atom.
There is no doubt that the process which takes place in the emission
of radiation from the radioactive elements is a _transformation of
the element_, an explosion of the atoms accompanied by the emission
either of double-charged helium atoms or of electrons, and the forming
of the atoms of a new element. The energy of the rays is an internal
atomic energy, freed by these transformations. The element uranium,
with the greatest of all known atomic weights (238), passes, by several
intermediate steps, into radium with atomic weight 226; from radium
there comes, after a series of steps, lead, or, in any case, an element
which, in all its chemical properties, behaves like lead. We shall go
no further into this subject, merely remarking that the transformations
are quite independent of the chemical combinations into which the
radioactive elements have entered, and of all external influences.
When α-particles from radium are sent against a screen with a coating
of especially prepared zinc sulphide, on this screen, in the dark,
there can be seen a characteristic light phenomenon, the so-called
scintillation, which consists of many flashes of light. Each individual
flash means that an α-particle, a helium atom, has hit the screen. In
this bombardment by atoms the individual atom-projectiles are made
visible in a manner similar to that in which the individual raindrops
which fall on the surface of a body of water are made visible by the
wave rings which spread from the places where the drops meet the water.
This flash of light was the first effect of the individual atom to
be available for investigation and observation. The incredibility of
anything so small as an atom producing a visible effect is lessened
when, instead of paying attention merely to the small size or mass of
the atom, its kinetic energy is considered; this energy is proportional
to the square of the velocity, which is here of overwhelming magnitude.
For the most rapid α-particles the velocity is 2·26 × 10⁹ cm. per
second; their kinetic energy is then about ⁴/₃₀ of the kinetic
energy of a weight of one milligram of a substance at a velocity of
one centimetre per second. This energy may seem very small, but, at
least, it is not a magnitude of “inconceivable minuteness,” and it is
sufficient under the conditions given above to produce a visible light
effect. We must here also consider the extreme sensitiveness of the eye.
[Illustration: FIG. 19.—Photograph of paths described
by α-particles (positive helium ions) emitted from a radioactive
substance.]
More practical methods of revealing the effects of the individual
α-particles and of counting them are founded on their very strong
ionization power. By strengthening the ionization power of α-particles,
Rutherford and Geiger were able to make the air in a so-called
ionization chamber so good a conductor that an individual α-particle
caused a deflection in an electrical apparatus, an electrometre.
[Illustration: FIG. 20.—Photograph of the path of a β-particle
(an electron).
(Both 19 and 20 are photographs by C. T. R. Wilson.)]
With a more direct method the English scientist C. T. R. Wilson has
shown the paths of the α-particles by making use of the characteristic
property of ions, that in damp air they attract the neutral water
molecules which then form drops of water with the ions as nuclei. In
air which is completely free of dust and ions the water vapour is not
condensed, even if the temperature is decreased so as to give rise to
supersaturation, but as soon as the air is ionized the vapour condenses
into a fog. When Wilson sent α-particles through air, supersaturated
with water vapour, the vapour condensed into small drops on the ions
produced by the particles; the streaks of fog thus obtained could be
photographed. Fig. 19 shows such a photograph of the paths of a number
of atoms. When a streak of fog ends abruptly it does not mean that the
α-particles have suddenly halted, but that their velocity has decreased
so that they can no longer break the molecules of air to pieces,
producing ions. The paths of the β-particles have been photographed
in the same way, although an electron of the β-particles has a mass
about 7000 times smaller than that of a helium atom; the electron has,
however, a far greater velocity than the helium atom. This velocity
causes the ions to be farther apart, so that each drop of water formed
around the individual ions can appear in the photograph by itself (cf.
Fig. 20).