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the comprehensive but still more indefinite designation "mis-

667. Conductivity of Gases. Gases are ordinarily the most
perfect of non-conductors. In the list of these bodies given in
Par. 20, air was placed at the foot. However, under certain con-
ditions described below (Par. 680) their conductivity can be
greatly increased. Although some of these conditions have been
known for upwards of fifty years, it is only within comparatively
recent times that this subject has been systematically investi-
gated, and as a result of these studies much light has been thrown
both upon the mechanism of conduction and upon the ultimate
nature of electricity itself.

668. Discharge Through Moderate Vacua. Fig. 358 represents
the arrangement already described in Par. 525. AB is a long
glass tube into each end of which is sealed a platinum wire ter-
minating on the inside in a small disc. The platinum wires are
connected to the opposite sides of the spark gap GH of an indue-



tion coil. An air pump is attached to a small tube blown in one
side of the larger tube and the air is gradually exhausted. At
first, the sparks produced by the coil leap across the gap GH,
but as the air is exhausted from the tube these sparks cease and
a flickering light, like summer lightning, appears on the inside.



Fig. 358.

If the exhaustion be carried a little farther, or to a pressure
corresponding to about an inch of mercury, a luminous column,
the positive column, extends the entire length of the tube between
the anode and the cathode. The spark gap may now be very
materially decreased without a spark passing, thus showing that
the conducting power of the gas within the tube has been greatly

669. Effect of Magnetic Field on Positive Column. If while
the discharge is taking the form of the positive column the tube
be placed in a crosswise magnetic field, the column is deflected
for apportion of its length. Thus, if a horseshoe magnet be placed
as shown in Fig. 358 so that the tube is penetrated at right angles
by a magnetic field from rear to front, the portion of the column
between the poles of the magnet will be bent upward as indicated
by the dotted lines. Application of the left hand rule (Par. 352)
will show that in this respect the column behaves as if it were a
flexible conductor carrying a current.

670. Discharge Through High Vacua. If the exhaustion of
the tube described in Par. 668 be continued, when the pressure
has been reduced to about that of two millimeters of mercury,
the following changes are observed. The surface of the cathode
is covered with a thin luminous layer. Adjoining this there is a



dark space C (Fig. 359), the Crookes dark space, which enlarges
as the pressure diminishes, and adjoining this space there is a
luminous region D, the negative glow. Beyond this there is a
second dark space F, the Faraday dark space, followed by the
positive column E which is now broken up into striae, transverse

Fig. 359.

luminous discs. A potential sufficient to produce in air a spark
one-eighth ^of an inch in length will now cause a discharge through
a tube twenty inches long. Tubes exhausted to this extent are
called Geissler tubes.

If the exhaustion be carried to about one-millionth of an
atmosphere, the tube is called a Crookes tube. The luminous
spaces entirely disappear, the Crookes dark space spreading
throughout the tube, but the glass itself now begins to phosphor-
esce with a color which varies with its composition. Soda glass
glows with a fine green color; lead glass with a pale blue. The
resistance is now much greater and increases rapidly so that if
the exhaustion be carried slightly farther it becomes no longer
possible to send a discharge through the tube.

671. Cathode Rays. It was discovered by Crookes that the
phosphorescence of the glass tube described in the preceding
paragraph is produced by certain invisible radiations proceeding


Fig. 360.

from the cathode, and these have accordingly been named cathode
rays. It may be shown that they leave the cathode at right
angles to the latter's surface. In the V-shaped Crookes tube
shown in a, Fig. 360, whether A or B be used as the anode, only
B, the arm up which the cathode points, will phosphoresce.


If the cathode be given a concave shape, a piece of platinum
foil placed at the focus may be raised to a red heat by the rays.
Many substances, even when not highly heated, fluoresce or
emit brilliant light when placed in the path of these rays.

672. Nature of Cathode Rays. Investigations lead to the
belief that the cathode rays consist of minute material particles
carrying electrical charges and moving with a velocity so great
that they cause the bodies upon which they strike to emit light
or fluoresce. These particles have been 'variously named cor-
puscles, electrons and negative ions.

In the Crookes tube shown in 6, Fig. 360, a mica cross B is
mounted upon the anode A and when the tube is in operation a
distinct shadow of this cross appears upon the phosphorescent
background at D. B therefore screens the glass from the rays
from C. Since B is transparent, the cathode rays are not of the
nature of ordinary light.

If, instead of the cross, a very delicate little paddle wheel be
mounted at B and so placed that the rays from C strike upon
the vanes of one side only, it will take up a motion of rotation
as if it had been bombarded with small particles from C.

673. Effect of Magnetic Field on Cathode Rays. The cathode
rays are deflected by a magnetic field. In the Crookes tube

Fig. 361.

shown in Fig. 361, a diaphragm B with a narrow slit is placed in
front of the cathode. Beyond this diaphragm and lying along
the axis of the tube is a vertical sheet of mica coated with chalk.
The narrow beam of rays through the slit causes in this chalk
a bright line of fluorescence CD. If now a horseshoe magnet be
placed as shown in Fig. 358, the field running from rear to front,
the beam of rays will be deflected in the same direction as the
positive column (Par. 669). There is, however, a great difference
in the two cases. The positive column is simply deflected as it
passes through the field and beyond this field returns to its original
direction; the cathode rays, after passing beyond the field, continue


in their deflected direction and terminate upon the side of the tube

674. Effect of Electric Field Upon Cathode Rays. Cathode
rays are also deflected by an electric field. Thus, if the tube shown
in Fig. 361 be placed between two parallel metal plates, one above
and the other below, and if the upper plate be charged positively,
the rays will be deflected upward in the direction CE. The con-
clusion is that the corpuscles, or little particles of which the cathode
rays are composed, carry negative charges and are consequently
attracted by the positively charged and repelled by the negatively-
charged plate.

The same conclusion might have been drawn from the deflec-
tion produced in the cathode rays by a magnetic field. Since
these rays, although moving in opposite direction, were deflected
in the same direction as the positive column, they must have
constituted a current of negative electricity.

675. Nature of Charge Carried by Corpuscle. The correctness
of the above conclusion that the corpuscles carry negative charges

Fig. 362.

is experimentally confirmed as follows. In the two-chambered
Crookes tube shown in Fig. 362, B is a metal diaphragm pierced
with a narrow slit and together with A constituting the anode.
C is the cathode. The side tube D contains a metal cylinder with
a narrow opening at F and with a connection at G by which it may
be grounded. Within this cylinder but insulated from it there
is a second cylinder with a terminal at H. An electrometer is
connected to this terminal. Normally, the cathode rays pass
through the slit in the diaphragm and strike at E where they
produce a luminous spot. By means of a magnet these rays are


deflected. The instant that they are bent enough to enter the
opening at F, the electrometer indicates that the inner cylinder
has received a negative charge.

676. Positive Rays. If the cathode of a Crookes tube be
pierced with small holes, luminous rays passing through these
holes will be seen at the back of the cathode. These are found
to consist of positively-charged ions and are accordingly called
positive rays, sometimes also canal rays.

677. Lenard Rays. While the cathode rays do not penetrate
the Crookes tube in which they are produced, Lenard found that
if a small window of aluminum foil be let into the side of the tube,
the effect of the rays could be detected for a distance of several
inches in the air on the outside. Since the fact that these rays
apparently pass through metal appears contrary to the theory
that they consist of small material particles, these exterior rays
were at first considered to be something different and were called
Lenard rays. It is now known that they are identical with cathode
rays. It is thought that the cathode rays on the interior of the
tube do not actually penetrate the aluminum but strike it with
such energy that the percussion drives off ions from the outer

678. X-Rays. In addition to heating the objects upon which
they fall, the cathode rays cause these objects to emit rays of a
very remarkable penetrative power. Rontgen accidently dis-
covered this fact in 1895. He noticed that a covered photographic
plate in his laboratory became fogged by the rays from a Crookes
tube with which he was working. It was known that the effect
of the Lenard rays extended only a few inches beyond the tube
and he realized that he was dealing with an unknown form. He
therefore designated them as X-rays, though later, in his honor,
they were called Rontgen rays.

They travel with the velocity of light, penetrate all bodies to
some extent, are not reflected or refracted and are unaffected by
electric or magnetic fields. Their penetration into the metals
varies inversely as the atomic weights of these metals. Lead,
whose atomic weight is 207, is therefore the metal most frequently
used as a screen for these rays.

They excite powerful phosphorescence in many substances.
Advantage is taken of this in the fluoroscope. This consists of a



light-proof frame shaped like the frustum of a pyramid. Over
the larger end is spread a cardboard coated with barium platino-
cyanide. The smaller end of the frustum is arranged so as to be
applied to an observer's face as shown in Fig. 363. When exposed
to the X-rays, the barium salt glows with a yellowish color and
to the observer the effect is as if he were looking through a frosted
glass window of that color. If the hand be interposed between
the source of the X-rays and the fluoroscope and be applied to
the coated cardboard, the X-rays penetrate the flesh more easily
than they do the bones and the outline or shadow of the bones
is clearly seen. This instrument is used in surgery in the examina-
tion of fractures, location of foreign bodies, etc.

Fig. 363.

X-ray photographs or sciagraphs (shadow pictures) are made
in a similar manner. The sensitive plate enclosed in its holder is
usually placed on a table, the patient placing immediately above
the plate the part of his body to be photographed. Exposure to
the rays is then made and the plate is developed. In this way the
greatest steadiness is secured.

In making these sciagraphs a special form of Crookes tube,
a so-called focusing tube, is used. As shown in Fig. 363, the cathode
is concave and the anode located at its focus is a flat plate of
platinum inclined at an angle of 45 to the cathode rays. The
X-rays thus emanate from a small area and more clear-cut
shadows are produced.

These rays are particularly destructive to cells. They are
therefore used in medicine for the treatment of superficial forms
of cancer, tuberculosis and skin diseases, but they are not selec-
tive and destroy the healthy as well as the diseased, producing
burns which are very difficult to heal.


679. Becquerel Rays. In 1896 Becquerel in investigating the
properties of phosphorescent bodies discovered that the compounds
of the metal uranium emitted rays which partook of the nature
of both the cathode rays and the X-rays. It was soon found that
these Becquerel rays were not confined to uranium compounds
but were produced by other substances. Those bodies which
emit these rays are said to be radio-active.

The principal ore of uranium is the oxide, pitch blende. The
Curies found that the residue left after extracting the uranium
from this ore was more radio-active than the uranium itself, and
irTl898 they succeeded in separating from this residue a compound
of a new element, radium, whose radio-activity was over a mil-
lion times greater than that of uranium. Radium has not yet
been isolated but is known to be a metal chemically allied to
barium. It exists in such minute quantities that from a ton of
the ore only about two-tenths of a gram of the impure chloride
or bromide is obtained. Associated with it are polonium and
actinium, two still rarer metals possessing similar properties.

The Becquerel rays are complex but by passing them through
a magnetic field they may be resolved into three types called
the alpha, the beta and the gamma rays, respectively. The alpha
and the beta rays are deflected, but in opposite directions; the
alpha rays being positive rays, the beta rays being negative or
cathode rays. The gamma rays are unaffected by the magnetic
field and are allied to, if not identical with, the X-rays. They
have almost incredible penetrative power, being able to penetrate
upwards of a foot of solid iron.

680. Increase of Conductivity of Gases. While, as already
stated, the conductivity of a gas is normally very small, there
are many widely different ways in which it may be greatly in-
creased. Thus, a gas becomes a conductor if it be highly heated,
or if it be mixed with gas drawn from the vicinity of glowing
metals or of the electric arc, or if an electric spark be passed
through it, or if it be exposed to any of the cathode, Lenard,
Becquerel or X-rays described above, or if it be exposed to ultra-
violet light, etc., etc. This increase in conductivity is best shown
by means of a gold leaf electroscope. So long as the surrounding
air remains in its normal state, the leaves, if charged, remain
diverging, or if they fall together, do so very slowly. If, how-
ever, the air be rendered conductive, as for example by holding



within a foot or so of the leaves a minute quantity of a radium
salt, the leaves collapse at once. The following experiment
illustrates the production of conductivity by the X-rays. In
Fig. 364, A is an X-ray tube enclosed in a thick box of lead with
a small aperture in the top through which the rays may emerge.
Immediately above this opening there is an inverted funnel F

Fig. 364.

which communicates through a glass tube with the jar B in which
the electroscope is suspended. The X-rays render the air
through which they pass conductive, for if suction be applied to
the tube C so as to draw the air within F over into the jar B, the
leaves collapse as soon as this air enters the jar.

681. lonization of Gases. In the preceding paragraph we saw
how the air within the funnel F (Fig. 364) was rendered conduc-
tive by the action of the X-rays and how it retained this con-
ductivity after it had been drawn over into the jar B. If, however,
there be placed in the tube between F and B a plug of glass wool,
the air from F after passing through this plug will be found to
have lost its conductivity. The same thing happens if this air
be drawn through a metal tube of fine bore, or if it be caused to
bubble through water. Since, therefore, the conductivity of
the air may be thus removed by filtration, it is but natural to
ascribe it to the presence of material particles. Furthermore, the
conductivity is removed if the air be passed between two parallel
metal plates between which a strong electric field is maintained.
These material particles must therefore carry electric charges,
and since the air as a whole shows no sign of a charge, there must
be an equal amount of positive and negative charges present.
The theory was therefore advanced by Thomson that the con-



ductivity of a gas is due to the presence of particles or part mole-
cules, called ions, some positively and others negatively charged.
These negatively-charged particles are found to be identical with
the corpuscles of the cathode ray. The production of these ions,
or ionization, is brought about by any of the agents mentioned
in the preceding paragraph. The explanation advanced is that
the ions or corpuscles associated with these various agencies are
moving with such velocity that when they come into collision
with the molecules of the gas through which they pass, they
break these molecules up into other ions.

682. Investigation of Corpuscles. From the time that the
theory was advanced that the cathode rays consisted of charged
corpuscles moving with high velocity, efforts were directed to
determine the mass of these corpuscles, the charge which they
carry and the velocity with which they move. In the solution
of this problem the work of J. J. Thomson has been especially
noteworthy. We can do no more than give a bare outline of his

His first step was to determine the relation between these
three quantities, and this he did as follows. In the neck of the
two-chambered Crookes tube shown in Fig. 365, B and D are two

Fig. 365.

thick metal diaphragms pierced by an opening about a millimeter
in width. The diaphragm B forms a part of the anode A. The
rays from the cathode C pass in a narrow line through the slits
in B and D and produce a small luminous spot at E on the far
side of the other end of the tube.

Let the mass of each corpuscle be ra grams, its velocity be v
centimeters per second and its charge q electro-magnetic absolute
units (Par. 536). Each moving corpuscle is equivalent to a cur-
rent whose strength is vq absolute units. If the tube be placed in
and at right angles to a uniform magnetic field of intensity H,
each corpuscle will be acted upon by a force vqH at right angles


to its path (Par. 356). Now it is shown in mechanics that if a
body moving with uniform velocity be acted upon by a constant
force at right angles to its path, then the body will move upon the
arc of a circle. The radius of this circle is given by the expression

r = -j->S being the force at right angles to the path. The deflected
corpuscles therefore move upon an arc whose radius is

_ mv 2 mv
~ vqH~^ qH

If the positive direction of the field be from front to rear, the
rays DE will be curved downward to DF. DE, the tangent
to the arc, and EF are measured,
whence the radius of the circle is de-
termined thus:

The triangles CDE and DFE (Fig.
366) being similar


EF'.DE ::DE:2r + EF

whence Fig 366

The intensity of the field being measured, we have H and r, and

the value of becomes known.

683. Velocity of Corpuscles. The velocity v of the moving cor-
puscles was determined by Thomson as follows. Coils were placed
in front and rear of the tube shown in Fig. 365 and a uniform trans-
verse magnetic field H established. If the positive direction of
this field was from front to rear, the rays were deflected downward
by a force vqH.

The parallel metal plates P and N were connected to the ter-
minals of the battery, thus establishing in the tube a vertical
electric field F. If the plate P were positively charged, the rays
would be deflected upward with a force Fq (Par. 674). By vary-
ing either field (generally the magnetic field) they could be so
adjusted that the tendency of one to bend the rays down was


exactly balanced by the tendency of the other to bend the rays
upward. At the instant

vq H Fq

whence v = -77


and knowing F and H, the

velocity v becomes known. This velocity, when the tube is highly
exhausted, is about one-tenth of the velocity of light, and is in-
dependent of the nature of the gas within the tube.

By inserting this value of v in the expression deduced in the
preceding paragraph we obtain the value of m/q, or the ratio of
the mass of the corpuscle to the charge which it carries. More
frequently, the reciprocal of this ratio, or the ratio of q/m is given.
According to the latest determinations it is 1.7x 10 7 . It can be
shown that in ordinary electrolysis the ratio of q/m for the hydrogen
atom is about 10 4 .

684. Mass of Corpuscle. Having thus found the value of q/m,
if either one of these quantities be separately obtained the value
of the other follows at once. The charge q is the one usually
determined directly. The actual process involves many steps
into which we can not go in detail. It is based upon the following
principles. If a volume of saturated vapor be suddenly expanded
its temperature falls and there is a tendency for condensation to
ensue. If in such supersaturated space microscopic particles of
dust be introduced, a fog is produced at once, the particles of dust
facilitating condensation by serving as nuclei upon which the
drops form. Now, if a closed vessel with an aluminum cover be
exposed to certain radiations, such as those from radium salts,
or to X-rays, corpuscles are produced in the gas within the vessel.
These corpuscles act just like the dust in that they serve as nuclei
for drops and in a supersaturated space cause a fog to form at
once. These drops of mist are very minute but may be seen
through a glass and by suitable observations the velocity with
which they slowly settle can be determined. Knowing this veloc-
ity and the density of the gas within the vessel, by the applica-
tion of known formulae the size, and hence the mass, of the drops
can be calculated. The vapor within the vessel contains both
positive and negative ions but it has been found that if the ex-
pansion is not greater than one-quarter of the original volume


that only the negative ions serve as nuclei and are carried down.
Each slowly-falling drop therefore has a corpuscle as a nucleus.
The charge carried by this corpuscle has been determined in
several ways. If the drop falls between two horizontal parallel
metal plates, the lower plate can be given a negative charge so
that its repulsion will counterbalance the force of gravity on the
drop, or may drive it upward. By measuring the upward velocity,
the force exerted upon it can be found, and hence the charge which
it carries.

Within the limits of experimental error, this charge is found to
be the same as the charge carried by the hydrogen atom in ordi-
nary electrolysis. Since we saw in the preceding paragraph that
the ratio of q to m for the corpuscle is 1.7 X 10 7 , and for the hydrogen
atom is 10 4 , and since q is shown to be the same in the two cases,
the mass of the corpuscle is 1700 times less than that of the hydrogen
atom. On the other hand, the mass of the positive ions is found
to agree with the mass of the corresponding atoms.

685. Nature of Corpuscles. Upon the nature of the negative
ions or corpuscles, scientists are not entirely agreed. From what-
ever substance produced, they appear to have the same mass.
Some therefore maintain that they are the true atoms of a uni-
versal single matter and that the fact that the weights of the
majority of the ordinary chemical atoms may be expressed in
whole numbers is simply an expression of a law of multiples
which would follow from these atoms being composed of definite
numbers of corpuscles. It would seem therefore that we had
confirmed the belief of the alchemists that all matter was composed
of a single and ultimate element.

On the other hand, a few deny that they are matter and claim
that they are portions of ether in rapid movement.

Whatever be the nature of the corpuscles themselves, it is quite
certain that the charges which they carry are electric atoms in the
sense that they are all the same and that no smaller charge has
yet been obtained. The atomic character of these electrons has
already been mentioned (Par. 280).



Online LibraryWirt RobinsonThe elements of electricity → online text (page 42 of 46)