Physics for university students (Volume 2) online

but the best results are obtained with tubes exhausted by
a mercurial air-pump to a pressure of about 2 mms. of mer-
cury and permanently sealed. Platinum electrodes are
melted into the glass at the two ends. The celebrity of
the tubes made by Geissler gave to them the name " Geiss-
ler tubes." Some of the patterns are shown in Fig. 197.

The luminous effects are more intense in the narrow
connecting tubes than in the larger bulbs. The cathode
exhibits a bluish or violet
glow, while the light at the
anode is of smaller extent,
but brighter. The colors
given by a gas depend on
its nature. The narrow por-
tions of a tube containing
hydrogen glow with a bril- p . JQ7

Kant crimson. Vapor of

water gives the same color, indicating the dissociation
of the vapor by the discharge. Tubes containing carbon
dioxide emit a pale gray light, but show splendid stratifica-
tions. The glow when examined by the spectroscope gives
the lines characteristic of the gas in the bulb.

Fluorescent materials in Geissler tubes are beautifully
luminous. Uranium glass, and solutions of quinine, ses-
culin, and naphthaline-red in tubes surrounding the ex-
hausted one, are among the best examples of fluorescent
bodies. Kerosene oil also shows marked fluorescence.

The strice or stratifications of the tube consist of portions
of greater luminosity separated by darker intervals. They
originate apparently at the positive and become more
numerous up to a definite point of the exhaustion, after
which they broaden out and diminish in number. J. J.




392 ELECTRICITY AND MAGNETISM.

Thomson has produced strise throughout a tube 50 feet
long, except near the cathode. They present a peculiar
flickering unstable motion, similar to that sometimes
observed during auroral displays. The striae are hotter
than the darker spaces between them.

.\A-

345. Discharges in High Vacua. When the ex-
haustion of a bulb is carried to a millionth of an atmos-
phere, the phenomena of the electric discharge entirely
change character. Such tubes can scarcely be said to con-
duct at all, apparently because of some difficulty which
the discharge encounters at the electrodes, for J. J. Thom-
son has shown that high vacua are good conductors. 1
These tubes have been investigated by Crookes with great
skill, and they are therefore called " Crookes tubes."

When the exhaustion has been
carried to a millionth of an atmos-
phere, the mean free path of the
molecules is increased a million
fold and becomes comparable with
the dimensions of the containing-

198

vessel. The disorderly motions
of the molecules of the residual gas may then be directed
by electrical or thermal means along definite paths.
The characteristic light of a Geissler tube then almost
entirely disappears by the broadening of the dark space
about the cathode till it reaches the opposite wall
of the bulb. The residual electrified gas is projected
entirely across the bulb in radiant streams, and the bom-
barded walls of the tube exhibit remarkable phosphorescent
effects, the color depending on the kind of glass and on

1 Electrician, June 7, 1895.




ELECTROMAGXETK' INDUCTION.



393



the substances, such as diamond, ruby, or various sul-
phides, subjected to this molecular cannonade (Fig 198).

Evidence is abundant that the projected molecules of
the residual gas move in straight lines, except as they are
deflected by a magnet or by mutual repulsion. The dis-
charge in a Geissler tube is acted on by a magnet like a




Fig. 199.



flexible conductor conveying a current ; but the stream of
radiant matter, as Crookes calls it, when once deflected
by a magnet does not recover its former direction of
motion after passing the magnet (Fig. 199).




Fig. 200.



Any obstructions placed in the path of these " cathode
rays " appears to stop them and casts a shadow by protect-
ing the wall of the tube behind it from the bombardment
(Fig. 200). If such obstruction consists of delicately



394



ELECTRICITY AND MAGNETISM.




Fig. 201.



poised vanes, they are set moving by this molecular wind.

If the cathode is made concave, the paths of the molecules
cross at the focus, and glass or even plati-
num may be fused at this point (Fig.
201).

Two parallel streams of such flying
molecules are deflected by a magnet, but
repel each other like charges of the same
sign. Hence their velocity is probably
less than that of light, for at this speed
they would act like two currents; but
their electrostatic repulsion is not offset
by their electrodynamic attraction as par-
allel currents.

346. Cathode Rays. The projection
of electrified molecules of the residual gas from the cathode
plate of a Crookes tube is not the only action going on at
that electrode. Hertz discovered that the emanations or
" rays " from the cathode are not transmitted through mica,
glass, or other transparent substances, but that they do pass
through metallic foil. By means of vacuum tubes with
a small window of aluminium foil at one end, Lenard
demonstrated that the " rays " from the cathode pass
through aluminium into the air, where they retain the
remarkable property of exciting phosphorescence. Ap-
parently these rays can be produced only in a good
vacuum ; but when they have passed through a medium
pervious to them into the air they retain their character-
istic properties. Professor Rontgen, of Wiirzburg, has just
discovered that these cathode rays, or some unknown radia-
tions from the phosphorescent glass, pass through opaque
bodies like wood, paper, hard rubber, aluminium, etc., and



EL E< TttO.V". 1 (f \ T ETIC INDUCTION. 395

tliat they affect a sensitized photographic plate. In this
way it has been found possible to photograph objects
entirely concealed from view, such as the bones of the
living hand or coins in a leather purse. These pictures
are silhouettes or shadows. The unknown rays producing
this effect seem not to be refrangible, and as far as now
known are not reflected. Rontgen says that they originate
at the part of the tube which exhibits bright phosphores-
cence ; if so, they are not the cathode rays of Lenard,
from which they are differentiated in several ways. The
Lenard rays are deflected by a magnet, while the Rontgen
rays are not; the former are quickly quenched in air at
atmospheric pressure, while the latter can be detected at a
distance of two metres from the source ; the former do not
pass through glass, while the latter do. Aluminium is
permeable to both. Rontgen has shown that his unknown
rays will pass through 200 times as thick a sheet of alumin-
ium as of platinum.

It has long been suspected that there are longitudinal as
well as transverse vibrations in the ether ; some physicists
have contended that they must exist. Rontgen is inclined
to ascribe the remarkable phenomenon that he has discov-
ered to such longitudinal disturbances in the ether.

347. The Telephone. - The transmitter and the re-
ceiver for the electric transmission of speech may be
identical instruments, but in practice they are usually
different. The transmission is commonly effected by
having a rounded platinum pin pressed lightly by a deli-
cat c spring against a polished carbon surface and mounted
in contact with an elastic diaphragm. This platinum-
carbon contact forms part of a local electric circuit. The
contact resistance is varied by the vibrations of the dia-



396



ELECTRICITY AND MAGNETISM.



phragm, so that the strength of the current is modified in
accordance with the aereal movements constituting sound
in the neighborhood of the mouthpiece of the instrument.
The current, thus moulded by the voice, passes through
the primary of a small induction coil, while the secondary
pulses are sent to the transmitting line.

The receiver (Fig. 202) consists of a thin iron dia-
phragm D held in close proximity to the pole of a small
electromagnet BB, which in turn is mounted on the end
of a permanent magnet M. The electric pulses coming

through the line actu-
ate the electromagnet
and so vary the mag-
netic field at the pole.
When the current
runs in one direction
the attraction be-
tween the magnet and
the disk is increased; when it flows in the other direction
it is diminished. The disk is thus forced to repeat the
vibrations of the diaphragm in the transmitter, and it
throws the air in contact with it into similar vibrations
and reproduces the sounds.

The receiver may also be used as a transmitter. The
to-and-fro motion of the iron disk, in conforming to the
sound-waves impinging on it, varies the magnetic induc-
tion between it and the pole. A movement of the lines of
force in the field near the end of the magnet is thus
brought about ; and this variation in the magnetic flux
through the coil produces induced currents in it, which
are transmitted to the distant station, where they actuate
the receiver in the manner described.




Fig. 202.



Dl'XAMO.-; AX/} MOTORS.



307



CHAPTER XXV.



DYNAMOS AND MOTORS.

348. Ideal Simple Dynamo. A dynamo is a machine
for converting the energy of mechanical motion into the
energy of an electric current. It is a generator of electro-
motive force, and is based on the principles of electromag-
netic induction discovered by Faraday. It consists of a
system of conductors, called an armature, revolving in




Fig. 203.



a magnetic field in such a way as to vary continuously
the magnetic flux through them.

Suppose a single loop of wire to revolve in a uniform
magnetic field between the poles N8 of a magnet (Fig.
-<))) around a horizontal axis in the direction of the
arrow. The loop of wire in the position in the figure
encloses the maximum magnetic flux. When it has re-
volved through an angle 6 the flux through it will be
reduced to < cos #, where ^> is the maximum ; for the pro-



398 ELECTRICITY AND MAGNETISM.

jection of the loop on the plane perpendicular to the field
varies as the cosine of the angle of displacement from that
plane. After a quarter turn the loop does not enclose
any lines of force ; as it revolves further they thread
through in the opposite direction, and this is equivalent
to a continued diminution of the magnetic flux through
the loop. During the second half-revolution the opposite
changes take place ; when the loop has revolved through
360 it returns to its initial relation to the magnetic field.

The magnetic flux through the loop varies therefore as
the cosine of the angle of displacement 0. During the
first half-revolution a direct current flows around the loop
in the direction of the arrows ; during the second half it is
reversed. The E.M.F. therefore changes sign twice every
revolution. Such a loop, or a coil composed of a number
of parallel turns, generates an alternating electromotive
force.

349. Law of the Electromotive Force. The induced
electromotive force is not equal to the total magnetic flux
through the circuit, but to the rate of change of that flux.
Now the total flux varies as the cosine of the angle de-
fining the position of the loop ; and when the flux is a
maximum, its rate of change is a minimum and conversely.
Hence when 6 is zero or 180 the E.M.F. generated is
zero ; while for the positions 90 and 270 the E.M.F. is a
maximum. The trigonometrical function that is related
in this way to the cosine is the sine. 1 Hence the law of
the variation of the electromotive force, generated by the
revolution of the loop in a uniform magnetic field, is
the same as the variation in the value of the sine of the
angle of position. If, therefore, we plot uniform distances

1 The differential oCthe cosine is minus the sine.



DYNAMOS A XI) MOTORS.



399



along a straight line to represent equal increments of 0,
and erect perpendiculars to denote the values of the cor-
responding sines of 0, the curve connecting the extremities
of the ordinates will be a sine curve. lu Fig. 204 the
heavy line / is the cosine curve, representing the changes




Fig. 204.

in the magnetic flux ; the light line II is the sine curve,
whose ordinates denote the rate of change of the flux, or
the induced E.M.F. Their maximum values differ by
90, or a quarter of a period. When the magnetic flux
decreases through its zero value at B, its rate of change is
greatest and there the E.M.F. is a maximum.

The current in such a loop is an alter-
nating one, having alternately numeri-
cally equal values in opposite directions
through the loop. To make it uni-
directional in the external circuit a two-
part commutator must be used (Fig.
iiO">). The two parts of the split tube,
insulated from each other and mounted
on the shaft, are connected with the two terminals of the
rotating coil. The brushes leading to the external circuit
are so placed that they exchange contacts with the two
commutator segments in passing through the positions
where the current changes its direction through the coil.
The pulses are then all in one direction in the external




Fig. 205.



400 ELECTEICITY AND MAGNETISM.

circuit. Alternate loops of the sine curve are thus re-
versed, so that all of them lie on the same side of the
zero line.

350. The Drum Armature. The modern Drum
Armature for direct currents (Fig. 206) is an evolution
from the shuttle armature with a single coil. If a second
coil be wound around an iron core with its plane at right
angles to the first, it will generate electromotive forces
differing in phase from the first by a quarter of a period.




Fig. 206.

When the two are rectified in the external circuit they
combine to give a fluctuating current of twuce the fre-
quency of either, superposed on a current . of constant
value, so that the resulting current never drops to zero.
By increasing the number of sections of the coils wound
at equal angular distances around the outside of the arma-
ture core, the E.M.F. and current are rendered nearly
constant in the external circuit. The sections are all
joined in series and the junctions between them are con-
nected to the commutator bars, which are insulated from
one another. When the brushes bear on opposite bars, it
will be readily seen that the current has two paths through
the armature ; so that one brush is constantly positive and
the other negative. By this arrangement the potential
difference betAveen the brushes is kept up to the highest
value given by half the coils in series. The brushes must



DYNAMOS AND MOTORS.



401



be placed near that part of the field where the E.M.F. in
any coil passes through zero and reverses.

351. The Field-Magnet. - - The magnetic field in
direct current machines is produced by a large electro-
magnet excited by a cur-
rent from the armature.
The residual magnetism of
the cores is sufficient to
start the induced current ;
and when the entire cur-
rent is carried around the
coils of the field-magnet,
the dynamo is connected in
series. The circuit of a
series dynamo is shown in
Fig. 2M 7. Such a machine
is adapted to furnish con-
stant currents only; it is
employed in arc-lighting.

The field may also be ex-
cited by a shunt winding^
consisting of many turns of wire connected as a shunt to the
external circuit. In Fig. 208 this shunt circuit is shown
connected to the brushes. It is employed on circuits
requiring a constant potential difference between the main
conductors. When the current changes as a result of a
change in the external resistance, the excitation of the
field-magnet remains "nearly the same and the E.M.F.
generated is therefore nearly constant.

A compoundrwovmd dynamo consists of a combination of
shunt and series coils on the field-magnet. It is designed
to maintain the potential difference more nearly constant




Fig. 207.



402



ELECTRICITY AND MAGNETISM.



than is possible with a simple shunt machine. When a
current flows through the armature, there is in conse-
quence of its resistance a loss of potential difference be-
tween the brushes. This
loss occasions a further
loss of voltage by reduc-
ing the exciting current
through the field-magnet.
Hence by carrying the
whole current around the
field-magnet in a series
coil of a few turns, the
increased excitation thus
produced makes up for
the loss of potential in
the armature and main-
tains a constant potential
difference between the
brushes. If the armature
were without resistance,
compounding would not be necessary to keep the potential
difference constant at the brushes, except for the demag-
netizing effect of the armature considered as an electro-
magnet.

An over-compounded machine has enough series turns to
more than compensate for the loss of potential when a
larger current flows through the armature. Hence the
potential difference between the brushes will increase with
an increase of load. The object is to compensate for a
further loss of potential in the mains, so as to maintain
the potential difference constant at some distant centre of
distribution.




Fi:>. 208.



T)Y \AVOS AXD MOTORS. 403

352. The Gramme Ring. The Gramme ring is a
different type of armature. It is a laminated iron ring
wound continuously with a large number of turns of wire,
all coiled in one direction and joined in series. Fig. 209
shows diagrammatically the relation of the several parts
of the machine. The eight coils are wound right-handedly,
and each junction between coils is joined to a commutator
bar. In this figure the upper brush is the positive, and the
current flows from it around the external circuit back to
the lower brush.
When a coil is in
the highest posi-
tion in the fig-
ure, the maxi-
mum flux passes
through it ; as
the ring rotates
the flux through
the coil d e -

creases, and after a quarter revolution all the lines are
taken out ; they then begin to thread through the other
wav. The current through each coil reverses twice dur-
ing each revolution, and there are two circuits through
the armature, exactly as in the drum type. In both cases
the iron, which is used to increase the magnetic flux
through the armature coils, must be laminated by planes
at right angles to the axis of rotation, for the purpose of
preventing induction currents in the iron. These cur-
rents would heat it and waste energy.

353. Reactions in the Field of a Dynamo and a
Motor. An electric motor for direct currents is con-
structed in the same manner as a generator. The study




404



ELECTltlCITY AND MAGNETISM.



of a magnetic field through which a current is passing
throws much light on the interactions between the field
and the armature. Fig. 210 is the field between unlike
poles distorted by a current through the loop of wire which
came up through one hole and went down through the
other. The lines of force are so distorted that some of
them thread through the loop. Now if we conceive this
loop to rotate counter-clockwise around an axis perpen-
dicular to the plane of the paper, then it is clear that




Fig. 210.

mechanical force must be applied to keep up the motion,
because the tension along the lines of force drags the loop
back. The armature therefore turns against the magnetic
forces or torque of the field acting on it. When used as
a generator, the field of the machine is distorted in the
direction of the rotation.

If, on the contrarj% we conceive this loop of wire to
rotate as an armature under the action of the magnetic
stress on it, then the relative density of the lines in differ-
ent parts of the field remains the same and the armature
reverses its motion.

When the machine is used as a generator mechanical



DF.NM.VOS A.\D IfOTORS. 405

power is converted into electrical energy, because the
rotation of the armature is kept up against the internal
magnetic actions in the field. Work is then done on the
machine as a generator. When it is used as a motor
electrical energy is converted into mechanical energy,
because the rotation takes place in the direction of the
magnetic effort between the field and the armature. Work
is then done by the machine as a motor.

354. Direction of Rotation as a Motor. A series
machine when used as a motor runs in the opposite direc-
tion to its motion as a generator. Its rotation will be in
the same direction whether the current goes through it
one way or the other, since it is reversed through the
armature when it is reversed through the field.

A shunt machine runs in the same direction as a motor
and as a generator. If in Fig. 208 the current from an
external source enters by the lower brush, as in the figure,
its direction through the armature remains unchanged ;
but it goes through the field coils in the opposite direction
to the arrows, and the armature and the field are now in
parallel with reference to the external source ; when used
as a generator the external circuit and the field are in
parallel with respect to the armature as the source of
the electric pressure. The field is therefore reversed.
But as a motor the machine runs with the magnetic torque,
and as a generator (njain*t it; so that running with the
torque when the field only is reversed is the same as run-
ning against it before the field is reversed. ^ It is clear
then that the shunt machine runs in the same direction
whether it is used as a motor or a generator.

The same is true of a compound-wound machine so long
as the ampere-turns of the shunt coil overbalance those of



406



ELECTRICITY AND MAGNETISM.



the series ; the two coils act against each other when the
machine runs as a motor.

355. Counter Electromotive Force in a Motor.
The armature of an electric motor revolves in a magnetic
field and generates an E.M.F. A little consideration will
show that this E.M.F. must be an opposing one tending to
reduce the current through it. In Fig. 211 a generator
and a motor are connected together. The direction of





Fig. 211.

rotation in the two machines is the same. The direction
of the electromotive forces generated in both armatures
is shown by the arrows. They are toward the lower
brush in both, because both armatures revolve in the
same direction in similar fields. But in the generator
the current runs in the same direction as the E.M.F.
generated in its armature, while in the motor it runs
against this generated E.M.F. Its own E.M.F. therefore
opposes the current.

If the motor is provided with a fly-wheel to keep up its
speed when the current from the generator is cut off, a
voltmeter placed across its terminals, as V in the figure,
will show only a slightly diminished E.M.F. immediately
after the circuit is broken, if there is no loajd on the motor



OYNAMOS A XI) MOTORS. 407

to produce a quick slackening of the speed. The volt-
meter shows no reversal of the current when the generator
is cut off. This fact shows that the positive brush of the
generator is connected to the positive of the motor, or that
the E.M.F. of the motor is a back E.M.F. The voltmeter
may be replaced by an incandescent lamp ; it will glow
nearly as brightly for a few seconds directly after the main
circuit is opened as before.

356. "Work done by a Motor. We have seen in
Articles 235 and 320 that the work done against an
opposing E.M.F. is measured electrically by the product
of this E.M.F. and the current. Now the total work done
on the motor is the product of the E.M.F. applied to its
terminals and the current, or EL The difference between
the two, El E'l, is the energy converted into heat
(236). With an electricalh r perfect motor, therefore, the
work done by it is the difference between the whole energy
applied to it and the waste in heat, or the work done
against its counter E.M.F., E'L

The two factors of the power, measured median idaVly,
are the torque and the speed. The torque is the moment
of the couple producing the rotation ;. it is proportional
both to the strength of the field and 'the current in the
armature. If the field is kept constant, the torque is
proportional to the current, and the E.M.F. to the speed.
Hence \vc may write

IE'=AnT,

where T is the torque, n the number of rotations per
second, and A a constant.

When the motor is working under a fixed load, an
increase of the field increases the torque and therefore
decreases the speed n ; weakening the field on the other



408 ELECTRICITY AND MAGNETISM.

hand diminishes the torque and increases the speed. Both
these conclusions follow from the constancy of the product
nT under the assumed condition of a fixed load. In both
cases the speed changes till the counter E.M.F. acquires
the same value that it had before the change was made in
the field.

357. Electrical Efficiency of a Motor. If IF and W