The American Institute of Electrical Engineers at its last meeting of
the season, held June 25, again considered the subject of electrical
traction, the paper presented by Mr. Leo Daft being based upon some
recent electrical work on the elevated railroads and its bearing on
the rapid transit problem. The _Railroad Gazette_ gives the following
abstract:
He introduced the subject with a tribute to the efficiency of
the elevated railroad system as it is now operated by steam,
with special reference to that section of it known as the Ninth
Avenue line, upon which his experiments with the electric motor
have been conducted, over which passengers are now conveyed a
distance of five miles in 26 minutes for five cents, which he
considered the best and cheapest municipal rapid transit in the
world, and which is operated with a higher degree of safety than
any other railroad in the world making an equal number of stops
per 100 miles. On a recent holiday, April 30 last, 835,720
passengers were carried upon the entire system without
noticeable detention or accident. The rapidly increasing traffic
makes the demand for better facilities a pressing one, and as
the average half million now carried daily will soon become a
million, it appears doubtful if any method can be devised of
providing for the growth by the use of steam motors on the
present structures, which are now taxed to their utmost. To the
mind of the mechanical engineer, having in view the ordinary
coefficients of tractive ability, there is no remedy for this.
The speaker stated that these coefficients were not entirely
trustworthy. He reiterated his previously expressed opinion,
based on frequent experiments, that there is a decided increase
in traction gained by the passage of the electric current from
the wheels to the rails, giving the details of one test where a
motor with a load making a total of 600 lb. climbed a gradient
of 2,900 ft. per mile, starting from a state of rest. He stated
that some of those people who had ridiculed his statements had
finally admitted that they were true.
The motor Ben Franklin, which had been used in making these
tests on the elevated roads, weighed 10 tons, and performed
service nearly equal to the steam motors weighing 18 tons. The
object of these tests was the determination of coal economy.
Tests with a Prony brake showed that the motor developed 128
H.P. The piece of track on which the experiments were conducted
embraced 2,200 ft. of level track and 1-8/10 miles of gradients,
varying from 11-3/10 to 98-7/10 ft. per mile, while at Thirtieth
street the station is at the foot of the steepest grade, thus
testing to the utmost the tractive capacity of the motor. The
experiments were begun in October, 1888, and carried on between
the hours of 9 P.M. and 4 A.M., beginning with one or two cars,
the load being increased nightly until it was finally made up of
eight coaches of 12 tons each, which were hauled up the 98 ft.
grade at a speed of 7½ miles per hour, the entire distance being
covered at the rate of 14-6/10 miles per hour. The maximum speed
obtained on level with that train was 16.36 miles per hour.
Seventy trips were subsequently made with a 70 ton train
operated between the steam trains under 3 minutes headway, but
the work was considered too critical on account of the absence
of suitable brakes. A number of experiments made about this time
showed that the mean speed with a three-car train running
express on the up-town track was about 24 miles per hour,
although the ability of the motor on a level with a similar
train was nearly 28 miles per hour. This, however, was not the
maximum speed, as the level track was not long enough to permit
of its attaining the highest rate. It was the opinion of the
speaker, however, that the speed attained could not be exceeded
with prudence on the elevated structure.
The measurements of speed were made by dividing the track into
19 sections of 500 ft., each section being provided with a
circuit-closing plate connected with a chronograph which was
carefully tested. The indicator cards were taken at the central
station by Mr. Idell and his assistants, and the dynamometer
used was of the liquid type made by Mr. Shaw, of Philadelphia.
The diagrams prepared from the data obtained were then explained
by the speaker, who stated that there was not a marked
difference between the 10 ton motor and the 18 ton locomotive in
the initial effort on the level, as will be seen by comparing a
run observed by a railroad officer on March 9 with a steam motor
and a load of about 57½ tons. The steam motor required 1 min.
and 29 sec. to make the distance from 14th to 23d streets, while
the electric motor with a train of 70 tons made the same trip in
1 min. and 50 sec.; the absence of power brakes compelled the
current to be taken off at 19th street, while it was probable
that the throttle of the steam locomotive was not closed until
it reached 23d street, this being the usual practice. The data
obtained in these experiments shows that 29,940 h.p. is required
to operate the Ninth avenue railroad for the 16 hours' service,
or an average of 1,871 h.p. per hour, or 2,181 h.p., adding
station friction. The varying requirements of the traffic during
the day shows that the service could be advantageously divided
up between four stationary engines of 800 h.p. each, there being
but five hours of the day when all of them would be required.
The fuel consumption per day, allowing 22 lb. of coal per h.p.
per hour at $2.25 per ton, would make a total of $92.25 per diem
for fuel, the coal being a mixture deliverable at the dock for
about $1.80 per ton. The weight of coal used for the present
locomotives is about the same, viz., 40 tons per day, but
practice has shown it to be most economical to use coal of the
best quality, costing $5 per ton, making the cost of fuel about
double that required for the electric system. Without entering
into other economies which the speaker claimed were in favor of
electricity, and ignoring the plan suggested by Sir William
Siemens of braking the train by converting the motor into a
dynamo and thus utilizing the energy of momentum, he believed
that the economy in fuel alone was sufficient to prove that the
application of power by electricity was preferable to direct
steam propulsion for the elevated railroad service.
* * * * *
MAGNETISM IN ITS RELATION TO INDUCED ELECTROMOTIVE FORCE AND
CURRENT.[1]
[Footnote 1: A paper read before the American Institute of
Electrical Engineers, New York, May 22, 1889.]
By ELIHU THOMSON.
There is perhaps no subject which at the present time can have a
greater interest to the physicist, the electrician, and the electrical
engineer than the one which heads this paper. The advances which have
been made in the study from its purely theoretical or scientific side,
and the great technical progress in the utilization of the known facts
and principles concerning magnetic inductions, can but deepen and
strengthen that interest.
On the side of pure theory we find the eager collection of
experimental data to be submitted to the scrutiny of the ablest and
brightest minds, to be examined and reasoned upon with the hope of
finding some clew to satisfying explanations, and on the side of
practice we find the search for new facts and relations no less
diligent, though often stimulated by practical problems presented for
solution. Indeed, the urgency for results is often the greater on the
practical side, for theory can wait, practice cannot, at least in the
United States.
We must look for continued triumphs in both directions, and the most
welcome of all will be the framing of a theory or explanation which
will enable us to interpret magnetic and electric phenomena. The
recent beautiful experiments of Hertz on magnetic waves have opened a
fertile region for investigation.
It would seem that the study of magnetism and electricity will give us
the ability to investigate the ether of space, which medium has been
theorized upon at great length, with the result of leaving it very
much where it was before, a mysterious necessity.
Faraday says, speaking of magnetism:
"Such an action may be a function of the ether, for it is not at
all unlikely that if there be an ether it should have other uses
than simply the conveyance of radiations." 3,075. Vol. III.,
Exp. Res.
"It may be a vibration of the hypothetical ether, or a state of
tension of that ether equivalent to either a dynamic or a static
condition," etc. 3,263. Vol. III., Exp. Res.
Faraday again says, speaking of the magnetic power of a vacuum:
"What that surrounding magnetic medium deprived of all material
substance may be I cannot tell, perhaps the ether." 3,277. Vol.
III., Exp. Res.
Modern views would seem to point that through a study of magnetic
phenomena we may take a feeble hold upon the universal ether.
Magnetism is an action or condition of that medium, and it may be that
electrical actions are the expression of molecular disturbances
brought about by ether strains or interferences. The close relations
which are shown to exist between magnetism and light tend to
strengthen such views. Indeed, it would not be too much to expect that
if the mechanics of the ether are ever worked out, we should find the
relation between sensible heat and electric currents to be as close as
that of light to magnetism, perhaps find ultimately the forms of
matter, the elements and compounds to be the more complex
manifestations of the universal medium--aggregations in stable
equilibrium. It is a difficult conception, I confess, and a most
shadowy and imperfect one, yet facts and inferences which favor such
views are not wanting.
Our science of electricity seems almost to be in the same condition
that chemistry was before the work of Lavoisier had shed its light on
chemical theory. Our store of facts is daily increasing, and
apparently disconnected phenomena are being brought into harmonious
relation. Perhaps the edifice of complete theory will not be more than
begun in our time, perhaps the building process will be a very gradual
one, but I cannot refrain from the conviction that the intelligence of
man will, if it has time, continue its advance until such a structure
exists.
I have been led to make these general allusions to electrical theory
in order to emphasize the fact that in the present paper no unraveling
of the mystery is to be attempted, but rather the presentation of some
few considerations upon a subject of absorbing interest.
The conception of Faraday in regard to the existence of lines of
magnetic force representing directions of magnetic strain or tension
in a medium has not only lost nothing of its usefulness up to the
present time, but has continually been of great service in the
understanding of magnetic phenomena. We need spend no time in showing,
as Faraday and others have done, that these lines are always closed
circuits, polarized so that the direction of the lines cannot be
reversed without reversal of the actions. Nor need we take time to
show that in any medium the lines are mutually repellent laterally if
of the same direction of polarization. Opposing this tendency to
separation or lateral diffusion of magnetic force is the strong
apparent tendency of the lines to shorten themselves in any medium.
These actions are distributed by the presentation of a better medium,
as iron instead of space or air. Lines of force will move into the
better medium, having apparently the constant tendency to diminish the
resistance in their paths.
The peculiar and mysterious nature of media, such as iron, is to
permit an extraordinary crowding of lines on account of slight
resistance to their passage through it. We need not, in addition, do
more than refer to the other well-known facts of an electric current
developing magnetic lines encircling the conductor, as being the
general type, which includes all forms of magnetic field or
electro-magnets, sustained by currents, and the fact of a development
when magnetic lines or circuits and material masses are in relative
movement of electromotive forces transversely to the direction of the
lines of magnetism, and also transversely to the direction of relative
movement, as in the case of electric conductors traversing or cutting
through a field, or of a field traversing or being moved across a
conductor. We must not forget that even insulators, as well as
conductors, cutting lines of force, have the electromotive force
developed in them. The action simply develops potential difference,
and this generates the current where a circuit exists. While we are in
the habit of saying that a conductor moved across a field of lines, or
_vice versa_, generates electric current, I think the statement
incomplete. The movement only sets up a potential difference, and the
power expended in effecting the movement generates C × E. The current
is energy less the potential, or the energy expended gives the two
effects of potential or pressure and current or rate of movement.
Consequently an insulator, or an open-circuited conductor, traversing
a field, consumes no energy, potential difference only being produced.
Nevertheless, as will be shown, the magnetic circuits or lines
themselves may furnish the energy for their own movement across a
conductor, and so develop current as well as potential.
This occurs in the effort of lines to shorten their paths, to lessen
their density, to pass to better media. Indeed, a close examination
will show that wherever power is expended in developing current in a
circuit, cutting lines of force, the energy expended is first employed
in stretching the lines, which thus receive the energy required to
permit them, in shortening, to cut the conductor and set up currents
in the electric circuit in accordance with the potential difference
developed in that circuit and its resistance.
I think we may also say, though I do not remember to have seen the
statement so put, that whenever electric potential is set up
inductively, as in self-induction, mutual induction, induction from
one circuit to another, and induction from magnets or magnetic field,
it is set up by the movement of lines of force laterally across the
body, mass or conductor in which the potential is developed, and that
whenever current is set up in a wire or an existing current prolonged,
or an existing current checked by induction, self-induction, or
induction from magnets, the action is a transfer of energy,
represented by strained lines of force shortening or lessening their
resistance, or lengthening and increasing the resistance in their
paths. The magnetic field is like an elastic spring--it can in one
condition represent stored energy--it can be strained and will store
energy--it can be made to relieve its strain and impart energy.
[Illustration: Fig. 1.]
Let us examine some known phenomena in this light. Take the case of a
simple wire, conveying current, say, in a line away from observer,
Fig. 1. There exists a free field of circular magnetism (so called),
shading off away from the wire, and which is represented by concentric
circles of increased diameter. The superior intensity or strength of
the lines near the wire may also be represented by their thickness.
This is often shown also by crowding the lines near the wire, though I
am disposed to regard Fig. 1 as more nearly expressing the condition,
unless we are to regard the lines as simply indicating a sort of
atmosphere of magnetic effect whose density becomes less as we proceed
outward from the wire, in which case either form of symbol suffices.
The direction of polarization of the lines may be indicated by an
arrow head pointing in a direction of right-handed rotation in the
path of the lines. This is the typical figure or expression for all
forms of simple magnetic circuit--the form of the lines, their length,
position, density, will depend on the shape of the conductor or
conductors (when more than one) and the materials surrounding or in
proximity to the wire or wires.
If the current traversing the conductor is constant, the magnetic
field around it is stable and static, unless other influences come in
to modify it. The cutting off of the current is followed by
instability of the field whereby it can and must produce dynamic
effects. I say _must_ because the field represents stored energy, and
in disappearing _must_ give out that energy. To throw light on this
part of the subject is one of the objects of the present paper.
Cutting off the current supply in the case assumed leaves the
developed magnetic lines or strains unsupported. They at once shorten
their paths or circuits, collapsing upon the conductor as it were, and
continuing this action, cut the section of the conductor, and
apparently disappear in magnetic closed circuits of infinitesimal
diameter but of great strength of polarization. It appears to me that
we must either be prepared to give up the idea of lines of force or
take the position that the magnetic circuits precipitate themselves in
shortening their circuits and disappearing upon and cut the conductor.
It was Hughes who put forward the idea that an iron bar in losing its
apparent magnetism really short-circuits the lines in itself as
innumerable strongly magnetized closed circuits among the molecules.
In becoming magnetic once more these short circuits are opened or
extended into the air by some source of energy applied to strain the
lines, such as a current in a conductor around the bar.
May not this idea be extended, then, to include the magnetic medium,
the ether itself? Does it contain intensely polarized closed circuits
of magnetism which are ready to be stretched or extended under certain
conditions by the application of energy, which energy is returned by
the collapse of the extended circuits? This is doubtless but a crude
expression of the real condition of things, for the lines are only
symbols for a condition of strain in a medium which cannot be
represented in thought, as we know nothing of its real nature. There
is one point in this connection which I must emphasize. The strained
lines, Fig. 1, are indications of stored energy in the ether, and the
lines _cannot_ disappear without giving out that energy. Ordinarily,
it makes its appearance as the extra current, and adds itself so as to
prolong the current which extended the lines when an attempt is made
to cut off such current. Were it conceivable that the current could be
cut off and the wire put on open circuit while the lines still
remained open or strained, the energy must still escape when the field
disappears. It would then produce such a high potential as to be able
to discharge from the ends of the conductor, and if the conductor were
of some section, part of the energy would be expended in setting up
local currents in it. The field could not disappear without an outlet
for the energy it represents. But we cannot cut off a current in a
wire so as to leave the wire on open circuit with the lines of the
magnetic circuit remaining around it without iron or steel or the like
in the magnetic circuit. We can approach that condition, however, by
breaking the circuit very quickly with a condenser of limited capacity
around the break. This is done in the Ruhmkorff coil primary; the
condenser forms a sort of blind alley for the extra current on its
beginning to flow out of the primary coil. But the condenser charges
and backs up and stops the discharge from the primary, even giving a
reverse current. The lines of magnetic force collapse, however, and
have their effect in the enormous potential set up in the secondary
coil.
Take away the secondary coil so as to stop that outlet, the energy
expends itself on the iron core and the primary coil. Take away the
iron core, and the energy of magnetization of the air or ether core
expends itself on the wire of the primary and, possibly, also on the
dielectric of the condenser to some extent. The extra current becomes
in this instance an oscillatory discharge of very high period back and
forth through the primary coil from the condenser, until the energy is
lost in the heat of C2 × R. This conversion is doubtless rendered
all the more rapid by uneven distribution of current and eddy current
set up in the wire of the coil.
The considerations just given concern the loss of field or the
shortening and apparent disappearance of the magnetic lines or
circuits, as giving rise to the self-induction or increased potential
on breaking. Where the energizing current is slowly cut off or
diminished the energy is gradually transferred to the wire in
producing elevation of potential during the decrease; and the collapse
and cutting of the wire by the collapsing circuits or lines is then
only more gradual.
Let the current be returned to the wire after disappearance of
magnetism, and the lines again seem to emanate from the wire and at
the same time cut it and produce a counter potential in it, which is
the index of the abstraction of energy from the circuit, and its
storing up in the form of elastically strained lines of magnetism
around the conductor. The effect is that of self-induction on making
or upon increase of current, the measure of the amount being the
energy stored in the magnetic circuits which have been extended or
opened up by the current. The greater the current and the shorter the
path for the lines developed around the axis of the conductor, the
greater the energy stored up. Hence, a circular section conductor has
the highest self-induction, a tube of same section less as its
diameter increases, a flat strip has less as its width increases and
thickness diminishes, a divided conductor much less than a single
conductor of same shape and section. Separating the strands of a
divided conductor increases the length of magnetic paths around it,
and so diminishes the self-induction. A striking instance of this
latter fact was developed in conveying very heavy alternating currents
of a very low potential a distance of about three feet by copper
conductors, the current being used in electric welding operations.
The conductors were built up of flat thin strips of copper for
flexibility. When the strips were allowed to lie closely together, the
short conductor showed an enormous self-induction, which cut down the
effective potential at its ends near the work. By spreading apart the
strips so as to lengthen a line around the conductor, the
self-induction could be easily made less than 35 per cent. of what it
had been before. The interweaving of the outgoing and return conductor
strands as one compound conductor gets rid almost entirely of the
self-inductive effects, because neither conductor has any free space
in which to develop strong magnetic forces, but is opposed in effect
everywhere by the opposite current in its neighbor.
Where a number of conductors are parallel, and have the same direction
of current, as in a coil or in a strand, it is evident that statically
the conductor may be considered as replaceable by a single conductor
with the same external dimensions and same total current in the area
occupied, the magnetic forces or lines surrounding them being of same
intensity. But with changing current strength the distribution of
current in the conductor has also a powerful effect on the energy
absorbed or given out in accordance with the magnetism produced. Hence
the self-induction of a strand, coil or conductor of the same section
varies with the rapidity of current changes, owing to the conduction
being uneven.
The uneven distribution of current, or its tendency to flow on the
outer parts of a conductor when the rate of variation or alternation
is made great, is in itself a consequence of the fact that less energy
is transferred into magnetism in this case than when the current flows
uniformly over the section, or is concentrated at the center. In other
words, when a uniform current traverses a conductor of the same
section, the circular magnetism, or surrounding magnetic lines, are to
be found not only outside the conductor, but also beneath its
exterior. Since in forming these lines on passage of current the
middle of section would be surrounded by more lines than any other
part of the conductor, the current tends to keep out of that part and
move nearer the exterior in greater amount. Hence, in rapidly
alternating currents the conductor section is practically lessened,
being restricted largely to the outer metal of the conductor. If the
round conductor, Fig. 2, were made of iron, the magnetism interior to
it and set up by a current in it would be very much greater, the
section of the conductor being filled with magnetic circuits or lines
around the center. The total magnetism, external and internal, would
be much greater in this case for a given current flow, and the energy
absorbed and given out in formation and loss of field or the
self-induction would be much increased. This could, however, be
greatly diminished by slitting the conductor radially or making it of
a number of separate wires out of lateral magnetic contact one with
the other, Fig. 3. In these cases the resistance of the interior
magnetic circuits would be increased, as there would be several breaks
in the continuity around the center of the conductor. The total
magnetism which could be set up by a current would be lessened, and
the self-induction, therefore, lessened.
[Illustration: Fig. 2.]
[Illustration: Fig. 3.]
The moment we begin the bringing of iron into proximity with an
electric conductor conveying current, we provide a better medium for
the flow or development of magnetic lines or circuits. In other words,
the lines may then be longer, yet equally intense, or more lines may
be crowded into a section of this metal than in air or space. Figs.
4a, 4b, 4c show the effect brought about by bringing iron of
different forms near to the conductor.
[Illustration: Fig. 4a.]
[Illustration: Fig. 4b.]
[Illustration: Fig. 4c.]
It shows, in other words, the development of the ordinary
electro-magnet of the horseshoe form, and the concentration of the
lines in the better medium. The lines also tend to shorten and
diminish the resistance to their passage, so that attraction of the
iron to the conductor takes place, and if there is more than one piece
of iron, they tend to string themselves around the conductor in
magnetic contact with one another.
When copper bars of 1 inch diameter are traversed by currents of
40,000 to 60,000 amperes, as in welding them, the magnetic forces just
referred to become so enormous that very heavy masses of iron brought
up to the bar are firmly held, even though the current be of an
alternating character, changing direction many times a second.
[Illustration: Fig. 5]
[Illustration: Fig. 6]
When a conductor is surrounded by a cast iron ring, as in Fig. 5, the
current in such conductor has an excellent magnetic medium surrounding
it. A large amount of energy is then abstracted on the first impulse
of current, which goes to develop strong and dense magnetic lines
through the iron ring and across the gap in it. On taking off the
current the energy is returned as extra current, and its force is many
times what would be found with air alone surrounding the conductor. We
have then greatly increased the self-induction, the storing of energy
and opposition to current flow at the beginning, the giving back of
energy and assistance to the current flow on attempting to remove or
stop the current. Let us now complete the ring, by making it of iron,
endless, Fig. 6, with the conductor in the middle.
We now find that on passing current through the conductor it meets
with a very strong opposing effect or counter potential. The evolution
of magnetic lines, or the opening out of magnetic circuits, goes on at
a very rapid rate. Each line or magnetic circuit evolved, and cutting
the conductor, flies at once outward, and locates itself in the iron
ring. This ring can carry innumerable lines, and they do not crowd one
another. It permits the lines even to lengthen in reaching it, and
yet, on account of its low resistance to their passage, the
lengthening is equivalent to their having shortened in other media. We
will suppose the current not sufficient to exhaust this peculiar
capacity for lines which the iron has. Equilibrium is reached, the
conductor has opened up innumerable closed circuits, and caused them
to exist in the ring still closed; but in iron, not space or ether
merely. The current passing has continued its action and storage of
energy until to emit another line in view of the resistance now found
in the crowded iron ring is impossible.
Now let us cut off the current. We are surprised to find a very weak
extra current, a practical absence of self-induction on breaking, or,
at least, a giving out of energy in nowise comparable to that on
making. Let us put on the current as it was before. Another curious
result. But little self-induction now on making energy not absorbed.
Now cut off the current again. Same effect as before. Now let us put
on the current reversed in direction. At once we find a very strong
counter potential or opposing self-induction developed.
The ring had been polarized, or retained its magnetic energy, and we
are now taking out one set of lines and putting in reversely polarized
lines of force. This done, we break the reversed current without much
effect of self-induction. The ring remains polarized and inert until
an opposite flow of current be sent through. Iron is then a different
medium from the ether.
The ring once magnetized must, in losing its magnetism, permit a
closure of the lines by shortening. This involves their passage from
the iron across the space in the center of the ring, notwithstanding
its great resistance to the lines of force. As passage from iron to
air is equivalent to lengthening of the lines, it is readily seen that
such lengthening may oppose more effect than a slight shortening due
to leaving iron, for air or space may give in provoking a closure and
disappearance of the lines. Looked at from another standpoint, the
lines on the iron may actually require a small amount of initial
energy to dislodge them therefrom, so that after being dislodged they
may collapse and yield whatever energy they represent.
I must reserve for the future further consideration of the iron ring,
but in thinking upon this matter I am led to think that the production
of a magnetic line in an iron ring around a conductor may represent a
sort of wave of energy, an absorption of energy on the evolution of
the line from the conductor, and a slight giving out of energy on the
line reaching that position of proximity to the iron ring, that its
passage thereto may be said to be a shortening process or a lessening
of its resistance.
The magnetism in air, gases, and non-magnetic bodies, being assumed to
be that of the ether, this medium shows no such effects as those we
get with the ring. It does not become permanently polarized, as does
even soft iron under the condition of a closed ring. The iron
possesses coercive force, or magnetic rigidity, and a steel ring would
show more of it. The molecules of the iron or steel take a set. If we
were to cut the soft iron ring, or separate it in any way, this
introduction of resistance of air for ether in the magnetic circuit
would cause the lines to collapse and set up a current in the
conductor. The energy of the ring would have been restored to the
latter. The curious thing is that physically the polarized ring does
not present any different appearance or ordinary properties different
from those of a plain ring, and will not deflect a compass needle. Its
condition is discoverable, however, by the test of self-induction to
currents of different direction. As a practical consideration, we may
mention in this connection that a self-inductive coil for currents of
one direction must be constructed differently from one to be used with
alternating currents. The former must have in its magnetic circuit a
section of air or the like, or be an imperfectly closed circuit, as it
were. The latter should have as perfectly closed a magnetic circuit as
can be made. We see here also the futility of constructing a Ruhmkorff
core coil on the closed iron magnetic circuit plan, because the
currents in the primary are interrupted, not reversed.
The considerations just put forward in relation to the closed iron
ring, and its passive character under the condition of becoming
polarized, are more important than at first appears. It has been found
that the secondary current wave of a closed iron circuit induction
coil or transformer, whose primary circuit receives alternating
current, is lagged from its theoretical position of 90 degrees behind
the primary wave an additional 90 degrees, so that the phases of the
two currents are directly opposed; or the secondary current working
lamps only in its circuit is one half a wave length behind a primary,
instead of a quarter wave length, as might have been expected.
But when it is understood that the iron core polarized in one
direction by the primary impulse does not begin to lose its magnetism
when that impulse simply weakens, but waits until an actual reversal
of current has taken place, it will be seen that the secondary
current, which can only be produced when magnetic lines are leaving
the core and cutting the secondary coil, or when the lines are being
evolved and passing into the core from the primary coil, will have a
beginning at the moment the primary reverses, will continue during the
flow of that impulse, and will end at substantially the same time with
the primary impulse, provided the work of the secondary current is not
expended in overcoming self-induction, which would introduce a further
lag. Moreover, the direction of the secondary current will be opposite
to that of the primary, because the magnetic circuits which are opened
up by the primary current in magnetizing the core, or which are closed
or collapsed by it in demagnetizing the core, will always cut the
secondary coil in the direction proper for this result. Transformers
of the straight core type with very soft iron in the cores and not too
high rates of alternation should approximate more nearly the
theoretical relation of primary and secondary waves, because the
magnetic changes in the core are capable of taking place almost
simultaneously with the changes of strength of the primary current.
This fact also has other important practical and theoretical bearings.
Let us assume a plain iron core, Fig. 7, magnetized as indicated, so
that its poles, N, S, complete their magnetic circuits by what is
called free field or lines in space around it. Let a coil of wire be
wound thereon as indicated. Now assume that the magnetism is to be
lost or cease, either suddenly or slowly. An electric potential will
be set up in the coil, and if it has a circuit, work or energy will be
produced or given out in that circuit, and in any other inductively
related to it. Hence the magnetic field represents work or potential
energy. But to develop potential in the wire the lines must cut the
wire. This they can do by collapsing or closing on themselves. The bar
seems, therefore, to lose its magnetism by gaining it all, and in
doing so all the external lines of force moving inward cut the wire.
The magnetic circuits shorten and short-circuit themselves in the bar,
perhaps as innumerable molecular magnetic circuits interior to the
iron medium. To remagnetize the bar we may pass an electric current
through the coil. The small closed circuits are again distended, the
free field appears, and the lines moving outward cut across the wire
coil opposite to the former direction and produce a counter potential
in the wire, and consequent absorption of the energy represented in
the free field produced. As before studied, the magnetism cannot
disappear without giving out the energy it represents, even though the
wire coil be on open circuit, and therefore unable to discharge that
energy. The coil open-circuited is static, not dynamic. In such
assumed case the lines in closing cut the core and heat it. Let us,
however, laminate the core or subdivide it as far as possible, and we
appear to have cut off this escape for the energy. This is not really
so, however. We have simply increased the possible rate of speed of
closure, or movement of the lines, and so have increased for the
divided core the intensity of the actions of magnetic friction and
local currents in the core, the latter still receiving the energy of
the magnetic circuit. This reasoning is based on the possibility in
this case of cutting off the current in the magnetizing coil and
retaining the magnetic field. This is of itself probably impossible
with soft iron. That the core receives the energy when the coil cannot
is shown in the well known fact that in some dynamos with armatures of
bobbins on iron cores, the running of the armature coils on open
circuit gives rise to dangerous heating of the cores, and that under
normal work the heating is less. In the former case the core
accumulates the energy represented in the magnetic changes. In the
latter the external circuit of the machine and its wire coils take the
larger part of the energy which is expended in doing the work in the
circuit. In this case, also, the current in the coils causes a
retardation of the speed of change and extent of change of magnetism
in the iron cores, which keeps down the intensity of the magnetic
reaction. In fact, this retardation or lag and reduction of range of
magnetic change may in some machines be made so great by closing the
circuit of the armature coils themselves or short-circuiting them that
the total heat developed in the cores is much less than under normal
load.
[Illustration: Fig. 7.]
I wish now, in closing, to refer briefly to phenomena of moving lines
of force, and to the effects of speed of movement. In order to
generate a given potential in a length of conductor we have choice of
certain conditions. We can vary the strength of field and we can vary
the velocity. We can use a strong field and slow movement of
conductor, or we can use a weak field and rapid movement of the
conductor. But we find also that where the conductor has large section
it is liable to heat from eddy currents caused by one part of its
section being in a stronger field than another at the same time. One
part cuts the lines where they are dense and the other where they are
not dense, with the result of difference of potential and local
currents which waste energy in heat. We cannot make the conductor move
in a field of uniform density, because it must pass into and out of
the field. The conditions just stated are present in dynamos for heavy
current work, where the speed of cutting of lines is low and the
armature conductor large in section.
But we find that in a transformer secondary we can use very large
section of conductor, even (as in welding machines) 12 to 15 square
inches solid copper, without meeting appreciable difficulty from eddy
currents in it. The magnetic lines certainly cut the heavy conductor
and generate the heavy current and potential needed. What difference,
if any, exists? In the transformer the currents are generated by
magnetic field of very low density, in which the lines are moving
across the conductor with extreme rapidity. The velocity of emanation
of lines around the primary coil is probably near that of light, and
each line passes across the section secondary conductor in a
practically inappreciable time. There is no cause then for differences
of potential at different parts of the section heavy secondary. Then
to avoid eddy currents in large conductors and generate useful
currents in them, we may cause the conductor to be either moved into
and out of a low density field with very great speed, or better, we
must cause the lines of a very low or diffused field to traverse or
cut across the conductor with very high velocity.
It is a known fact that, in dynamos with large section armature
conductors, there are less eddy currents produced in the conductors
when they are provided with iron cores or wound upon iron cores than
when the conductors are made into flat bobbins moved in front of field
poles. Projections existing on the armature between which the
conductors are placed have a like effect, and enable us to employ
heavy bars or bundles of wire without much difficulty from local
currents. The reason is simple. In the armatures with coils without
iron in them, or without projections extending between the turns, the
conductor moves into and out of a very dense field at comparatively
low velocity, so that any differences of potential developed in the
parts of the section of conductor have full effect and abundant time
to act in setting up harmful local currents. In the cases in which
iron projects through the coil or conductor, the real action is that
the lines of the magnetic circuits move at high speeds across the
conductor, and the conductor is at all times in a field of very low
density. Figs. 8 and 9 will make this plain. In Fig. 8 we have shown a
smooth armature surface, having a heavy conductor laid thereon, and
which is at a just entering a dense field at the edge of the pole,
N, and at b leaving such field. It will be seen that when in such
position the conductor, if wide, is subjected to varying field
strength, and moves at a low speed for the generation of the working
potential as it passes through the field, thus giving rise to eddy
currents in the conductor.
[Illustration: Fig. 8.]
In Fig. 9 the conductors are set down between projections, in which
case both armature and field poles are laminated or subdivided. As
each projection leaves the edge of field pole, N, the lines which it
had concentrated on and through it snap backward at an enormous speed,
and cross the gap to the next succeeding projection on the armature,
cutting the whole section of the heavy armature conductor at
practically the same instant. This brisk transfer of lines goes on
from each projection to the succeeding one in front of the field pole,
leaving a very low density of field at any time between the
projections. The best results would be obtained when the armature
conductor does not project beyond or quite fill the depth of groove
between the projections. Of course there are other remedies for the
eddy current difficulty, notably the stranding and twisting of the
conductor on the armatures so as to average the position of the parts
of the compound conductor.
[Illustration: Fig. 9.]
Perhaps the most extreme case of what may be called dilution of field
by projections and by closed magnetic circuits in transformers would
be that of a block of iron, B, Fig. 10, moved between poles, N and S,
and having a hole through it, into and through which a conductor is
carried. The path through the iron is so good that we can scarcely
consider that any lines cross the hole from N to S; yet as B moves
forward there is a continual snapping transfer of lines from the right
forward side of the hole to the left or backward side, cutting the
conductor as they fly across, and developing an electromotive force in
it. I have described this action more in detail because we have in it
whatever distinction in the manner of cutting the lines of the field
is to be found between wire on smooth armatures and on projection
armatures and modifications thereof; and also between flat, open coils
passing through a field and bobbins with cores of iron. The
considerations advanced also bring out the relation which exists
between closed iron circuit transformers and closed iron circuit
(projection) dynamos, as we may call them.
[Illustration: Fig. 10.]
I had intended at the outset of this paper to deal to some extent with
the propagation of lines of magnetism undergoing retardation in
reference to alternating current motor devices, transformers with
limited secondary current, or constant average current, an alternating
motor working with what I may term a translation lag, etc.; but it was
soon found that these matters must remain over for a continuation of
this paper at some future time. My endeavor has been in the present
paper to deal with the lines of force theory as though it were a
symbol of the reality, but I confess that it is done with many
misgivings that I may have carried it too far. Yet, if we are to use
the idea at all it has seemed but right to apply it wherever it may
throw any light on the subject or assist in our understanding of
phenomena.
* * * * *
ELECTRIC LIGHTING AT THE PARIS EXHIBITION--THE OERLIKON
WORKS.
Immediately on entering the Machinery Hall by the _galerie_ leading
from the central dome, and occupying a prominent position at the
commencement of the Swiss section, is a very important plant of
dynamos, motors, and steam engines, put down by the Oerlikon Works, of
Zurich. During the time the machinery is kept running in the hall,
power is supplied electrically to drive the whole of the main shafting
in the Swiss section and part of that in the Belgian section,
amounting in all to some 200 ft., a large number of machines of
various industries deriving their power from these lines of shafting,
while during the evening a portion of the upper and lower galleries
adjoining this section is lit by some twenty-five arc lamps run from
this exhibit. Steam is supplied from the Roser boilers in the motive
power court. The whole of the generating plant is illustrated in one
view, and a separate view is given of the motor employed to drive the
main shafting, this latter view showing the details of connection to
the same. On the extreme right hand side of the first view is a direct
coupled engine and dynamo of 20 horse power, a separate cut of which
is given in Fig. 3. The engine is of the vertical single cylinder
type, standing 5 ft. high, and fitted, as are the other two engines
exhibited, with centrifugal governor gear on the fly wheel, acting
directly on the throw of the cutoff valve eccentric. The two
standards, supporting the cylinder and forming the guide bars,
together with the entire field magnets and pole pieces of the dynamo,
and the bed plate common to both, are cast in one piece.
[Illustration: FIG. 3 ENGINE AND DYNAMO FOR STEAMSHIPS.]
The machine is specially designed for ship lighting, and with the view
of preventing any magnetic effect upon the ship's compass, the field
is arranged so that the armature, pole pieces, and coils are entirely
inclosed by iron. Any tendency to leakage of magnetic lines will
therefore be within the machine, the iron acting as a shield. This
build of field--shown in Fig. 3A--is also advantageous as a mechanical
shield to the parts of the machine most likely to suffer from rough
handling in transport, and it will be seen that the field coils are
easily slipped on before the armature is mounted in its bearings.
[Illustration: FIG. 3A]
The winding is compound, and in such a direction that the two opposite
horizontal poles have the same polarity; it follows from this that
there will be two consequent poles in the iron, these being opposite
in name to the horizontal poles and at right angles to them, viz.,
above and below the armature. Opposite sections of the commutator are
connected together internally as in most four-pole machines, so that
only two brushes are necessary, at 90 deg. apart.
The section of iron in the field is 60 square inches and rectangular
in form, and the whole machine measures 4 ft. 3 in. in length, and 2
ft. in height, without including the height of the bed plate. The
armature is 17 in. in length and the same in diameter, measured over
the winding, and develops at the machine terminals 70 volts and 200
amperes at 480 revolutions. The moving parts of the engine are well
balanced, and run remarkably well and without noise at this high rate
of speed.
This dynamo serves to develop power to run a motor in an adjoining
inclosure, containing some fine specimens of lathes and machine tools
constructed by the Oerlikon Works. These are driven by the motor
through the medium of a countershaft, and the power and speed are
controlled from the switch board seen at the left of the exhibit, and
in Fig. 11. The resistance, R1, serves to vary the intensity of the
shunt field of the dynamo, the volts being indicated by the voltmeter
V1, and a resistance separate from the switch board is inserted in
the main circuit of the two machines. The ammeter, A2, is directly
connected to the dynamo, and therefore indicates the current, whatever
circuit this machine is running.
[Illustration: Figs. 5-9, 11 plus THE PARIS EXHIBITION--STAND OF THE
OERLIKON WORKS.]
A larger combined engine and dynamo, seen in the center of the stand,
serves to run the lighting of the galleries. The engine is a 60 horse
power compound, running at 350 revolutions, and fitted with a governor
on the fly wheel, like that described above.
The dynamo is a two-pole machine, the upper pole and yoke being cast
in one, and the lower pole, yoke, and combined bed plate forming a
separate casting. The two vertical cores, over which the field bobbins
are slipped, are of wrought iron, and are turned with a shoulder at
either end, the yokes being recessed to fit them exactly. The cores
are then bolted to the yokes vertically from the top and horizontally
below. The field of this machine is shunt-wound, and in order to
maintain the potential constant a hand-regulated resistance--R2 on
the switch board--is added in circuit with the shunt field. The
voltmeter, V2, immediately above this resistance, serves to
indicate the difference of potential at the machine terminals. Both
voltmeters are fitted with keys, so that they are only put in circuit
when the readings are taken.
The main terminals of this machine are fitted on substantial
insulating bases, fixed one at each end of the top yoke. These connect
to the external circuit by a heavy cable--the machine being capable of
developing 500 amperes--and to the shunt circuit, and regulating
resistance by small wires; while the two connections to the brushes
are by four covered wires in parallel on each side. This mode of
connection is more flexible than a short length of heavy cable, and
looks well, the wires being held neatly together by vulcanized fiber
bridges. The dynamo is a low tension machine, the field being
regulated to give 65 volts when running the lamp circuits.
[Illustration: Fig. 10.]
The illustration, Fig. 10, represents the automatic
re-regulator--C.E.L. Brown's patent. Motion is imparted to the cores
of two electro-magnets at the ends by the pulleys, W W1. The cores
have a projection opposite to the spindle, ab, which latter is
screw-threaded. By a relay one or other electro-magnet is put in
action, and the rotating core, which is magnetized, causes rotation of
the spindle by attraction, resulting in the movement of the contact
along the resistance stops. The relay is acted upon directly by the
potential of the dynamo, and the variable resistance is included in
the shunt field of the machine, so that changes in the potential,
resulting from changes in load or speed, are compensated for.
The arrangements of the lamp circuits and the lamp itself may now be
described. The lamps are all run in parallel circuit, but are divided
into groups of five, each group being controlled by a separate switch
on the board--Figs. 11 and 11A. These switches are not in
direct communication with the dynamo, but make that connection through
a large central switch, S2, which therefore carries the whole
current. The returns from each group are brought to the connections
seen between the two resistances, where the circuits may be
disconnected if desired, and the main current then passes through the
ammeter, A3, to the other terminal of the machine. One of the smaller
switches at the top, Fig. 11A, is directly connected with one terminal
of the 20 horse power dynamo before mentioned, and the other side of
the switch to the motor in the machine tool exhibit. Also one of the
switches in connection with the central switch, S2, is connected to
the same motor, and therefore the latter may be run by either machine,
or, in fact, any combination of machines, lamps, and motor be made as
required.
The form of switch made by the Oerlikon Works is illustrated in Fig.
7. Two thick semicircular bands of copper are screwed at one end to
opposite sides of a square block which is turned round by the switch
handle. The block has a projection at each corner, and two strong,
flat, stationary springs are attached to the framework of the switch
and press on opposite sides of the block. The ends of the springs
engage in the projections and prevent the switch being turned round
the wrong way, while the pressure of the springs on opposite sides
forces the copper bands to take up a position exactly in line with the
terminal contacts when the switch is closed, or at right angles to
them when it is opened.
[Illustration: FIG. 4A]
[Illustration: FIGS. 4, 4B and 4C]
Further, each lamp has its own separate adjustable resistance, fuse,
and switch. These are of special construction, combined in one, and
are illustrated in Figs. 4 and 4A; the other figures, 4B and 4C,
showing some of the details of the same. The wires, W W, lead from and
to one lamp. The current enters at one wire, passes through the fuse,
f--Figs. 4C and 4A--down the center of the cylinder to a divided
contact, into which a switch arm can be shot. When this is so, a
connection is made to the upright brass rod, T, which serves to grip
the band, R, passing round the body of the cylinder. The current then
passes through all the turns of wire above the band, and out at the
other terminal. The resistance can be varied by raising or lowering
the band. Fig. 4B shows the manner of tightening the band against the
wires on the cylinder. The upright rod, T, is seen in section, and is
fixed in one position to the frame of the apparatus. Abutting against
this, and working in the block to which the two ends of the band are
screwed, is a thumb screw, S, by turning which the band may be
loosened for adjusting, and tightened when the right position is
found. The cylinder is covered with asbestos sheet, and the wire,
which is of nickel, and measures altogether from 3 to 4 ohms, is wound
helically round this. The switch arm, to which the handle is attached
below, does not itself make and break the circuit, but carries a
spring, as shown, which, when the arm is at the end of its movement,
pulls over the contact lever with a rapid action, shooting the same
between the divided contact piece, and making a perfect contact. The
switchboard forms one side of a closed wooden case or cupboard, with
sufficient room for a man to enter and adjust the resistances or
switches for each lamp. These are screwed to the inside of the case in
rows, to the number of twenty-five. The greatest care has been taken
in the fixing of the connections to the inside of this case, and no
leading wires of different potential are allowed to cross each other.
[Illustration: FIG. 11A]
The Oerlikon lamp, which is designed to work with constant potential,
is shown partly in section in Fig. 8. There is only one solenoid, A,
through which all the current passes, and whose action is to strike
the arc and maintain the current constant. The soft iron core, C, is
suspended from the inside of the tube, T, in which it has an up and
down movement checked by an air piston in the tube. An end elevation
of the brake wheels and solenoid is given in Fig. 9, where it will be
seen that the spindle carrying these wheels also carries between them
a pinion engaging with the rack rod, R. The top carbon attached to the
rack rod falls by its own weight, and is therefore in contact with the
lower carbon before the lamp is switched in circuit. When this is done
the core is instantly magnetized, and attracted to the soft iron brake
wheels, which it holds firmly. The air cushion in the tube prevents
the core being drawn up until it has fairly gripped the sides of the
wheels. The subsequent raising of the core therefore turns the wheels,
raises the rack rod, and strikes the arc. The feed is operated by the
weakening of the magnetic field of the coil, which causes the core to
lose its grip of the wheels, and allows the top carbon to descend. The
catch, L, Fig. 8, has a lateral play, and serves to engage in the
teeth of the rack rod, so as to prevent its falling when being
trimmed. Each carbon when in position is held against two rectangular
guide bars by the pressure of a wire spring--see figure. In this way
the carbon is pressed against two parallel knife edges, and is
therefore always in true alignment. The action of the lamp is very
simple, the working parts are few and solidly constructed, and the
regulation, as exhibited by the lamps running in the galleries, is
exceptionally steady.
The transmission of power plant consists of two 250 horse power
dynamos--C.E.L. Brown's patent--the generator being driven by a
vertical compound condensing engine of the same power, running at 180
revolutions. The dynamo generator is a four-pole 600 volt direct
current machine, series wound, and may be distinguished in the
engraving next to the switch board; while the motor receiver
connected to it, and erected in another portion of the Swiss section,
is of exactly the same size and type. The field, which is hexagonal in
shape, is cast in two pieces, bolted together horizontally, the
cross-sectional area of iron being 170 square inches. The armature is
cylindrical, and built up of flat rings stamped out of soft sheet
iron, eight notches in the same being provided to fit over the arms of
the spider keyed to the shaft. The spider is in halves, which are
bolted together longitudinally after the rings are in position. It is
Gramme wound, and measures over the winding 7 in. radial depth, 37 in.
outside diameter, and 22 in. in length. The current is collected by
four brushes. The fitting and mechanical build of the dynamos leaves
nothing to be desired. All the working parts of the dynamos and
engines are turned up to gauge and template, so as to be
interchangeable. As an instance of this, the armature of the generator
was built in the works, while the field magnets were being erected in
the exhibition, and, on arrival, fitted in position perfectly, and ran
at once without trouble.
The energy taken off on the motor shaft is close on 200 horse power,
but varies according to the machines at work; the speed of the motor
does not, however, vary more than 3 per cent., and the brushes need no
adjustment. About 6 ft. of shafting is coupled on in line with the
motor shaft, and an extra plummer block fixed at the end. This
shafting carries at its extremity an additional 2 ft. pulley, the
power being delivered by belting from these pulleys to two large
pulleys on the main shaft.
The machines run by this transmission consist of the looms of Rieter &
Co., of Winterthur; the large flour mill and lift of A. Millot & Co.;
the flour milling machinery of Frederick Wegmann & Co., of Zurich; the
brick and tile making machines of the Rorschach foundries; and the
looms of Messrs. Houget & Teston, of Verviers, in the Belgian section.
A 15 horse power two-pole Oerlikon dynamo is also run by a belt from
the main shaft, and generates power to drive a motor of similar type
in the Swiss section of the upper gallery. This runs a length of
countershafting supplying power to three silk-weaving machines
constructed by Benninger Frères; six weaving machines from the Ruti
works, near Zurich; and one knitting machine exhibited by Edward
Dubied & Co., of Couvet.
The dynamo and motor are connected to the main cable by switches of
the type shown in Fig. 5. These are specially designed to destroy the
extra current on breaking circuit by the formation of an arc which
gradually increases the resistance till the break occurs, rendering it
less sudden. One wire passes through the handle and makes contact with
the springs, and the other is attached to the clamp in which the
carbon rod is held. The current is made to enter at the carbon rod, so
that the arcs formed cause consumption of the carbon. A magnetic
cut-out--Fig. 6--is also provided to each machine; this consists of an
electro-magnet, through which the main current passes, provided with
side pole pieces. A flat soft iron plate armature is hinged so as to
come up against the pole pieces when attracted. When the current is
not sufficiently strong to cause the plate to be attracted, a hole in
the center of the latter engages over a small projection in the top of
a weighted arm hinged in the center of the board, and keeps it
upright. If now the current exceeds the limits of safety to the
machine, due to a too heavy load being thrown on, the armature is
attracted and releases the vertical arm, which falls over and enters
with considerable force between the two spring contacts below. These
contacts are connected to the field terminals, which are, therefore,
short-circuited, and prevent the dynamo generating any current. A
retractile spring can be adjusted to cause cut-off at any required
current. These details are indicated in our illustrations mounted on
their respective switch boards.
Since the erection of plant by these works at Solothurn for
transmitting 50 horse power five miles distant, which attracted so
much interest some time ago, several important works have been carried
out. Among these we may mention a 280 horse power transmission at 1½
kilom. distance to a cotton mill at Derendingen in Switzerland, a 250
horse power transmission at ½ kilom. distance, carried out for Gaetano
Rossi at Piovene in Italy, and a 300 horse transmission at 6 kilom.
distance installed for Giovanni Rossi, in which the power is given off
at two different stations.--_The Engineer._
* * * * *