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Scientific American Supplement No. 822, October 3, 1891 online

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engineering, and in such works as the Severn Tunnel, the Forth and Tay
Bridges, and the Manchester Ship Canal, which was now in progress of
construction. In mining, the progress had been slow, and it was a
remarkable fact that, with the exception of pumping, the machinery in
use in connection with mining operations in Great Britain had not, in
regard to economy, advanced so rapidly as had been the case in our
manufactures and marine. This was probably due, in metalliferous
mining, to the uncertain nature of the mineral deposits not affording
any adequate security to adventurers that the increased cost of
adopting improved appliances would be reimbursed; while in coal
mining, the cheapness of fuel, the large proportion which manual labor
bore to the total cost of producing coal, and the necessity for
producing large outputs with the simplest appliances, explained the
reluctance with which high pressure steam compound engines, and other
modes embracing the most modern and approved types of economizing
power had been adopted. Metalliferous mining, with the exception of
the working of iron ore, was not in a prosperous condition; but in
special localities, where the deposits of minerals were rich and
profitable, progress had been made within a recent period by the
adoption of more economical and efficient machinery, of which the
speaker quoted a number of examples. Reference was also made to the
rapid strides made in the use of electricity as a motive power, and to
the mechanical ventilation of mines by exhaustion of the air.


COAL MINES.

Summarizing the position of mechanical science, as applied to the coal
mining industry in this country, Mr. Brown observed that there was a
general awakening to the necessity of adopting, in the newer and
deeper mines, more economical appliances. It was true it would be
impracticable, and probably unwise, to alter much of the existing
machinery, but, by the adoption of the best known types of electrical
plant, and air compression in our new and deep mines, the consumption
of coal per horse power would be reduced, and the extra expense, due
to natural causes, of producing minerals from greater depths would be
substantially lessened. The consumption of coal at the collieries of
Great Britain alone probably exceeded 10,000,000 tons per annum, and
the consumption per horse power was probably not less than 6 lb. of
coal, and it was not unreasonable to assume that, by the adoption of
more efficient machinery than was at present in general use, at least
one-half of the coal consumed could be saved. There was, therefore, in
the mines of Great Britain alone a wide and lucrative field for the
inventive ingenuity of mechanical engineers in economizing fuel, and
especially in the successful application of new methods for dealing
with underground haulage, in the inner workings of our collieries,
more especially in South Wales, where the number of horses still
employed was very large.


STEAM TRAMS AND ELECTRIC TRAMS.

Considerable progress had within recent years been made in the
mechanical appliances intended to replace horses on our public tram
lines. The steam engine now in use in some of our towns had its
drawbacks as as well as its good qualities, as also had the endless
rope haulage, and in the case of the latter system, anxiety must be
felt when the ropes showed signs of wear. The electrically driven
trams appeared to work well. He had not, however, seen any published
data bearing on the relative cost per mile of these several systems,
and this information, when obtained, would be of interest. At the
present time, he understood, exhaustive trials were being made with an
ammonia gas engine, which, it was anticipated, would prove both more
economical and efficient than horses for tram roads. The gas was said
to be produced from the pure ammonia, obtained by distillation from
commercial ammonia, and was given off at a pressure varying from 100
to 150 lb. per square inch. This ammonia was used in specially
constructed engines, and was then exhausted into a tank containing
water, which brought it back into its original form of commercial
ammonia, ready for redistillation, and, it was stated, with a
comparatively small loss.

* * * * *




IMPROVED CHANGEABLE SPEED GEARING.


This is the invention of Lawrence Heath, of Macedon, N.Y., and relates
to that class of changeable speed gearing in which a center pinion
driven at a constant rate of speed drives directly and at different
rates of speed a series of pinions mounted in a surrounding revoluble
case or shell, so that by turning the shell one or another of the
secondary pinions may be brought into operative relation to the parts
to be driven therefrom.

The aim of my invention is to so modify this system of gearing that
the secondary pinions may receive a very slow motion in relation to
that of the primary driving shaft, whereby the gearing is the better
adapted for the driving of the fertilizer-distributers of grain drills
from the main axle, and for other special uses.

Fig. 1 is a side elevation. Fig. 2 is a vertical cross section.

[Illustration: FIG. 1.]

[Illustration: FIG. 2.]

A represents the main driving shaft or axle, driven constantly and at
a uniform speed, and B is the pinion-supporting case or shell, mounted
loosely on and revoluble around the axle, but held normally at rest by
means of a locking bolt, C, or other suitable locking device adapted
to enter notches, _c_, in the shell.

D is the primary driving pinion, fixed firmly to the axle and
constantly engaging the pinion, E, mounted on a stud in the shell. The
pinion, E, is formed integral with or firmly secured to the smaller
secondary pinion, F, which in turn constantly engages and drives the
center pinion, G, mounted to turn loosely on the axle within the
shell, so that it is turned in the same direction as the axle, but at
a slower speed.

F', F_{2}, F_{3}, F_{4}, etc., represent additional secondary pinions
grouped around the center pinion, mounted on studs in the shell, and
made of different diameters, so that they are driven by the center
pinion at different speeds. Each of the secondary pinions is formed
with a neck or journal, _f_, projected out through the side of the
shell, so that the external pinion, H, may be applied to any one of
the necks at will in order to communicate motion thence to the gear,
I, which occupies a fixed position, and from which the fertilizer or
other mechanism is driven.

In order to drive the gear, I, at one speed or another, as may be
demanded, it is only necessary to apply the pinion, H, to the neck of
that secondary pinion which is turning at the appropriate speed and
then turn the shell bodily around the axle until the external pinion
is carried into engagement with gear I, when the shell is again locked
fast. The axle communicates motion through D, E, and P to the center
pinion, which in turn drives all the secondary pinions except F. If
the external pinion is applied to F, it will receive motion directly
therefrom; but if applied to either of the secondary pinions, it will
receive motion through or by way of the center pinion. It will be seen
that all the pinions are sustained and protected within the shell.

The essence of the invention lies in the introduction of the pinions D
and E between the axle and the series of secondary pinions to reduce
the speed.

* * * * *




ELECTRICAL STANDARDS.


_Nature_ states that the Queen's Printers are now issuing the Report
(dated July 23, 1891) to the President of the Board of Trade, of the
Committee appointed to consider the question of constructing standards
for the measurement of electricity. The committee included Mr.
Courtenay Boyle, C.B., Major P. Cardew, R.E., Mr. E. Graves, Mr. W.H.
Preece, F.R.S., Sir W. Thomson, F.R.S., Lord Rayleigh, F.R.S., Prof.
G. Carey Foster, F.R.S., Mr. R.T. Glazebrook, F.R. S., Dr. John
Hopkinson, F.R.S., Prof. W.E. Ayrton, F.R.S.

In response to an invitation, the following gentlemen attended and
gave evidence: On behalf of the Association of Chambers of Commerce,
Mr. Thomas Parker and Mr. Hugh Erat Harrison; on behalf of the London
Council, Prof. Silvanus Thompson; on behalf of the London Chamber of
Commerce, Mr. R. E. Crompton. The Committee were indebted to Dr. J.A.
Fleming and Dr. A. Muirhead for valuable information and assistance;
and they state that they had the advantage of the experience and
advice of Mr. H. J. Chaney, the Superintendent of Weights and
Measures. The Secretary to the Committee was Sir T.W. P. Blomefield,
Bart.

The following are the resolutions of the Committee:


_Resolutions._

(1) That it is desirable that new denominations of standards for the
measurement of electricity should be made and approved by Her Majesty
in Council as Board of Trade standards.

(2) That the magnitudes of these standards should be determined on the
electro-magnetic system of measurement with reference to the
centimeter as unit of length, the gramme as unit of mass, and the
second as unit of time, and that by the terms centimeter and gramme
are meant the standards of those denominations deposited with the
Board of Trade.

(3) That the standard of electrical resistance should be denominated
the ohm, and should have the value 1,000,000,000 in terms of the
centimeter and second.

(4) That the resistance offered to an unvarying electric current by a
column of mercury of a constant cross sectional area of 1 square
millimeter, and of a length of 106.3 centimeters at the temperature of
melting ice may be adopted as 1 ohm.

(5) That the value of the standard of resistance constructed by a
committee of the British Association for the Advancement of Science in
the years 1863 and 1864, and known as the British Association unit,
may be taken as 0.9866 of the ohm.

(6) That a material standard, constructed in solid metal, and verified
by comparison with the British Association unit, should be adopted as
the standard ohm.

(7) That for the purpose of replacing the standard, if lost,
destroyed, or damaged, and for ordinary use, a limited number of
copies should be constructed, which should be periodically compared
with the standard ohm and with the British Association unit.

(8) That resistances constructed in solid metal should be adopted as
Board of Trade standards for multiples and sub-multiples of the ohm.

(9) That the standard of electrical current should be denominated the
ampere, and should have the value one-tenth (0.1) in terms of the
centimeter, gramme, and second.

(10) That an unvarying current which, when passed through a solution
of nitrate of silver in water, in accordance with the specification
attached to this report, deposits silver at the rate of 0.001118 of a
gramme per second, may be taken as a current of 1 ampere.

(11) That an alternating current of 1 ampere shall mean a current such
that the square root of the time-average of the square of its strength
at each instant in amperes is unity.

(12) That instruments constructed on the principle of the balance, in
which, by the proper disposition of the conductors, forces of
attraction and repulsion are produced, which depend upon the amount of
current passing, and are balanced by known weights, should be adopted
as the Board of Trade standards for the measurement of current,
whether unvarying or alternating.

(13) That the standard of electrical pressure should be denominated
the volt, being the pressure which, if steadily applied to a conductor
whose resistance is 1 ohm, will produce a current of 1 ampere.

(14) That the electrical pressure at a temperature of 62° F. between
the poles or electrodes of the voltaic cell known as Clark's cell may
be taken as not differing from a pressure of 1.433 volts by more than
an amount which will be determined by a sub-committee appointed to
investigate the question, who will prepare a specification for the
construction and use of the cell.

(15) That an alternating pressure of 1 volt shall mean a pressure such
that the square root of the time average of the square of its value at
each instant in volts is unity.

(16) That instruments constructed on the principle of Sir W. Thomson's
quadrant electrometer used idiostatically, and for high pressure
instruments on the principle of the balance, electrostatic forces
being balanced against a known weight, should be adopted as Board of
Trade standards for the measurement of pressure, whether unvarying or
alternating.

We have adopted the system of electrical units originally defined by
the British Association for the Advancement of Science, and we have
found in its recent researches, as well as in the deliberations of the
International Congress on Electrical Units, held in Paris, valuable
guidance for determining the exact magnitudes of the several units of
electrical measurement, as well as for the verification of the
material standards.

We have stated the relation between the proposed standard ohm and the
unit of resistance originally determined by the British Association,
and have also stated its relation to the mercurial standard adopted by
the International Conference.

We find that considerations of practical importance make it
undesirable to adopt a mercurial standard; we have, therefore,
preferred to adopt a material standard constructed in solid metal.

It appears to us to be necessary that in transactions between buyer
and seller, a legal character should henceforth be assigned to the
units of electrical measurement now suggested; and with this view,
that the issue of an Order in Council should be recommended, under the
Weights and Measures Act, in the form annexed to this report.


_Specification referred to in Resolution 10._

In the following specification the term silver voltameter means the
arrangement of apparatus by means of which an electric current is
passed through a solution of nitrate of silver in water. The silver
voltameter measures the total electrical quantity which has passed
during the time of the experiment, and by noting this time the time
average of the current, or if the current has been kept constant, the
current itself, can be deduced.

In employing the silver voltameter to measure currents of about 1
ampere, the following arrangements should be adopted. The kathode on
which the silver is to be deposited should take the form of a platinum
bowl not less than 10 cm. in diameter, and from 4 to 5 cm. in depth.

The anode should be a plate of pure silver some 30 square cm. in area
and 2 or 3 millimeters in thickness.

This is supported horizontally in the liquid near the top of the
solution by a platinum wire passed through holes in the plate at
opposite corners. To prevent the disintegrated silver which is formed
on the anode from falling on to the kathode, the anode should be
wrapped round with pure filter paper, secured at the back with sealing
wax.

The liquid should consist of a neutral solution of pure silver
nitrate, containing about 15 parts by weight of the nitrate to 85
parts of water.

The resistance of the voltameter changes somewhat as the current
passes. To prevent these changes having too great an effect on the
current, some resistance besides that of the voltameter should be
inserted in the circuit. The total metallic resistance of the circuit
should not be less than 10 ohms.

_Method of making a Measurement._ - The platinum bowl is washed with
nitric acid and distilled water, dried by heat, and then left to cool
in a desiccator. When thoroughly dry, it is weighed carefully.

It is nearly filled with the solution, and connected to the rest of
the circuit by being placed on a clean copper support, to which a
binding screw is attached. This copper support must be insulated.

The anode is then immersed in the solution, so as to be well covered
by it, and supported in that position; the connections to the rest of
the circuit are made.

Contact is made at the key, noting the time of contact. The current is
allowed to pass for not less than half an hour, and the time at which
contact is broken is observed. Care must be taken that the clock used
is keeping correct time during this interval.

The solution is now removed from the bowl, and the deposit is washed
with distilled water and left to soak for at least six hours. It is
then rinsed successively with distilled water and absolute alcohol,
and dried in a hot-air bath at a temperature of about 160° C. After
cooling in a desiccator, it is weighed again. The gain in weight gives
the silver deposited.

To find the current in amperes, this weight, expressed in grammes,
must be divided by the number of seconds during which the current has
been passed, and by 0.001118.

The result will be the time average of the current, if during the
interval the current has varied.

In determining by this method the constant of an instrument the
current should be kept as nearly constant as possible, and the
readings of the instrument taken at frequent observed intervals of
time. These observations give a curve from which the reading
corresponding to the mean current (time average of the current) can be
found. The current, as calculated by the voltameter, corresponds to
this reading.

* * * * *




THE TWO OR THREE PHASE ALTERNATING CURRENT SYSTEMS.

By CARL HERING.


The occasion of the transmission of power from Lauffen to Frankfort
has brought to the notice of the profession more than ever before the
two or three phase alternating current system, described as early as
1887-88 by various electricians, among whom are Tesla, Bradley,
Haselwander and others. As to who first invented it, we have nothing
to say here, but though known for some years it has not until quite
recently been of any great importance in practice.

Within the last few years, however, Mr. M. Von Dolivo-Dobrowolsky,
electrical engineer of the Allgemeine Elektricitats Gesellschaft, of
Berlin, has occupied himself with these currents. His success with
motors run with such currents was the origin of the present great
transmission of power exhibit at Frankfort, the greatest transmission
ever attempted. His investigation in this new sphere, and his ability
to master the subject from a theoretical or mathematical standpoint,
has led him to find the objections, the theoretically best conditions,
etc. This, together with his ingenuity, has led him to devise an
entirely new and very ingenious modification, which will no doubt have
a very great effect on the development of alternating current motors.

It is doubtless well known that if, as in Fig. 1, a Gramme ring
armature is connected to leads at four points as shown and a magnet is
revolved inside of it (or if the ring is revolved in a magnetic field
and the current led off by contact rings instead of a commutator),
there will be two alternating currents generated, which will differ
from each other in their phases only. When one is at a maximum the
other is zero. When such a double current is sent into a similarly
constructed motor it will produce or generate what might be called a
rotary field, which is shown diagrammatically in the six successive
positions in Fig. 2. The winding here is slightly different, but it
amounts to the same thing as far as we are concerned at present. This
is what Mr. Dobrowolsky calls an "elementary" or "simply" rotary
current, as used in the Tesla motors. A similar system, but having
three different currents instead of two, is the one used in the
Lauffen transmission experiment referred to above.

[Illustration: FIG. 1.]

[Illustration: FIG. 2.]

In investigating this subject Mr. Dobrowolsky found that the best
theoretical indications for such a system would be a large number of
circuits instead of only two or three, each differing from the next
one by only a small portion of a wave length; the larger their number
the better theoretically. The reason is that with a few currents the
resulting magnetism generated in the motor by these currents will
pulsate considerably, as shown in Fig. 3, in which the two full lines
show the currents differing by 90 degrees. The dotted line above these
shows how much the resulting magnetism will pulsate. With two such
currents this variation in magnetism will be about 40 degrees above
its lowest value. Now, such a variation in the field is undesirable,
as it produces objectionable induction effects, and it has the evil
effect of interfering with the starting of the motor loaded, besides
affecting the torque considerably if the speed should fall slightly
below that for synchronism. A perfect motor should not have these
faults, and it is designed to obviate them by striving to obtain a
revolving field in which the magnetism is as nearly constant as
possible.

[Illustration: FIG. 3.]

If there are two currents differing by 90 degrees, this variation of
the magnetism will be about 40 per cent.; with three currents
differing 60 degrees, about 14 per cent; with six currents differing
30 degrees it will be only about 4 per cent., and so on. It will be
seen, therefore, that by doubling the three-phase system the
pulsations are already very greatly reduced. But this would require
six wires, while the three-phase system requires only three wires (as
each of the three leads can readily be shown to serve as a return lead
for the other two in parallel). It is to combine the advantages of
both that he designed the following very ingenious system. By this
system he can obtain as small a difference of phase as desired,
without increasing the number of wires above three, a statement which
might at first seem paradoxical.

Before explaining this ingenious system, it might be well to call
attention to a parallel case to the above in continuous current
machines and motors. The first dynamos were constructed with two
commutator bars. They were soon found to work much better with four,
and finally still better as the number of commutator bars (or coils)
was increased, up to a practical limit. Just as the pulsations in the
continuous current dynamos were detrimental to proper working, so are
these pulsations in few-phased alternating current motors, though the
objections manifest themselves in different ways - in the continuous
current motors as sparking and in the alternating current motors as
detrimental inductive effects.

The underlying principle of this new system may be seen best in Figs.
4, 5, 6, 7 and 8. In Fig. 4 are shown two currents, I_{1} and I_{2},
which differ from each other by an angle, D. Suppose these two
currents to be any neighboring currents in a simple rotary current
system. Now, if these two currents be united into one, as shown in the
lower part of the figure, the resulting current, I, will be about as
shown by the dotted line; that is, it will lie between the other two
and at its maximum point, and for a difference of phases equal to 90
degrees it will be about 1.4 times as great as the maximum of either
of the others; the important feature is that the phase of this current
is midway between that of the other two. Fig. 5 shows the winding of a
cylinder armature and Fig. 7 that, of a Gramme armature for a simple
three-phase current with three leads, with which system we assume that
the reader is familiar.

[Illustration: FIG. 4.]

[Illustration: FIG. 5.]

[Illustration: FIG. 6.]

[Illustration: FIG. 7.]

[Illustration: FIG. 8.]

The two figures, 4 and 5 (or 7), correspond with each other in so far
as the currents in the three leads, shown in heavy lines, have a phase
between those of the two which compose them. Referring now to Fig. 6
(or 8), which is precisely like Fig. 5 (or 7), except that it has an
additional winding shown in heavy lines, it will be seen that each of
the three leads, shown in heavy lines, is wound around the armature
before leaving it, forming an additional coil lying _between_ the two
coils with which it is in series. The phase of the heavy line currents
was shown in Fig. 4 to lie between the other two. Therefore, in the
armature in Fig. 6 (or 8) there will be six phases, while in Fig. 5
there are only three, the number of leads (three) remaining the same
as before. This is the fundamental principle of this ingenious
invention. To have six phases in Fig. 5 would require six leads, but
in Fig. 6 precisely the same result is obtained with only three leads.
In the same way the three leads in Fig. 6 might again be combined and
passed around the armature again, and so on forming still more phases,


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Online LibraryVariousScientific American Supplement No. 822, October 3, 1891 → online text (page 2 of 11)