Scientific American Supplement, No. 303

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It will thus be seen that electricity can only be used as a means of
transmitting power from one place to another, or for storing power up
at one time to be used at a subsequent period; but it cannot be used to
originate power in the way coal can be used. It possesses no inherent
potential. It is incapable of performing work unless something is done
to it first. We have spoken of it as a fluid, but only for the sake of
illustration. As we have said, no one knows what it is, but the theory
which bids fair for acceptance is that it is a mode of motion of the
all-pervading ether. Very curious and instructive experiments are now
being carried out in Paris by Dr. Bjerkness, of Christiania, in the
Norwegian section of the electrical exhibition. This gentleman submerges
thin elastic diaphragms in water, and causes them to vibrate, or rather
pulsate, by compressed air. He finds that if they pulsate synchronously
they attract each other. If the pulsations are not simultaneous, the
disks repel each other. From this and other results he has obtained,
it may be argued that the ether plays the part of the water in Dr.
Bjerkness' tank, and that when special forms of vibration are set up
in bodies they become competent to attract or repel other bodies. This
being so, it will be seen that the power of attraction or repulsion of
an electrical body depends in the first instance on the motion set up
in the body attracted or repulsed, and this motion is, of course, some
function of the work originally done on the body. We need not pursue
this argument further. Among the most scientific investigators of the
day it is admitted that the efficiency of electricity as a doer of work,
or a producer of action at a distance, must depend for its value on the
performance of work in some one way or another on the electricity itself
in the first instance. It may be worth while here to dispel a popular
delusion. It is held very generally that electricity can be made, as,
for instance, by the galvanic battery. There is no reason to believe
anything of the kind; but whether it is or is not true that electricity
is actually made by the combustion of zinc in a galvanic trough, it is
quite certain that this electricity, unless it possesses potential, can
do no work, no matter how great its quantity. Of course, it is to be
understood that all electric currents possess potential. If they did
not, their presence would be unknown; but the potential of a current
is in all cases the result of work done on electricity, either by the
oxidation of zinc, or in some other way. This is a broad principle, but
it is strictly consistent in every respect with the truth. Electricity,
then, is, as we have said, totally different from coal; and it can never
become a substitute for it alone. Water power, air power, or what we
may, for want of a better phrase, call chemical power, combined with
electricity, can be used as a substitute for coal; but electricity
cannot of itself be employed to do work. It is true, however, that
electricity, on which work has already been done, may be found in
nature. Atmospheric electricity, for example, may perhaps yet be
utilized. It is by no means inconceivable that the electricity contained
in a thunder cloud might be employed to charge a Faure battery; but up
to the present no one has contemplated the obtaining of power from the
clouds, and whether it is or is not practicable to utilize a great
natural force in this way does not affect our statement. The use of
electricity must be confined to its power of transmitting or storing up
energy, and this truth being recognized, it becomes easy to estimate the
future prospects of electricity at something like their proper value.

It has been proved to a certain extent that electricity can be used to
transmit power to a distance, and that it can be used to store it up.
Thus far the man of pure science. The engineer now comes on the stage
and asks--Can practical difficulties be got over? Can it be made to pay?
In trying to answer these questions we cannot do better than deal with
one or two definite proposals which have been recently made. That with
which we shall first concern ourselves is that trains should be worked
by Faure batteries instead of by steam. It is suggested that each
carriage of a train should be provided with a dynamo motor, and that
batteries enough should be carried by each to drive the wheels, and so
propel the train. Let us see how such a scheme would comply with working
conditions. Let us take for example a train of fifteen coaches on the
Great Northern Railway, running without a stop to Peterborough in one
hour and forty minutes. The power required would be about 500 horses
indicated. To supply this for 100 minutes, even on the most absurdly
favorable hypothesis, no less than 25 tons of Faure batteries would be
required. Adding to these the weight of the dynamo motors, and that
unavoidably added to the coaches, it will be seen that a weight equal to
that of an engine would soon be reached. The only possible saving would
be some 28 to 30 tons of tender. In return for this all the passengers
would have to change coaches at Peterborough, as the train could not be
delayed to replace the expended with fresh batteries. This is out of
the question. The Faure batteries must all be carried on one vehicle or
engine, which could be changed for another, like a locomotive. Even then
no advantage would be gained. As to cost, it is very unlikely that the
stationary engines which must be provided to drive the dynamo machines
for charging the batteries would be more economical than locomotive
engines; and if we allow that the dynamo machine only wasted 10 per
cent. of the power of the engine, the Faure batteries 10 per cent. of
the power of the dynamo machines, and the dynamo motors 10 per cent. of
the power of the batteries--all ridiculously favorable assumptions--yet
the stationary engines would be handicapped with a difference in net
efficiency between themselves and the locomotive--admitting the original
efficiency per pound of coal in both to be the same--of some 27 per
cent., we think we may relegate this scheme to the realms of oblivion.
Another idea is that by putting up turbines and dynamo machines the
steam engine might be superseded by water power. Now it so happens that
if all the water power of England were quadrupled it would not nearly
suffice for our wants. It may be found worth while perhaps to construct
steam engines close to coalpits and send out power from these engines by
wire; but the question will be asked, Which is the cheaper of the two,
to send the coal or to send the power? On the answer to this will
depend the decision of the mill owners. Another favorite scheme is that
embodied in the Siemens electrical railway. We believe that there is a
great future in store for electricity as a worker of tramway traffic;
but the traffic on a great line like the Midland or Great Northern
Railway could not be carried on by it. As Robert Stephenson said of the
atmospheric system, it is not flexible enough. The working of points
and crossings, and the shunting of trains and wagons, would present
unsurmountable difficulties. We have cited proposals enough, we think,
to illustrate our meaning. Sir William Armstrong, Sir Frederick
Bramwell, Dr. Siemens, Sir W. Thomson, and many others may be excused if
they are a little enthusiastic. They are just now overjoyed with success
attained; but when the time comes for sober reflection they will, no
doubt, see good reason to moderate their views. No one can say, of
course, what further discoveries may bring to light; but recent speakers
and writers have found in what is known already, materials for sketching
out a romance of electricity. It is but romancing to assert that the end
of the steam engine is at hand. Wonderful and mystical as electricity
is, there are some very hard and dry facts about it, and these facts are
all opposed to the theory that it can become man's servant of all work.
Ariel-like, electricity may put a girdle round the earth in forty
minutes; but it shows no great aptitude for superseding the useful old
giant steam, who has toiled for the world so long and to such good
purpose--_The Engineer_.

* * * * *

ON A METHOD OF OBTAINING AND MEASURING VERY HIGH VACUA WITH A MODIFIED
FORM OF SPRENGEL-PUMP.

By Ogden N. Rood, Professor of Physics in Columbia College.

In the July number of this Journal for 1880, I gave a short account of
certain changes in the Sprengel-pump by means of which far better vacua
could be obtained than had been previously possible. For example, the
highest vacuum at that time known had been reached by Mr. Crookes, and
was about 1/17,000,000, while with my arrangement vacua of 1/100,000,000
were easily reached. In a notice that appeared in _Nature_ for August,
1880, p. 375, it was stated that my improvements were not new, but had
already been made in England four years previously. I have been unable
to obtain a printed account of the English improvements, and am willing
to assume that they are identical with my own; but on the other hand,
as for four years no particular result seems to have followed their
introduction in England, I am reluctantly forced to the conclusion that
their inventor and his customers, for that period of time, have remained
quite in ignorance of the proper mode of utilizing them. Since then I
have pushed the matter still farther, and have succeeded in obtaining
with my apparatus vacua as high as 1/390,000,000 without finding
that the limit of its action had been reached. The pump is simple in
construction, inexpensive, and, as I have proved by a large number of
experiments, certain in action and easy of use; stopcocks and grease are
dispensed with, and when the presence of a stopcock is really desirable
its place is supplied by a movable column of mercury.

_Reservoir_.--An ordinary inverted bell-glass with a diameter of 100 mm.
and a total height of 205 mm. forms the reservoir; its mouth is closed
by a well-fitting cork through which passes the glass tube that forms
one termination of the pump. The cork around tube and up to the edge of
the former is painted with a flexible cement. The tube projects 40 mm.
into the mercury and passes through a little watch-glass-shaped piece of
sheet-iron, W, figure 1, which prevents the small air bubbles that creep
upward along the tube from reaching its open end; the little cup is
firmly cemented in its place. The flow of the mercury is regulated
by the steel rod and cylinder, CR, Figure 1. The bottom of the steel
cylinder is filled out with a circular piece of pure India-rubber,
properly cemented; this soon fits itself to the use required and answers
admirably. The pressure of the cylinder on the end of the tube is
regulated by the lever, S, Figure 1; this is attached to a circular
board which again is firmly fastened over the open end of the
bell-glass. It will be noticed that on turning the milled head, S, the
motion of the steel cylinder is not directly vertical, but that it tends
to describe a circle with c as a center; the necessary play of the
cylinder is, however, so small, that practically the experimenter does
not become aware of this theoretical defect, so that the arrangement
really gives entire satisfaction, and after it has been in use for a few
days accurately controls the flow of the mercury. The glass cylinder is
held in position, but not supported, by two wooden _adjustable_ clamps,
_a a_, Figure 2. The weight of the cylinder and mercury is supported by
a shelf, S, Figure 2, on which rests the cork of the cylinder; in this
way all danger of a very disagreeable accident is avoided.

[Illustration: MODIFIED FORM OF SPRENGEL PUMP.]

_Vacuum-bulb_.--Leaving the reservoir, the mercury enters the
vacuum-bulb, B, Figure 2, where it parts with most of its air and
moisture; this bulb also serves to catch the air that creeps into the
pump from the reservoir, even when there is no flow of mercury; its
diameter is 27 mm. The shape and inclination of the tube attached to
this bulb is by no means a matter of indifference; accordingly Figure
3 is a separate drawing of it; the tube should be so bent that a
horizontal line drawn from the proper level of the mercury in the bulb
passes through the point, _o_, where the drops of mercury break off. The
length of the tube, EC, should be 150 mm., that of the tube, ED, 45 mm.;
the bore of this tube is about the same as that of the fall-tube.

_Fall-tube and bends_.--The bore of the fall-tube in the pump now used
by me is 1.78 mm.; its length above the bends (U, Figure 2) is 310 mm.;
below the bends the length is 815 mm. The bends constitute a fluid valve
that prevents the air from returning into the pump; beside this, the
play of the mercury in them greatly facilitates the passage of the
air downward. The top of the mercury column representing the existing
barometric pressure should be about 25 mm. below the bends when the pump
is in action. This is easily regulated by an adjustable shelf, which is
also employed to fill the bends with mercury when a measurement is taken
or when the pump is at rest. On the shelf is a tube, 160 mm. high and 20
mm. in diameter, into which the end of the fall-tube dips; its side has
a circular perforation into which fits a small cork with a little tube
bent at right angles. With the hard end of a file and a few drops of
turpentine the perforation can be easily made and shaped in a few
minutes. By revolving the little bent tube through 180 deg. the flow of the
mercury can be temporarily suspended when it is desirable to change the
vessel that catches it.

_Gauge_.--For the purpose of measuring the vacua I have used an
arrangement similar to McLeod's gauge, Figure 4; it has, however, some
peculiarities. The tube destined to contain the compressed air has a
diameter of 1.35 mm. as ascertained by a compound microscope; it is not
fused at its upper extremity, but closed by a fine glass rod that fits
into it as accurately as may be, the end of the rod being ground flat
and true. This rod is introduced into the tube, and while the latter
is gently heated a very small portion of the cement described below is
allowed to enter by capillary attraction, but not to extend beyond the
end of the rod, the operation being watched by a lens. The rod is
used for the purpose of obtaining the compressed air in the form of a
cylinder, and also to allow cleansing of the tube when necessary. The
capacity of the gauge-sphere was obtained by filling it with mercury;
its external diameter was sixty millimeters; for measuring very high
vacua this is somewhat small and makes the probable errors rather
large; I would advise the use of a gauge-sphere of about twice as great
capacity. The tube, CB, Figure 4, has the same bore as the measuring
tube in order to avoid corrections for capillarity. The tube of the
gauge, CD, is not connected with an India-rubber tube, as is usual,
but dips into mercury contained in a cylinder 340 mm. high, 58 mm. in
diameter, which can be raised and lowered at pleasure. This is best
accomplished by the use of a set of boxes of various thicknesses, made
for the purpose and supplemented by several sheets of cardboard and even
of writing-paper. These have been found to answer well and enable the
experimenter to graduate with a nicety the pressure to which the gas is
exposed during measurement. By employing a cylinder filled with mercury
instead of the usual caoutchouc tubing small bubbles of air are
prevented from entering the gauge along with the mercury. An adjustable
brace or support is used which prevents accident to the cylinder when
the pump is inclined for the purpose of pumping out the vacuum-bulb. The
maximum pressure that can be employed in the gauge used by me is 100 mm.

All the tubing of the pump is supported at a distance of about 55 mm.
from the wood-work; this is effected by the use of simple adjustable
supports and adjustable clamps; the latter have proved a great
convenience. The object is to gain the ability to heat with a Bunsen
burner all parts of the pump without burning the wood-work. Where glass
and wood necessarily come in contact the wood is protected by metal or
simply painted with a saturated solution of alum. The glass portions
of the pump I have contrived to anneal completely by the simple means
mentioned below. If the glass is not annealed it is certain to crack
when subjected to heat, thus causing vexation and loss of time. The
mercury was purified by the same method that was used by W. Siemens
(Pogg. Annalen, vol. ex., p. 20), that is, by a little strong sulphuric
acid to which a few drops of nitric acid had been added; it was dried by
pouring it repeatedly from one hot dry vessel to another, by filtering
it while quite warm, the drying being completed finally by the action of
the pump itself. All the measurements were made by a fine cathetometer
which was constructed for me by William Grunow; see this Journal, Jan.,
1874, p. 23. It was provided with a well-corrected object-glass having a
focal length of 200 mm. and as used by me gave a magnifying power of 16
diameters.

_Manipulation_.--The necessary connections are effected with a cement
made by melting Burgundy pitch with three or four per cent of gutta
percha. It is indispensable that the cement when cold should be so hard
as completely to resist taking any impression from the finger nail,
otherwise it is certain to yield gradually and finally to give rise to
leaks. The connecting tubes are selected so as to fit as closely as
possible, and after being put into position are heated to the proper
amount, when the edges are touched with a fragment of cold cement which
enters by capillary attraction and forms a transparent joint that can
from time to time be examined with a lens for the colors of thin plates,
which always precede a leak. Joints of this kind have been in use by me
for two months at a time without showing a trace of leakage, and the
evidence gathered in another series of unfinished experiments goes to
show that no appreciable amount of vapor is furnished by the resinous
compound, which, I may add, is never used until it has been repeatedly
melted. As drying material I prefer caustic potash that has been in
fusion just before its introduction into the drying tube; during the
process of exhaustion it can from time to time be heated nearly to the
melting point: if actually fused in the drying tube the latter almost
invariably cracks. The pump in the first instance is to be inclined at
an angle of about 10 degrees, the tube of the gauge being supported by
a semicircular piece of thick pasteboard fitted with two corks into the
top of the cylinder. This seemingly awkward proceeding has in no case
been attended with the slightest accident, and owing to the presence of
the four leveling-screws, the pump when righted returns, as shown by the
telescope of the cathetometer, almost exactly to its original place. In
the inclined position the exhaustion of the vacuum bulb is accomplished
along with that of the rest of the pump. The exhaustion of the
vacuum-bulb when once effected can be preserved to a great extent for
use in future work, merely by allowing mercury from the reservoir to
flow in a rapid stream at the time that air is allowed to re-enter the
pump. During the first process of exhaustion the tube of the gauge is
kept hot by moving to and fro a Bunsen burner, and is in this way
freed from those portions of air and moisture that are not too firmly
attached. After a time the vacuum-bulb ceases to deliver bubbles of
air; it and the attached tube are now to be heated with a moving Bunsen
burner, when it will be found to furnish for 15 or 20 minutes a large
quantity of bubbles mainly of vapor of water. After then production
ceases the pump is righted and the exhaustion carried farther. In spite
of a couple of careful experiments with the cathetometer I have not
succeeded in measuring the vacuum in the vacuum bulb, but judge from
indications, that is about as high as that obtained in an ordinary
Geissler pump. Meanwhile the various parts of the pump can be heated
with a moving Bunsen burner to detach air and moisture, the cement being
protected by wet lamp-wicking. In one experiment I measured the amount
of air that was detached from the walls of the pump by heating them for
ten minutes somewhat above l00 deg. C., and found that it was 1/1,000,000
of the air originally present. I have also noticed that a still larger
amount of air is detached by electric discharges. This coincides with an
observation of E. Bessel-Hagen in his interesting article on a new form
of Toepler's mercury-pump (Annalen der Physik und Chemie, 1881, vol.
xii.). Even when potash is used a small amount of moisture always
collects in the bends of the fall tube; this is readily removed by a
Bunsen burner; the tension of the vapor being greatly increased, it
passes far down the fall-tube in large bubbles and is condensed. Without
this precaution I have found it impossible to obtain a vacuum higher
than 1/25,000,000; in point of fact the bends should always be heated
when a high exhaustion is undertaken even if the pump has been standing
well exhausted for a week; the heat should of course never be applied at
a late stage of the exhaustion. Conversely, I have often by the aid of
heat completely and quickly removed quite large quantities of the vapor
of water that had been purposely introduced. The exhaustion of the
vacuum-bulb is of course somewhat injured by the act of using the pump
and also by standing for several days, so that it has been usual with me
before undertaking a high exhaustion to incline the pump and re-exhaust
for 20 minutes; I have, however, obtained very high vacua without using
this precaution.

During the process of exhaustion not more than one-half of the mercury
in the reservoir is allowed to run out, other wise when it is returned
bubbles of air are apt to find their way into the vacuum-bulb. In order
to secure its quiet entrance it is poured into a silk bag provided with
several holes. When the reservoir is first filled its walls for a day
or two appear to furnish air that enters the vacuum-bulb; this action,
however, soon sinks to a minimum and then the leakage remains quite
constant for months together.

_Measurement of the vacuum_.--The cylinder into which the gauge-tube
dips is first elevated by a box sufficiently thick merely to close the
gauge, afterwards boxes are placed under it sufficient to elevate the
mercury to the base of the measuring tube; when the mercury has reached
this point, thin boards and card-boards are added till a suitable
pressure is obtained. The length of the inclosed cylinder of air is
then measured with the cathetometer, also the height of the mercurial
"meniscus," and the difference of the heights of the mercurial columns
in A and B, figure 4. To obtain a second measure an assistant removes
some of the boxes and the cylinder is lowered by hand three or four
centimeters and then replaced in its original position. In measuring
really high vacua, it is well to begin with this process of lowering and
raising the cylinder, and to repeat it five or six times before taking
readings. It seems as though the mercury in the tube, B, supplies to the
glass a coating of air that allows it to move more freely; at all events
it is certain that ordinarily the readings of B become regular, only
after the mercury has been allowed to play up and down the tube a number
of times. This applies particularly to vacua as high 1/50,000,000 and to
pressures of five millimeters and under. It is advantageous in making
measurements to employ large pressures and small volumes; the correct
working of the gauge can from time to time be tested by varying the
relations of these to each other. This I did quite elaborately, and
proved that such constant errors as exist are small compared with
inevitable accidental errors, as, for example, that there was no
measurable correction for capillarity, that the calculated volume of the
"meniscus" was correct, etc. It is essential in making a measurement
that the temperature of the room should change as little as possible,
and that the temperature of the mercury in the cylinder should be at
least nearly that of the air near the gauge-sphere. The computation is
made as follows

n = height of the cylinder inclosing the air;
c = a factor which, multiplied by n, converts it into cubic
millimeters;
S = cubic contents of the meniscus;
d = difference of level between A and B, fig. 4;
= the pressure the air is under;
N = the cubic contents of the gauge in millimeters;
x = a fraction expressing the degree of exhaustion obtained; then

x=1/([N (760/d)]/[nc - S])

It will be noticed that the measurements are independent of the actual
height of the barometer, and if several readings are taken continuously,
the result will not be sensibly affected by a simultaneous change of the
barometer. Almost all the readings were taken at a temperature of about
20 deg. C., and in the present state of the work corrections for temperature
may be considered a superfluous refinement.

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