Many ingenious men have vainly attempted to apply what has been
erroneously called "centrifugal force" as a motive power, conceiving
that the effort made by bodies to fly off when whirled round in a
circle was occasioned by a force generated by their rotation. The
experiment of the "whirling table," which is commonly shown to
illustrate centrifugal action, tends to confirm the notion that force
is generated; for it is there seen that, when the velocity of rotation
is doubled, the centrifugal force is quadrupled, and that it continues
to increase in a geometrical ratio. It has, therefore, been conceived
that a power might be generated of indefinite amount; for as a double
velocity can be communicated by doubling the moving power, whilst the
tendency to fly off at the circumference is quadrupled, there appeared
to be a creation of power which, if properly applied, would realize
perpetual motion.
A working engineer known to the author was so fully possessed with
the notion that power might thus be created, and that its application
would be of the utmost benefit, that he imagined he had been specially
appointed to reveal the principle to man, as a boon of inestimable
value to the manufacturing arts. The plan he adopted was to employ
what he called a generating engine, consisting of a centrifugal pump;
and the force with which the water was projected from the ends of
two rotating horizontal arms was directed against pistons working in
cylinders, as the force of steam is in a steam engine. Having once
set this machine in action, he expected to be able, by means of the
self-creating centrifugal force, to generate the power that worked
the generating engine, and thus to have a reservoir of force of any
magnitude constantly at command. So completely satisfied was he of
the practicability of the plan, founded, as he supposed, upon one of
Newton's laws of motion, and he felt so happy in the thought of being
charged with an important mission for the benefit of mankind, that it
was almost cruel to attempt to correct his notions of the power of
centrifugal force. He spent all his money in endeavouring to realize
this impossible project, and even its failure did not convince him of
his error.
The simple kind of Centrifugal Pump applied in that chimerical scheme
was known upwards of one hundred years ago. It consisted of a vertical
hollow shaft, into which were inserted two horizontal arms. The shaft
was supported on a pivot at the bottom, and was turned by a handle at
the top, as represented in the accompanying drawing. The lower end
of the vertical shaft was immersed in water, and when rotary motion
was given to the machine, the centrifugal action propelled the water
from the ends of the arms, and the water rose in the vertical shaft to
supply its place.
[Illustration]
The effect in a pump of this construction is due to the pressure of
the atmosphere, for the outpouring of the water from the rotating arms
tends to produce a vacuum in the shaft, in the same manner as the
lifting of the plunger in a common pump. It is evident, therefore,
that a Centrifugal Pump of that construction could not raise a column
of water higher than the pressure of the atmosphere would force it up,
which would be about thirty feet.
[Illustration]
Mr. Appold's Centrifugal Pump, which constituted one of the most
remarkable features of the Machinery Department of the Great
Exhibition, is constructed on a different plan, though the principle
is the same. The rotating arms are immersed in the water to be raised,
and to diminish the resistance which would be produced by the rotation
in water of two or more exposed arms, they are enclosed within discs of
metal, about one foot in diameter, and three or four inches apart. The
arms are formed by curved partitions between the discs, which radiate
from the centre to the outer rim, towards which the space between the
discs is contracted. This pump is fixed on an axis, to which rapid
rotary motion can be given; and it is fitted into a case connected with
the pipe that conveys the water to the discharging orifice. The water
enters the rotating disc through a large aperture in the centre, and it
is forced through the spaces formed by the radical arms with increasing
velocity, until it escapes from the circumference. Sections of Mr.
Appold's pump are shown in the accompanying diagrams, in which A is the
central opening for the admission of water; C, C, C, the curved radical
partitions which form the arms by which motion is communicated to the
water, and through the ends of which it issues into the external case,
connected with the lift-pipe, D.
In the Great Exhibition there were two other Centrifugal Pumps shown
in action, one by Mr. Bessemer, and one by Mr. Gwynne, from America;
but neither of them exhibited such striking effects as Mr. Appold's,
which was so arranged as to throw out a continuous cascade of water
from an aperture six feet wide, at a height of twenty-six feet. The
Jury of Class V., who made numerous experiments to determine the
practical efficiency of Centrifugal Pumps, and the relative merits of
the three exhibited, reported very favourably of that of Mr. Appold,
to whom a Council Medal was awarded. When rotating at the rate of 788
revolutions in a minute, and lifting the water 19·4 feet, the greatest
practical effect, compared with the power employed, was attained. The
discharge of water per minute at that height, with the pump rotating
with a velocity of 788 revolutions, was 1,236 gallons; and with a
lift of 8 feet, 2,100 gallons per minute were discharged, when the
rotating velocity was 828 revolutions per minute. In Mr. Gwynne's and
Mr. Bessemer's pumps, which had straight vanes, the ratio of power to
the effect did not exceed 0·19. One of Mr. Appold's pumps, only one
inch in diameter (the exact size of the small diagram), will discharge
ten gallons per minute. The greatest height to which water has been
raised by the pumps that are one foot in diameter is 67·7 feet, with a
velocity of 4,153 feet per minute.
The velocity with which the pump should revolve depends upon the
height to which the water is to be raised. Beyond a certain height,
the required velocity is practically unattainable, but long before
that limit is reached the waste of power becomes so great, that the
pump is of no value, for the pressure on the circumference counteracts
the force with which the water is expelled. It is, therefore, only
at comparatively low levels that the Centrifugal Pump is a useful
engine. The absence of all valves renders it very suitable for draining
marshes, and for other similar purposes, as the muddy water and
suspended matters will not obstruct its action.
In the Report of the Jury the influence of the curved shape of the
radial arms is considered very important in producing the effects.
"If the vanes be straight," the Report observes, "it is evident, that
whatever may be the velocity of the water in the direction of a radius,
when it leaves the wheel its velocity in the direction of a tangent
will be that of the circumference of the wheel, so that the greater
the velocity of the wheel the greater will be the amount of _vis viva_
remaining in the water when discharged, and the greater the amount of
power uselessly expended to create that _vis viva_. If, however, the
vanes be curved backwards as regards the motion of the wheel, so as
to have nearly the direction of a tangent to the circumference of the
wheel at the points where they intersect it, the velocity due to the
centrifugal force of the water carrying over the surface of the vane in
the opposite direction to that in which the wheel is moving, and nearly
in the direction of a tangent to the circumference, will--if this
velocity of the water over the vane in the one direction be equal to
that in which the vane is itself moving in the other--produce a state
of absolute rest in the water, and entire exhaustion of _vis viva_." It
is an interesting fact in the history of the invention, that the curved
form was formerly adopted in some of the American pumps, and afterwards
abandoned.
There are competing claims to the invention of Centrifugal Pumps in
the form now adopted. This kind of pump is stated to have been used in
America in 1830. M. Charles Combe took out a patent in France for a
similar pump in 1838; but though Mr. Appold was later in the field with
his more perfect machine, he appears to have proceeded independently of
previous inventors.