June 20, 1913.

THE COLLIERY GUARDIAN.

1331

of using that method, the ordinary method of merely
taking the arithmetical mean of all the readings was
employed, errors were introduced.

Some engineers did not divide the area up, but merely
moved an anemometer to and fro along a zigzag line
over the entire section. If that method was carried out
by mechanical means, so as to secure uniformity of
motion, it might give fairly reliable results, but as
usually performed was liable to error. The best method,
no doubt, would be to determine the true mean velocity,
and, at the same time, the position of mean velocity of
the air-current should be determined once for all by a
set of careful experiments, and all measurements should
then be made at the point thus determined, when one,
or at the most two, determinations of the velocity would
be all that would be required. Obviously, if, as he
(Prof. Louis) strongly advised, a continuous record of
the velocity of the air-current was kept and an ordinary
recording instrument employed, it should be perma-
nently set up at the point in question. He would refer
to an entirely different principle of air measurement
later on.

Having thus determined where the velocity of an air-
current was to be measured, they had next to consider
the appliances for measuring it. In this country the
use of the ordinary anemometer was almost universal.
Although this instrument had no doubt been greatly
improved in many respects, there were grave objections
to the use of anemometers as a class. First and fore-
most, the calibration of the instrument presented great
difficulties; he did not know of any place in this country

Prof. Louis’ New Indicating Anemometer.

where a really correct method of calibrating anemo-
meters was in use. Even if the instrument were

calibrated, as it should be, in a flow of air of known
velocity, instead of on a revolving arm, it would be
necessary to re-calibrate it at relatively frequent intervals.
The anemometer was necessarily delicate, and its indica-
tions were only reliable as long as the angle of inclination
of the vanes remained as originally given to them by the
maker. It was evident that pressure of the air-current
when the instrument was in use would tend to alter that
angle; hence the need of frequent re-calibration.

Furthermore, there was a serious objection to the
method in which the . anemometer was generally
employed. The use of the anemometer required at
least two observers. Of course, the errors due to two
observers engaged upon the same observation were
likely to be far greater than with one observer alone;
he was aware that self-timing anemometers had been
introduced in order to get over this objection, but they
did not appear to have been found satisfactory. In
using the anemometer one at least of the observers
must stand close to the spot where the velocity was
being measured, and the resistance to the flow of air set
up by his body must affect the anemometer readings,
although it was rarely taken into account. Some of
these latter difficulties could be got over by constructing
an anemometer which would measure not quantity of
flow like the ordinary anemometer, but actual rate of
flow. This could be done by registering the actual
speed of revolution of the anemometer spindle. It
appeared to him that one of the simplest methods of

doing that would be to attach to the anemometer spindle
a small armature revolving in the field of a permanent
magnet. The electric current thus set up could be
measured by an ordinary voltmeter, and as the voltage
would be proportional to the rate of revolution, the
velocity of the air current could be directly read off on
a voltmeter suitably calibrated. Furthermore, the
voltmeter could be connected with the anemometer by
leads of any desired length, so that the observer could
be stationed sufficiently far away from the instrument
for his body not to affect the air current, or, if desired,
the voltmeter might be placed at bank—e.g., in the
manager’s office, or in the engine-room, or in both
places. Above all, it would be easy to obtain a con-
tinuous record from such an instrument. Recording
anemometers were largely used in Germany; one of the
favourite methods there appeared to be to furnish the
anemometer with a contact maker which transmitted an
electric current after the anemometer had made a
definite number of revolutions. This method was quite
satisfactory, but it was not so easy to read as the
arrangement suggested by himself.

At this stage Prof. Louis introduced to the members
his own anemometer, the working of which he demon-
strated.

One of the most useful instruments for reading off
directly the air current, said Prof. Louis, was to be
found in the Pitot tube, which was largely used both on

TRANSVERSE SECTION

3.

V

Prof. Louis’ New Indicating Anemometer.

the Continent and in America, though but rarely seen
over here. Darcy modified the Pitot tube by combining
it with a water-gauge in such a manner that the water-
gauge read directly the difference between above sum
and the static pressure, so that the pressure due to the

velocity head alone was read off on the water-gauge.
The form of Darcy tube proposed by Dr. Brabee was a
very convenient one for practical purposes ; he had so
shaped the tube that the co-efficient K = 1, so that for
ordinary temperatures and pressures they could use
the formula in its simplest form, and could determine
the velocity (V) of flow of air in feet per second from the
water-gauge h in inches from the formula V = 66 >Jh.
It was easy to employ a water-gauge graduated in
accordance with this formula, so that the velocities could

be read off directly without calculation. The well- taper in the opposite direction, so as to convert the entire
known corrections must be introduced for variations in drift into a Venturi meter, the difference of pressure

temperature and pressure, if great accuracy was
required ; but for ordinary mining conditions these
could quite well be neglected. The only objection to
this method lay in the fact that the graduations were
very small for low velocities, and it was advisable to use
some specially sensitive form of water-gauge in order to
read them accurately. There were several such gauges
on the market, especially in Germany, where they were
sold under the name of “ micromanometers.” A favourite
form was the inclined tube water-gauge, and another
was the well-known differential water-gauge, a con-
venient form of which was manufactured in this eountry

by Casella. Instruments on the aneroid principle and
on the diving bell principle had also been made, the two
latter being especially convenient for registering
purposes so that a continuous record of the quantity
of air flowing in any given airway could thus be obtained.

A still greater improvement was produced by com-
bining a reversed Pitot tube with an ordinary one.
This produced a more open scale, which was easier to
read. A convenient form of this tube was that due to
Kreil, known as the Kreil pneumometer, which was
largely used in Germany, and appeared to give very
reliable results. Obviously any form of delicate water-
gauge, preferably registering, might be used in combina-
tion with this modification of the Pitot tube as readily
as with the other form.

A novel method for measuring the velocity of air had
quite recently been proposed by several electricians, one
of the most recent applications being by Prof. J. T.
Morris, of London. As far as he (Prof. Louis) knew, it
had not yet been employed in mines, but he saw no
reason why it should not be adapted for that purpose
also. If an electric current was sent through two
exactly similar and similarly-situated wires, each would
be heated by it to the same extent, and each would
therefore show the same increase of electrical resistance

due to such heating. If now one of the wires be exposed
to an air-current it would thereby be cooled, and its
electrical resistance would be proportionately decreased.

It had been proved that the cooling effect was
proportional to the rapidity with which the air flowed
past the wire, hence the difference between the electrical
resistance of two similar wires, one of which was exposed
to the air-current, whilst the other was shielded from

it, was a measure of the velocity of the air-current. It
was certainly advisable that this new method should
undergo an exhaustive series of experiments in the pit,
as it might be a question to what extent the presence of
coaldust in the air might interfere with its accuracy.

Another totally different method, which, as far as he
knew had never been applied in mining work, was to
measure the flow of air in an airway as a whole. For
example, if a fan drift be contracted slightly with a
gradual taper facing the flow of air, and a more abrupt

between the air in the wide and narrow portions of the
drift could be measured by a sensitive water-gauge,
which would thus give a measure of the quantity of air
flowing, and this could be continuously recorded in
exactly the same way that the flow of water in the mains
of waterworks was continuously recorded by the Venturi
meter, this being quite a common practice. In practice
there would be no sensible error in assuming that the
quantity of air flowing varied very nearly as the square
root of the difference of pressure, and there would be
no practical difficulty in graduating the differential
water-gauge so as to read off directly velocities or