May 1, 1914.

THE COLLIERY GUARDIAN.

955

The experiments were carried out in such a manner
that either the bell or the signal wires, by the short-
circuiting of which the bell could be caused to ring,
could be enclosed in a chamber containing an explosive
mixture of methane and air. It was found, that ignition
at the bell, due to the maintained spark at the spring
contact, could be obtained with a battery of five cells,
the current when the circuit was closed being 0'70
amperes, and the voltage across the terminals of the
bell being 7'5. Of more importance to determine was
the number of cells required to cause ignition by the
flash produced by short-circuiting the signal wires to
give a signal. With only the resistance due to the bell
(10 ohms) in the circuit, a battery of three cells was
sufficient to give a break-flash capable of igniting the
most sensitive methane-air mixtures (containing from
8 to 9'5 per cent, of methane).

To allow for the resistance afforded by the long
lengths of signalling wire usedin practice, non-inductive
resistance coils were inserted, as shown in the series of
experiments below, in which a mixture of methane and
air (containing 8'2 per cent, of methane) was employed.

Resistance in circuit.	No. of cells required for ignition by break-flash.	Current on closed circuit, amperes.	Voltage on open circuit.
Bell only (10 ohms)	3	0'45	4’5
Bell + 10 ohms		5	0 40	7 5
Bell + 20 ohms		7	0 35	105
Bell + 30 ohms		11	0'40	16'5
Bell + 50 ohms		15	035	225
A resistance of about 13 ohms is given by			one mile of

galvanised iron signalling wire of No. 8 gauge.

The series of experiments just described were made,
as already stated, with an 8'2 per cent, methane-air
mixture, which is one of the mixtures most readily
ignited. Mixtures containing smaller percentages of
methane are less easily ignited. The effect of methane-
content on the ease of ignition is better shown when
either the voltage or the amperage of the current is kept
constant. In the accompanying diagram (fig. 11) are given
the voltages required to cause the ignition of different
methane-air mixtures by the break-flash when the current
passing through the bell was kept constant at 0'60
ampere by the insertion of sufficient (non-inductive)
resistance. No ignition up to the highest voltage
available (80 volts) could be obtained of either the
lower or the higher-limit mixture of methane and air.

As Prof. Thornton has pointed out, “ with low-
voltage apparatus the inductance of the circuit is of the
first importance from the point of view of safety at
break.” This is illustrated by comparative tests made
with the “ Senghenydd ” bell and that lent by Prof.
Thornton (bell “ A ”), the inductances of which, accord-
ing to determinations kindly made by Prof. Thornton,
were :—“ Senghenydd ” bell: Mean inductance at all
currents 0'344 henry ; Bell “ A ” : Mean inductance at
all currents 0135 henry. Whereas with the “ Seng-
henydd ” bell in circuit a battery of three dry cells was
required to cause the ignition of an 8 per cent, methane-
air mixture by the break-flash, with bell “ A ” five cells
were required, the currents being 0'45 ampere at 4 5
volts and 0'7 ampere at 7'5 volts respectively. Both
bells had the same resistance, namely, 10 ohms.

The influence of amperage can be appreciated from
an examination of the series of experiments with an 8'2
per cent, methane-air mixture, using the “ Senghenydd ”
bell, in which resistances of different values were
included in the circuit. It would appear that if the
current flowing round the signalling circuit could be
reduced below 0'30 ampere by introducing suitable
non-inductive resistances, there would be but little risk
of ignition by break-flashes below 25 volts pressure.
The use of Leclanche cells, therefore, owing to their
high internal resistance, should afford greater compara-
tive safety than the use of dry batteries. Comparative
experiments illustrative of this were made using an 8'2
per cent, methane-air mixture and the “ Senghenydd ”
bell, the results being as follow :—

No. of cells
required Current on
for ignition closed circuit,
by break-flash.

Dry cells.............. 3	...	0'45

Leclancl e cells ...... 6	...	0'50

Whilst with bell “ A ” the results were :—

Dry cells.............. 5	...	0'70

Leclanche cells....	9	...	0 60

Voltage.

45

90

7'5

13'5

The opportunity arose of testing a device of Prof.
Thornton’s, intended to prevent sparking at the signal
wires and at the bell. The bell used was bell “ A,”
already referred to, and the circuit was modified by the
addition of two circuits termed respectively “ Simplex ”
and “ Duplex,” which could be connected to the bell
separately or together. The tests with bell “ A ” alone
have already been given. The effect of adding the special
circuits was as follows:—

Ignition op an 8 pbb cent. Methane-air Mixture

& Amperes.

S o

“ V ° tn
£.2 2

O'H

o 5/S ?

Bell + “ simplex ” circuits	15	... 2 225

Bell + “ simplex ” and

“duplex” circuits......	15	... 2 275

o

>

22'5

22 5

The above results were obtained when a battery of dry
cells was used. When Leclanche cells were employed
no ignition of an 8'5 per cent, methane-air mixture could
be effected with the maximum number of cells available
■—namely, 24 (current on closed circuit 1 ampere, voltage
34'5). The addition of either the “ simplex ” or the
“ duplex ” circuit practically eliminated sparking at the
bell.

The preliminary experiments were made with an
explosion chamber of about 1 cubic foot capacity, in
which either the bell or the signalling contacts could
be placed. The last-named consisted of two steel rods
of J in. diameter, one fixed firmly in position across the
box and the other running parallel with it for a few
inches and then turned at right angles. This bent rod
could be moved round or backwards and forwards by
band, so that a rubbing contact or a rapid make and
break of circuit could be made. For the later compara-
tive experiments a mechanical device for making and
breaking circuit in an explosive mixture was employed,
as being more convenient and giving greater uniformity
of results. The results obtained by the two methods
of experiment were the same; but whereas with the
hand-breaks some 30 or 40 contacts had to be made at the
limiting voltages before ignition occurred—the nature
of the break-flash depending greatly on the rapidity
with which contact was made and broken, and on
whether the break at the signal wires synchronised with
the break at the bell—with the mechanical break
ignition was obtained at the limiting voltages at the
first (or sometimes at the second) break. The apparatus
is shown in the photographs in figs. 12 and 13. It
consisted of a gastight wooden box of about half a cubic
foot capacity, fitted with a glass window and having
one end removed. This end was covered by a sheet of
oiled paper and a measured volume of methane was
passed in from a gasholder, displacing air. The mixture
was then made by rapidly revolving a small motor-driven
fan for a given time, and a sample drawn out over
mercury into the glass vessel shown on the right of the
photograph, whence it was transferred to an apparatus
for explosion analysis. The methane used was prepared
from aluminium carbide and purified by passing through
ammoniacal cuprous chloride (to remove traces of acety-
lene) and over “ oxidised ” palladium precipitate heated at
212 degs. F. (to remove traces of hydrogen). Analysis

--LI

Fig. 1.—External View of Drum.





showed it to contain 98 per cent, of methane and 2 per
cent, of nitrogen. The mechanical contacts and breaks
were made by slowly revolving (by means of a motor
and gearing) a turntable carrying four metal rods, each
of which just touched the end of a strip of metal fixed
at right angles to them. For the majority of the
experiments the contacts were steel and steel, but a
comparative series was also made with zinc-zinc contacts
without any difference being obtained in the results.

[Extracts from the reports of Messrs. Robert Smillie
and Evan Williams will be given next week.]

A meeting of the central executive committee of
the Employers’ Parliamentary Association was held in
Manchester on April 23. Sir Charles W. Macara, Bart.,
president of the association, was in the chair. After a
full discussion of the effects of the operation of the
National Insurance Act, it was unanimously decided that
a letter should be forwarded forthwith to the members of
the association regarding the appointment of a non-party
commission of experts to enquire into the incidence of
taxation under the Act. With regard to the Trades
Disputes Act the following resolution was adopted :—
“ That this association, being entirely in accord with the
view expressed by 13 members of the Industrial Council,
who in an addenda to the report of the recent enquiry into
industrial agreements recorded their opinion that an
enquiry should be held into the effects of the Trades
Disputes Act, 1906, urges upon the Government the
desirability of adopting this recommendation without
delay.” Some discussion took place on the fencing and
safety precautions for transmission machinery, and it was
resolved that the secretary be instructed to request the
Board of Trade to extend the present distribution of
reports on matters relating to this subject, and to others of
general interest to employers to the Employers’ Associa-
tions included in the Board of Trade’s industrial directory.
The secretary reported that during the first quarter of the
year the membership of the association had been augmented
by the addition of nine further associations of employers.

AN INTERNALLY-DRIVEN COMPRESSED-AIR
HAULAGE.

(By an Engineering Correspondent.)

A novel and interesting type of haulage drum has
been recently introduced by Messrs. David Ashton and
Co., of Sheffield. The drum is driven by a rotating
compressed-air engine consisting of three single-acting
cylinders working on to a fixed crank pin and entirely
enclosed within the drum.

Fig. 1 shows an external view of the drum, from which
it will be seen that the only external parts are the
starting and reversing lever, the friction clutch, and the
hand brake. The interior of the drum forms an oil
chamber, in which the cylinders and gearing are con-
tained and run in a bath of oil. A slight positive
pressure of air is maintained within the drum, which
discharges outwards, so that no dust or grit can enter.

Fig. 2 shows a sectional elevation of the drum. A fixed
crank shaft contains two air passages, A and B, com-
municating with the air chest C through the slide valve
D. This valve is only used for reversing or stopping the
engine, and is controlled through a rack and pinion by
the control lever. When the valve is in its upper
position, B is the air-inlet passage and A the exhaust,
and this is reversed when the valve is brought to its
lowest position.

Two ports shown in section in fig. 3 form a communi-
cation between the passages A and B and the passages E
attached to and revolving with each cylinder. The
flaps F F1 carry out an important function. If the

passage A is acting as inlet, the air pressure keeps the
flap F in the position shown in fig. 3, and the cylinders
rotating in the direction of the arrow, air is cut off at about
two-thirds stroke. The flap F1, however, partially covering
the exhaust passage B automatically falls and allows
exhaust to continue throughout the stroke. On reversing
the engine the flap F falls and F1 rises, giving the
cut-off on the admission stroke.

The eccentrically-fixed crank pin G is surrounded by
a bronze bush, on which abut the steel ends of the
connecting rods, which are in the form of the arc of a
circle. The bush is fixed to one of the rods, and the
other rods have a slight relative movement upon it.
Straps H are fixed to another connecting rod, and hold
all the rods up to the bush.

The cylinders are carried between two discs mounted
on the crank shaft on ball bearings ; on the boss of the
right-hand disc is fixed a phosphor bronze pinion J
gearing into a pinion K, which latter is mounted in ball
bearings on an arm attached to a sleeve L. The sleeve
L surrounds the fixed crank shaft and on it is keyed
the clutch pulley M, which can be held or released by a
friction band operated by a hand wheel on the right
side of the drum. When the sleeve L is held by the
clutch, the motion of the pinion K is transmitted to an
internal gear wheel mounted on the inner side of the
drum side P through the pinion N, thus revolving the
drum through a double-reduction gear.

When the clutch M is released the sleeve L revolves
with the engine, the pinion N runs idly around the
internal gear and the engine and drum can rotate
independently. This arrangement allows the drum to
run freely when lowering down an incline. All the
gears are of steel and machine-cut, except the pinion J,
which is of phosphor bronze machine-cut.

The engine can be easily removed by taking off the