1182

THE COLLIERY GUARDIAN.

December A, 1914.

Leaving the causes of deterioration due to direct
attack, and turning to secondary causes, it is obvious,
and, moreover, well known, that the susceptibility of
alkaline attack must have a great influence under con-
tinuous current fault conditions which can produce
electrolytic action, because in most soil conditions it is
possible not only to form alkaline substances at parts
which constitute the virtual negative electrode of the
leakage circuit, but to force such substances by osmotic
pressure through the dielectric to the negative conductor.
Under these circumstances both the direct action on the
vulcanised bitumen of concentrated alkaline substances
and the aggravated conditions due to the passage of
current (producing continuous supplies of deleterious
matter and forcing them to the best position for attack)
come into play.

Details are given of experiments to test the source
of current producing saponification effects on negative
cables. From the results of these experiments it would
appear that a dielectric must first be rendered conduct-
ing, for example, by osmotic influences, incipient flaws,
damages, or general weakness, before it will allow suffi-
cient current to pass through it to form electrolytic pro-
ducts in its vicinity.

The author arrived at the conclusion several years ago
that there existed a distinct type of deterioration
different from the saponification effect. The evidence
came first in a very curious form. A fault had occurred
on a length of 37/13’s high-tension three-core solid
bitumen armoured cable working at 2,000 volts three-
phase installed in a pit shaft. ' The shaft was full of
steam, and the temperature varied between 75 degs. and
100 degs. Fahr. No burn-out had occurred, but on one
phase a leakage to earth of about 20 amperes had been
noted. The resistance of the fault was too high for it
to be located by a loop test, and in attempting to locate
it under working conditions by tracing the fall of poten-
tial in the armour, a heated area a few feet in length
was discovered. At this part a length of about 20 yds.
was cut out for investigation. When cold the resistance
of the fault was over 20 megohms, and there was no
trace of mechanical damage to the cable, or of an arc
having been at any time set up between the faulty core
and the armour. Dissection of the interior part of the
cable revealed an extraordinary condition of affairs, in
that each of the conductors was found to have been dis-
placed in an outward direction, one of them to a
greater extent than the other two. The normal ten-
dency of the conductors in a three-core cable is, of
course, to crush together, and some extraordinary con-
ditions must have arisen in the shaft to bring about this
peculiar distortion. This conclusion was confirmed
shortly after by the total collapse of the shaft, which
was a very old one. It was found that the softening
followed the faulty conductor closely, and also that it
tailed off as the normal position of the conductors was
reverted to. The coincidence of the softening with the
faulty core is important.

The factors which evidently produced the deteriora-
tion in this case were heat, current, and possibly
moisture, in conjunction, of course, with an unknown
time element. Prolonged heating by ordinary means
would not by itself permanently soften the vulcanised
bitumen to the consistency found in the softer parts.
Although the heat had been produced by the current in
this case, it was fairly evident from the coincidence
above referred to that the current had some effect other
than I2R heating.*

Chemical examination showed that there was an
absence of the usual formation of free alkali in the
fibrous wrappings of the cable, and the softened bitumen
showed no trace of saponification. This was not sur-
prising, because from the method of installation the
moisture contained in the tapes and jute bedding was
not likely to contain such substances in solution as
would usually be found in the vicinity of a cable laid in
the ground or drawn into ducts. These instances, there-
fore, afford a confirmation that softening effects on
bitumen cables could be produced which were appar-
ently physically similar but chemically different, and
brought about by means different from the commonly
known saponification effects. It may be added that
analysis did not reveal that anything of the character
of de-vulcanisation had occurred.

Considerable addition to the knowledge of facts was
made as the result of some investigations which the
author had the opportunity of making some time later
during a trip to the East. The first interesting feature
was that the softening was in no case found to affect a
whole length of cable. In some cases only a matter of
inches in length of deteriorated bitumen separated parts
which were -normal; in other cases yard after yard w7as
found to be affected, often more on one side of a cable
than on the other. This clearly showed that variations
in material could have no relation to the trouble.
Another interesting feature was that the local softening
occurred indiscriminately on positive, negative, and
neutral cables. In some cases badly deteriorated cables
were found adjacent to normal ones in the same trough
or feeder pillar, apparently subjected to the same con-
ditions and influences, showing that climatic influences
per se could not be responsible for the trouble. Soils,
waters, temperatures, insects, equipotential points and
zones on networks, geographical relationships to
generating or distributing centres, and other conceiv-
able factors were examined, with the object of deducing
some positive or negative conclusions. Eventually it
was found :—

(1)	That leakage current played a prominent part in

* Since the date of this investigation (1911) two instances
of similar softening effects at or near faults have come to
the author’s notice on unarmoured single-core cables fixed in
grooved wooden casing, and used on high-tension three-
phase circuits. In these cases I2R heating could not have
been an appreciable factor on account of the high resistance
of the faults.

bringing about softening of the vulcanised bitumen, and
that such currents even of a very minute order would
produce a type of softening other than that caused by
saponification in the well-known manner;

(2)	That moisture must have been a factor in the
case, although it was not proved that it had any other
function than that of rendering fibrous covering and
other paths conductive. The relation of the distribu-
tion of stains in tape coverings to that of softening of
the vulcanised bitumen is confirmatory evidence of its
presence;

(3)	It appeared likely that, in the presence of the two
preceding factors, this particular form of softening
occurred more readily in tropical than in temperate
climates, and that temperature was therefore also a
factor;

(4)	Time is obviously important in all changes of
the kind under consideration. The results therefore
confirmed the tentative conclusions drawn.

The results of a number of preliminary experiments
showed that the softening could be produced by the
combined action of heat and moisture. Also that the
time element, which even under faulty electrical con-
ditions might be years, could be’reduced to weeks or
even hours by varying the temperature. In the light
of experimental reproduction of the softening effects
by the combined agency of heat and moisture (steam),
hydrolysis of the glycerides of the fatty and hydroxy
acids contained in the vulcanised bitumen was clearly
indicated. The results of analysis of the softened
material, and a comparison thereof with the results
obtained with the original vulcanised bitumen, confirmed
this indication. The process of softening, however, was
found to be accompanied by an increase in the amount
of free acid, roughly in proportion to the degree of
softening, the amount of acid being approximately
doubled in the case of moderately softened material, and
trebled in cases of considerable softening.

After much experimental work it was found that high-
grade vulcanised rubber could be treated in such a
manner that it would fulfil the requirements as regards
incorporation with vulcanised bitumen, and would, in
addition, exert a surprising degree of protection against
hydrolytic and selective action, even when used in

Fig. 1.—Sectional Elevation of
Gas Detector (half size).

small proportions. In fact, there was apparently some
kind of mutual protective action, because the resistance*
to softening under the steam pressure conditions pre-
viously described was greater in the combined substance
than in either of its components. Remembering that
the softening action only occurs under faulty conditions,
and requires five or six years to develop to a troublesome
stage, the protection afforded by 5 to 10 per cent, of
the rubber substance would appear to be ample under
the worst conditions of practice.

The depressed state of the iron trade was discussed by
Mr. C. E. Rhodes on Friday, the 27th ult., at the meeting
of the shareholders of the Midland Iron Company, at
Rotherham. The iron trade, he said, had been unsatis-
factory throughout the year. Not only had prices shown a
continuous decrease—the total drop being Ils. 6d. as
compared with the year before—but in addition the output
was down 20 per cent, as against 1913, and 33 per cent*
three years ago. Notwithstanding these facts the costs
were practically the same as for the previous year, although
they had had to contend with an increase of Is. per ton in
the price of coal, owing to the local stoppage and the
necessity for going outside for fuel. Further, the standing '
charges were up considerably, due to State insurance, which
had involved an additional expenditure of something like
<£325. This, with the compensation fund, which last year
stood at <£620, meant that altogether, as compared with five
years ago, they were now paying an increase of close on
<£800 a year. The iron trade was in a very depressed state
at the present time, and orders were very difficult to obtain.
They had a scheme for the electrification of the works in
hand, but owing to the war coming forward they did not
see their way to proceed with it.

A FIREDAMP INDICATOR.*

By H. R. Webster.

The writer presents a description of a gas detector
which has engaged his attention for a considerable time.
In the first place, he wishes to acknowledge his indebted-
ness to Mr. Walter Hargreaves, the president of the
institute, for assistance and for the benefit of that
gentleman’s experience in furthering the writer’s efforts
to produce a practical instrument.

Any gas detector to be of service must be sufficiently
sensitive to indicate percentages very much below the
explosive limit of about 5 per cent., as 3 per cent, of
gas or less might under certain conditions be dangerous.
The writer has endeavoured in his experimental work to
produce an instrument sensitive down to 0-25 per cent.,
with a maximum reading of 6 per cent. The principle
of this instrument is based on that of the diffusion of
gases, sometimes termed “ osmosis.” There is nothing
new in applying that law; it has already been tried many-
times by inventors, but without success. The writer’s
first visit to a pit with an instrument of this kind, which
worked admirably in the laboratory, convinced him how
useless was such a device underground, unless the
peculiar effects of the pit air on the instrument could
be totally eliminated. Such unforeseen difficulties as
variations in temperature and pressure, humidity of
the air, etc., all influenced the instrument, but worst
of all were the excessive draughts met with underground
at velocities of 1,400 to 1,500 ft. per minute.

The instrument (figs. 1 and 2) consists of a porous pot
mounted on a flange. The interior of the pot is closed
at the bottom by a sensitive flexible diaphragm, whilst
at the top it is closed by a cover, on which is mounted
an automatic valve communicating with the interior of
the pot. The flange is supported on columns enclosing
a glass cylinder and the graduated instrument, which
latter is operated by the diaphragm. A porous non-
conducting and drying material is placed around the
pot, which material, in turn, is completely enclosed by
a protective gauze or gauzes. If two gauzes are used,
the space between them may be filled up with an absor-
bent for carbon dioxide. The whole of the upper portion
is then covered by an airtight cover, which as it is
screwed on the flange operates the valve, and, as will
be seen later, brings the instrument to zero. The action
of the instrument is as follows :—The instrument, con-
taining pure air, is taken to the place where gas is sus-
pected, the cover is taken off, and the maximum reading
noted. This occurs because the light methane diffuses
more quickly than the air comes out, and consequently
a pressure is set up which deflects the diaphragm. The
valve is then pressed, and the cover put on, whilst the
whole instrument is kept stationary. This ensures that
the gas in the pot is of the same composition as the gas
in the air.

Supposing that the reading was 2 per cent., then the

Fig. 2.—Plan of Gas
Detector (half size).

mixture in the instrument is a 2 per cent, mixture,
although the hand of the indicator is standing at zero.
If the instrument is now taken into a 3 per cent, mix-
ture, that is, 1 per cent, stronger than that in the pot,
when the cover is removed the indicating hand will
move to 1 per cent., showing that the gas present is
(2 + 1 = ) 3 per cent. By this method it is not abso-
lutely necessary to clean the instrument out thoroughly
with pure air after each test. It is, however, recom-
mended that whenever possible the instrument should
be filled with pure air, or air of known composition.

The protective layer of non-conducting material has
a remarkable effect, for it prevents any sudden fluctua-
tion in temperature from affecting the reading of the
instrument. With the cover on, the instrument can be
taken from a temperature of 30 degs. to one of 130 degs.
Fahr, without the slightest influence on the hand. The
gauze or gauzes assist in the breaking up of any draught,
which becomes entirely destroyed by the porous non-
conducting material. The latter also prevents any
foreign matter, such as coal, stone dust, etc., from
reaching the pores of the pot. The valve automatically
releases any pressure in the pot, and keeps the
diaphragm in a neutral position.

There is one point on which, so far, no stress has been
laid. Doubtless, it may be asked what effect other
gases found in the atmosphere of pits would have on the
readings of this instrument. The only one of import-
ance, namely, carbon dioxide (carbonic acid gas), might,
of course, be absorbed, and the instrument would be
quite unaffected by its presence. Its greater density
would be a set-off to the lesser density of the methane,
and to that extent the explosive properties of the latter
would be proportionately diminished. The instrument
when used at the surface in the return air for the esti-
mation of small percentages of gas, could with advantage

* From a paper read before the Midland Institute of
Mining. Civil and Mechanical Engineers.