1170 THE COLLIERY GUARDIAN. December, 4, 1914. Investigations into a System of Electric Bell Signalling for Use in Collieries.* By THOMAS G. WATTS, B.Sc. In a report on the ignition of methane and air mixtures by electric bells,f Dr. R. V. Wheeler, chief chemist to the Explosions in Mines Committee, makes the general conclusion that: “If the current flowing round the signalling circuit could be reduced below 030 ampere by introducing suitable non-inductive resistances, there would be 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 comparative safety than the use of dry batteries.” The general method of carrying out the experiments was seen by the writer at Eskmeals by the courtesy of Dr. Wheeler. With a view, not so much of deter- mining the scientific features of electric spark ignition of explosive gaseous mixtures, as to find some safe system of bell signalling for underground use, experi- ments were commenced at the laboratory at the Great Western Colliery Company’s Maritime Coke Works. In order to have an apparatus which, by reason of its size, would enable larger instruments, &c., to be thoroughly tested, a chamber was constructed having the following internal measurements—1 ft. 6 in. x 1 ft. 6 in. x 1 ft. 6 in. It is made throughout of 3-ply facings of walnut fixed to frames of red pine f in. thick. The opening to which the oiled tissue was glued was placed at one end instead of on the top, and was 15| ft. x 14|ft. in area. The base of the chamber was in one part with the ends and sides, and so all clamping was avoided. A sampling burette with platinum igniting wires and two-way cock traversed the base of the chamber. A sketch of this testing chamber is appended. 1 GM Fig. 1.—General View of Explosion Chamber, Great Western Colliery Company’s Maritime Coke Works. The investigations were directed to determining two points, viz.:— (1) The ignition of mixtures by the spark at the contact breaker of the bell. (2) The ignition by the spark at the circuit closing contact, which may be at any point on the line. Explosibility of Coke Oven Gas. The following preliminary experiments were then carried out:— (1) A set of experiments ranging up to about 20 were made by sending into the chamber various measured amounts of coke oven gas, withdrawing samples into the gas burette and noting speed of explosion flash. By this means it was determined that the most explosive mixture of the gas used was 15’7 per cent, gas and 84*3 per cent. air. The coke oven gas was analysed and found to be :—Carbon dioxide, 1*77 per cent.; ethylenes, benzines, etc., 0'64; carbon monoxide, 2T0; oxygen, 0’48 ; hydrogen, 45 50 ; methane, 29’50 ; nitrogen, 20’01 per cent. Attention should be given to the large hydrogen and methane contents, which is a result of the gas being coke oven gas and produced by higher temperature of carbonisation than that of ordinary town gasworks. The high content of hydrogen is the reason of the small percentage of coke oven gas required to form the most explosive mixture, the figures found by Clerk with Glasgow coal gas being 16’6 per cent, gas and 83’4 per cent, air, as compared with the 15’7 per cent, and 84’3 per cent, for oven gas. Using similar methods with methane (supplied by cylinder from the Cymmer colliery), which in the particular cylinder only contained 63 per cent, methane (analysed), it was found that 12’9 per cent, of the gas from the cylinder with 87’1 per cent, air proved to be the most easily ignitible and explosive mixture. This bears out in a striking manner Dr. Wheeler’s experi- ments in that 12’9 per cent, of a 63 per cent, methane is equal to an 812 per cent, mixture of pure methane. Throughout the investigations, therefore, wherever methane air mixtures were used, 12 9 per cent, cylinder gas, or as we may call it 8T2 per cent, methane air mixtures, were used. In the account following, of * From a paper read before the South Wales Institute of Engineers, and published in the Proceedings. t See Reports on The Senghenydd Explosion, Colliery Guardian, May 1, 1914, p. 954. experiments made, the mixture used is stated in terms of pure methane (by calculating from 63 per cent, methane to purity). First Series of Experiments.—To determine the relative ease of ignition of explosive mixtures of oven gas and methane by the spark at the contact breaker of an ordinary unprotected colliery bell, No. 1 experi- ment was repeated five times, and an explosion occurred when the bell was rung with four cells immediately the circuit closed. As far as ignition by spark is concerned we can, therefore, state as a fact that coke oven gas and air mixtures are (so long as they are in explosive proportion) more easily ignited than methane and air mixtures ; and, as a rider from this statement, that any system which proves to be safe in coke oven gas and air mixtures is absolutely safe in methane air mixtures. First Series. No. of No. Explosive mixture. ^cellJ1 in^ Effect. circuit. 1 ...15’7 per cent, coke oven gas... 4 ... Explosion and air. 2 ... 4’91 per cent, methane and air... 10 ... No explosion 3 ... 5’60 >> ••• io 4 ... 6’24 „ „ „ ... 10 • • • 99 5 ... 6-87 „ „ M ... 10 6 ... 7’49 „ „ „ ... 10 7 ... 8’13 ,, ,, ... 10andl2 99 * 8 ... 8’76 „ „ ,, ... io . . . a * 9 ... 9’39 >> » ... io 10 ...10’02 „ „ „ ... 10 • • • 19 Second Series of Experiments.—To determine the relative ease of ignition of explosive mixtures of oven gas and methane by sparks at the point of closing the circuit on the line. Second Series. Colliery bell outside chamber. “Knocking wires” inside. Contact made by rubbing wire with right angle bend against fixed wire. No. of No. Mixture. Result, cells in circuit. 1 ...15’7 percent, coke oven gas... 4 ... Explosion and air. 2 .. 8’12 per cent.methane and air... 9 ... ,, These experiments, therefore, point to the fact that there is a real danger in electric bell signalling under- ground if gas is present to an explosive extent at the point of making and breaking circuit, and that systems of signalling with unprotected bells and the ordinary method of circuit closing with a knife or other tool across galvanised wires is distinctly unsafe in such parts of a mine where gas is liable to occur. Dr. Wheeler’s experiments are sufficiently convincing without the added testimony of other observers, and the adoption of some safe method becomes a necessity. On this assumption (and well founded by experiment) attempts were then made to find some system which was safe in oven gas and air mixtures, this being a criterion of absolute safety in methane air mixtures. What is a Safe Bell? The first attention was directed to the bell. If by any means gaseous mixtures could be excluded from the contact breaker of the bell by means of an absolutely flame-tight cover, perfect safety in this direction should be attained. A bell of this type has been made by the General Electric Company. The electro-magnets, con- tact breaker, and hammer spring are enclosed in a cast iron case fitted with a cover, which is screwed up to an indiarubber joint between face and face. The motion of the contact breaker is conveyed to the external hammer by means of an attachment passing through a case filled with vaseline, so that the only point where internal motion is conveyed to the exterior (which debars a tight joint) is through a clot of vaseline, which is impervious to the passage of air or gaseous mixtures. Whether or not this vaseline clot, in the high tempera- tures sometimes found underground, would run out and so leave open a passage for gaseous mixtures to enter, remains to be investigated, and further experiments with this type of bell are still to be carried out. Between 10 and 20 experiments, consisting of ringing this bell with from 10 to 14 Leclanche cells, failed to produce the explosion of a 15 7 per cent, gas (oven gas) and air mixture. The order of things was then reversed. “ Knocking wires ” of the type mentioned before were introduced into the circuit with the aforementioned bell and with four cells, and an immediate explosion occurred. With three other similar bells ignition was brought about at the “ knocking wires ” with two cells and three cells respectively. Adopting the principle of the Rhumkoff coil condenser —that is to say, connecting through a condenser the circuit containing the “ make and break ” of the bell in order that the high E.M.F. due to self-inductance (which is distributed mainly along the spark-gap) is imparted to the coats of the condenser, and the energy which would have been spent in the transmission of a spark absorbed thereby—we ought to avoid an ignition of any explosive mixture that might be present. With this arrangement on the same bell as used in the first series of experiments, and which, it will be remembered, ignited the mixture of oven gas and air when rung by four Leclanche cells, it was found that absolutely no ignition of the oven gas and air mixture was brought about, even when the bell was rung with 12 Leclanche cells. We have, therefore, in response to the first query, “ What bell is safe in explosive mixtures ? ” two remedies. One is, as stated above, a flame-tight bell; the second, an ordinary bell fitted with a condensing system across the spark-gap. As to which is the better system several points arise in distinct favour of the condenser-fitted bell:— (1.) No alteration in temperature has the remotest effect on the sparkless condition brought about by the condenser. Temperature rise has not been proved to have any effect on the gas-tight bell, but it is obvious to anyone cognisant of the melting point of vaseline that the ordinary underground temperatures would produce a condition of mobility which in time would result in a running out of the vaseline, and, therefore, an inlet for the mixed gas. (2.) The reduction of the spark to zero means a longer life to the platinum contacts, for these are, beyond the small effect of wear due to constant tapping, the only source of wear in a bell. (3.) Any bell at present in use can be converted into a safe bell by the mere introduction of a condenser into the spark-gap circuit, thus avoiding any outlay except in condensers. (4.) No amount of “ tampering,” as is often practised by the uninitiated, would alter the effectiveness of a condenser-fitted bell, whereas the removal of the outer casing of a gas-tight bell immediately destroys its effectiveness as a protected bell. Having satisfactorily arrived at a method of eliminating the danger arising from the contact breaker spark, attention was then paid to the lines or “ knocking wires.” Knocking Wires. Three bells—one the old type, two others of the closed gas-tight type—were respectively connected up through a variable battery of one to 10 cells outside the chamber and the circuit-closing contact fitted inside the chamber. This circuit-closing contact consisted of one straight No. 8 S.W.G. galvanised wire fixed through one side of the chamber and rigid, the other a piece of No. 8 S.W.G. galvanised wire passing through the same side and bent at right angles inside the chamber, so that by a half- turn it could make contact with the fixed and rigid wire. This contact closed the circuit and rang the bell. The cells were put in circuit one after another. Type of bell. Eesult‘ Ordinary bell ...... 15’7 per cent, gas 4 ... Explosion (coke oven) Gas-tight bell, No. 1... ,, „ ... 3 ... ,, Gas-tight bell, No. 2... ,, „ ... 2 ... ,, To obviate any apparent discrepancy between these and Dr. Wheeler’s results, it should be noted that these are tests in oven gas-air mixtures and not methane air. The colliery bell (ordinary type) was then used as above with a condenser in the contact breaker as previously described, the onjy alteration in result being that five cells were necessary to produce an explosion at the knocking wires. Working on the theory that as a condenser connected across the spark-gap of the bell produces a safe bell, the same effect should be produced by placing a condenser across the “knocking wires,” this was tried externally from the chamber, and it was found that though there was no diminution of spark there was a distinctly characteristic change—the brilliant blue (which is due, no doubt, in some proportion, to volatilisation of zinc from the galvanising) changing to a red, surrounded by a corona of yellow spangles. The same experiment was tried within the chamber, using a 15’7 oven gas-air mixture and No. 2 flame-tight bell. Conditions were arranged as before. Knocking wires of No. 8 S.W.G. galvanised wire—one loose and bent at right angles, the other fixed and rigid. Although with the bell, ignition had occurred with two cells, no ignition, though repeated time after time, could be brought about by t he two galvanised wires making contact with a condenser across them and ten Leclanche cells in the circuit. The spark was vivid and of the characteristics previously mentioned. Apparently, therefore, the introduction of a condenser across the knocking wires had had some peculiar physical influence on the spark; and, theorising, one can only conclude it to be a matter of temperature. Before this could be accepted as a safe arrangement, however, conditions had to be brought more into line with colliery conditions, and so the method of making contact was altered from the rubbing of two galvanised wires in making contact to that of a file making contact on galvanised wire. To bring about this condition, a piece of an ordinary hard steel file was soldered to the loose wire and so arranged that by a half turn of same, the cutting face could be dragged up and down and against the end of the fixed wire. The chamber was again filled up with the explosive mixture and contact made. The variable battery was worked from one cell to two cells. Immediately on the introduction of the third cell explosion occurred. A repeat experiment had a similar result. The condensing system was increased from one micro-farad by gradual increments to ten micro-farads, but on each contact an explosion occurred on three or four cells being introduced into the circuit and the file being used for contact. The file was then removed and the contact made again with simply the galvanised wires, and no explosion occurred with even ten cells in the circuit. An explanation of this phenomenon is worthy of research, but could only be met by work with delicate apparatus such as does not find its way into the ordinary colliery laboratory. As a system of prevention, however, the condenser across the knocking wires is by these results proved to be useless. The Use of Relays. The only other outlet occurring to the mind of the experimenters was the introduction of relays. This system depends upon the completion of the bell circuit by the action of a relay worked by a separate circuit. If, therefore, one can operate a relay by means of so