October 16, 1914. THE COLLIERY GUARDIAN. 833 A Portable Electrical Gas=Detecting Device for Use with Miners’ Lamps.* By GEORGE J. RALPH. In October 1911 a similar device was brought by the author to the notice of the institute. The device then shown,f while acting in a very satisfactory way with most of the various inflammable gases, was a failure in detecting firedamp, as many other electrical devices have been. This failure was due to somewhat peculiar phenomena attending the combustion of firedamp, which were unknown to the author, and possibly to many others, at that time. Further experiments have now been made, which have proved successful up to a certain point. In the case of an initially-heated platinum wire, one may measure or estimate its change in brightness, length, temperature, or electrical resistance. In the device before the members, the alteration in the elec- trical resistance of the wire is estimated by comparing it with a similar wire sealed up in such a manner that the surrounding gassy atmosphere cannot act upon it. In the gas detector exhibited, the galvanometer, or voltmeter, whichever one prefers to style it, is of the moving coil pattern, which type of electrical instru- ment, as is well known, is the least affected by external magnetic disturbances. Obviously, a very long pointer cannot be used to indicate the very small angular move- ment of the galvanometer needle or coil and the instru- ment kept small enough to go easily into the pocket; while, on the other hand, if a short pointer be employed, the divisions on the scale would be so small that it would be very difficult to read them with accuracy. Mai Fig. 1.—Gas-detector. and Lamp Separate. Instead, therefore, of attempting to read the magni- tude of movement of the galvanometer pointer, the bridge circuit is so arranged that when the pointer shows a deflection it may be brought back to zero by moving a sliding contact on a wire that forms part of the circuit. It is the magnitude of the movement of this sliding contact necessary to bring the galvanometer pointer to zero that is read, and this acts, as it were, as a multi- plier to the length of the pointer. In other words, if the pointer of the galvanometer is 3 in. long, and moves T’s in. from zero, the amount of movement of the sliding contact may be as much as J in., or eight times as great, equivalent to a pointer (8x3 = ) 24 in. long. In order to economise current, it is essential to use wire of extremely small diameter for comparison of the resistances, and this in turn necessitates the use of very short lengths of wire, if the detector is to be used in conjunction with a 4-volt battery. For use on a 2-volt circuit, the length must obviously be still less if the diameter of the wire and its temperature are to remain approximately the same as on the 4-volt circuit. The temperature at which the active wire gives the best result is about a dull red heat. This temperature is considerably below the ignition point of firedamp or methane. There is the further advan- tage in working at this comparatively low temperature, that the wire will last a very long time without burning out, and there will be little or no permanent change in its resistance. Fig. 2 is a diagrammatic plan of the arrangement of the device. A is a battery, D, E, F, and G being the four arms of a Wheatstone bridge; H is a wire of low resistance compared with any of the four arms; and J is an electrical measuring instrument. D and F are connected together at one end to one pole of the bat- tery, and E and G are likewise connected together to the other pole of the battery, when it is desired to take a reading, by means of a spring key or switch P. The * From a paper read before the North of England Institute of Mining and Mechanical Engineers. f “ The Holmes-Ralph Gas-detecting Portable Electric Lamp,” by George J. Ralph, Trans. Inst M.E., 1911, vol. xlii., p. 201. free ends of F and G are connected together to one terminal of the galvanometer J. The free ends of D and E are connected together through the low resist- ance wire H. The other terminal of the galvanometer J is connected to some part of the wire PI through the contact arm K, which can be partly rotated or made to slide along the wire H. Assuming that under normal conditions (that is, when no inflammable gas is present in the air) the electrical resistance of D equals E and F equals G, then the con- tact K must be resting on a point of the wire H exactly midway between D and E. Let it be assumed that D is the platinum wire exposed to the atmosphere, and that E is a similar wire shielded from the atmo- sphere, while F and G are not necessarily made of a material that is affected by the state of the atmosphere, and remain equal in resistance to one another under all conditions. If, owing to the presence of some hydrocarbon gas, the resistance of D increases, there will no longer be a state of equilibrium in the circuit, and a certain amount of current will pass through the galvanometer, causing the pointer to show a deflection. By sliding the contact arm K away from the central position towards D, a position can be found which will restore the balance and allow no current to pass through the galvanometer, which consequently returns to the zero position. The new position of the arm K is then noted, and K can. be moved back to the central position ready for the next reading. The positions which K must occupy in order to bring the galvano- meter needle to zero are carefully marked when D is surrounded by air containing various measured per- centages of gas. In the actual apparatus D and E are contained in the projecting portion at the bottom of the instrument, the circular cover of which has openings covered with fine wire gauze, and in the centre of which is the spring- contact P. F and G, the galvanometer J, the slide wire H, and contact arm K are contained in the case. K is rotated by means of the knob, a pointer attached to which indicates on the outside calibrated dial the position of K inside the case, while the movement of the galvanometer pointer is observed through the glass covered aperture at the top of the ease. The present form of the device is adapted either for use with a small separate battery of its own, or for use in conjunction with an electric lamp, whichever may eventually prove the more convenient. There is every indication that the device will meet technical requirements, and that it can be produced at such a price as will render it commercially practicable for adoption by mining officials, deputies, and firemen. It may be pointed out that the instrument brought before the institute in 1911 required an 8-volt battery, whereas the present device is adapted to work on a 4-volt battery, and if necessary can be made to work on a 2-6-volt alkaline battery. It may be possible to adapt it to a 2-volt lead cell. Its weight is about 1 lb. without a battery. There is no risk attending its use, G Fig. 2.—Diagram showing Arrangement of Gas- detector. for the temperature of the exposed platinum wire is very considerably below the inflammation point of methane, and the spring contact is contained within the gauze protected chamber, so that any minute spark which may occur is safeguarded. If used in connection with an electric hand pump, a short length of flexible cord is necessary, together with a special plug, and a socket attached to the lamp bat- tery case. It was a matter which involved a consider- able amount of thought to devise a socket with a locked cap which would be practically unpickable. There is no danger of a spark occurring when the contact plug is being screwed in, as the electric circuit is not com- pleted until the spring key on the detector is pressed. Another difficulty that has to be overcome in appa- ratus of this nature is to compensate it fully for varia- tions of atmospheric temperature and pressure which may occur, and the inevitable variations of voltage when the current is obtained from a battery, the voltage of which must necessarily be greater when fully charged than when nearly discharged. Actual trials have been carried out in a mine, and also with natural pit gas from Cymmer Colliery, South Wales, as well as with artificially prepared methane, all the results of which agree. LETTERS TO THE EDITORS. The Editors are not responsible either for the statements made, or the opinions expressed by correspondents. All communications must be authenticated by the name and address of the sender, whether for publication or not. No notice can be taken of anonymous communications. As replies to questions are only given by way of published answers to correspondents, and not by letter, stamped addressed envelopes are not required to be sent. ELECTRIC SAFETY LAMPS COMPARED WITH OIL LAMPS. Sirs,—With reference to Mr. Hailwood’s recent lec- ture on the above interesting subject, will you allow me, as a man who has been employed in coal pits for 16 years, working up from the coal face to be road- man, fireman (deputy), overman, etc., and who has had a large experience of oil lamps of various makes (including Hailwood’s “ wonder lamp ”), and electric lamps, to deal briefly with the matter from a practical point of view? I admire Mr. Hailwood for his pluck in giving to an assembly of experienced mining men such a lot of “ theoretical piffle,” which no doubt would have been duly dealt with at once had there been time for a dis- cussion. Space, of course, prevents me from replying fully, but I should like to deal with the following points :—■ In the first place, I would say that the lecture would have carried more weight if it had been delivered by an independent expert instead of by Mr. Hailwood, who was evidently merely “ blowing his own horn,” and could not be expected to say anything bad regarding his own special production, “ Hailwood’s Combustion Tube Lamp.” It is many years ago since a similar production was introduced (i.e., an oil lamp with an additional glass cylinder placed round the flame), and this lamp is now almost out of the running, and I understand that in France also several similar types have been tried and abandoned. Mr. Hailwood talks about the breaking of a bulb causing an explosion. I should like to ask him if the breaking of a glass of a flame lamp could not cause an explosion. I would also ask him : “Is there any auto- matic arrangement on any flame lamp that would extinguish the flame?” Or are we to depend on the blow that the glass gets to extinguish the flame? I should then tell him that I have seen a flame safety lamp still burning after the glass was broken. In the case of the Ceag electric lamp, two separate glasses would have to be broken before an explosive mixture could reach the filament, and by that time, in the case of the Ceag, the filament would have cooled, making it impossible for it to ignite gas. This is effected by the fusing or automatic “ cut out ” arrange- ment in the Ceag lamp. Mr. Hailwood states that he has not been able to carry out further experiments with this lamp, and, having regard to his failure to create an explosion with ‘the Ceag lamp at a recent meeting somewhere in Scotland—previously referred to in the trade papers—I am not surprised that he now leaves this lamp out of his experiments. “ Once bitten twice shy.” Regarding the light-giving powers of Hailwood’s combustion tube lamp, if this lamp is not kept in an absolutely vertical position — which is impossible in every-day practical use — the glass becomes so black (due to the “ drawn ” flame burning against this so- called combustion tube) that I have heard miners say, when describing this lamp after it has burned four or five hours, “ it was just like a candle burning in a black bottle.”’ Mr. Hailwood refers to an explosion at La Haye, Belgium, but with regard to the cause of the explosion we have again to stand or fall by what he “ under- stands ”; at least he gives no data by which to prove his statement. I suppose this is also “ owing to the war. ’ ’ With regard to the lighted taper being waved about in a mixture containing 4| per cent, of explosive gas without causing an explosion, I can only say that I con- sider it very unfortunate that Mr. Hailwood should make such a statement in public. It would be little short of criminal to lead miners to assume that a naked light could be used without grave danger anywhere where there is the slightest risk of finding even minute traces of gas, and any remark which might give the contrary impression should be carefully avoided. As to Mr. Hailwood’s remark about the teaching in schools with regard to heated gauze and ignition of gas being all wrong, I hope that all our eminent mining lecturers will please note. Then his point about the Chief Mines Inspector’s conclusions being groundless makes one wonder whether in his opinion anyone’s view is worth considering—except that of Mr. E. A. Hailwood. I admire the self-confidence which makes him put aside lightly the opinions of men who have served a lifetime in and about mines, and who have carried on scientific and practical investigations in the sole interest of the miners, and to check the great loss of life which still continued in spite of flame “ safety ” lamps. Air. Hailwood has apparently “ weighed these men in the balance and found them wanting ”—truly a sad state of affairs. He states that 6 per cent, of methane would “ enlarge ” the flame of a safety lamp. I should just think it would enlarge the flame—and the lamp, too. Then those two eases (not reported) of “ men being hauled out by their mates.” It must be awful, the underhand work which goes on in the mining world; and with the mines inspectors staff so greatly aug- mented, too. I will only say that it is somewhat strange that Mr. Hailwood never brings proof of his “ facts,” but always refers to “ private information,” or that he “ understands,” etc.