July 3, 1914. THE COLLIERY GUARDIAN. 21 would appear from this that the limits of inflammability of the paraffin series, and possibly of others, may be controlled by dilution of the most inflammable mixture symmetrically on either side of combustion intermediate between CO and C02. In the case of carbon disulphide and hydrogen there is explosion rather than relatively slow combustion, and the conditions are not the same. Thus while continuous current ignition is easier in mix- tures approaching perfect combustion, with alternating current an excess of combustible gas favours inflamma- tion; and there is no change of type, but a regular fall and rise of the igniting currents between the limits. The alternating current ignition curves for alcohol have the same parabolic form as the paraffins, but, unlike them, have the same minimum current, in this resembling the continuous current paraffin curves. With alternating current arcs the most easily ignited benzene mixture is at 4*5 per cent. The incomplete combustion of benzene in explosion indicated by these results may account for the apparently unavoidable deposit of carbon in engines driven by benzol. The carbon disulphide curve is remarkable for its resemblance to the contin- uous current curve. The ignition of hydrogen by alternating current break- sparks has several interesting features. The lower limit is about 10 per cent. From this the.curve falls as if to a minimum at 42 per cent., the point for perfect com- bustion, and almost midway between the limits usually given of 10 and 72 per cent, when ignition is by high tension sparks. But at 32 per cent, the ignition sud- denly changes type, the curve becomes a straight line passing nearly through the origin and extending to 70 per cent., at which point there is evidence of a more rapid rise of current. There is here a more than usually clear combination of the alternating and continuous types of ignition. Carbon monoxide shows a transition between the types; the continuous current curve between the paraf- fins and hydrogen, the alternating current curve between hydrogen and the symmetrical parabolic curve. Absence of Influence of External Ionisation. Although it would appear from the nature of the results that the influence of the arc is confined to a small volume—-a few molecules thick perhaps—-around it, yet within that space the action must be intense. The effect upon the igniting current of exposing the gas in the explosion vessel to any ionising source such as X-rays or strong ultra-violet light (through quartz win- dows) is too small to observe when there is no platinum present. It was less than 0-5 per cent, with illuminating gas at atmospheric pressure, which containing a large proportion of hydrogen, might be expected to be sensi- tive to ionisation. A glowing static discharge from needle points had also no perceptible influence on ignition. Summary of Alternating Current Results. The difference between the continuous and alternating current results arises from the rapidly-reversing electric field across the spark dragging the electrons or ions from pole to pole and preventing their escape into the gas. Alternating igniting currents have therefore to be much greater than continuous currents at the same voltage. This difference is increased by the approach of the period of the current to the time of duration of the break-spark. The latter varies with the voltage in such a manner that the igniting current remains constant in value over a long range of voltage. The igniting currents of the paraf- fins increase as the order rises, the product of the minimum current for each gas and the percentage at which it occurs being constant. The product of the number of atoms in a molecule of combustible gas and of the above percentage is also constant, so that the number of atoms in unit volume in all of the most inflammable mixtures is the same, The igniting current is most nearly proportional to the number of atoms in the group of one molecule of com- bustible gas and the oxygen required for combustion forming an explosive unit. The paraffin gases, after methane, are most easily ignited at a percentage giving combustion midway between carbon monoxide and dioxide, and both limits of inflammability are symmetri- cal from this point. Benzene is most easily ignited when burning to carbon monoxide, alcohol to carbon dioxide. Carbon disulphide and hydrogen have the forced or con- tinuous current type of ignition. External ionisation which does not itself cause ignition has no measurable influence on the least igniting currents by break-sparks. Conclusion. On the whole it would appear to be probable from the great differences in magnitude and type between con- tinuous and alternating current ignition that some kind of ionisation precedes combustion for the evidence is strong that a relation between electrification and chemi- cal change not unlike that of ordinary ionisation occurs at ignition, caused possibly by the violent collisions which must occur in the gas in contact wtih high-tem- perature sparks. That a gas has a particular tempera- ture of inflammation may mean that ionisation by collision begins at this temperature with absorption of energy from the source or, later, from that liberated by recombining atoms. On this view, when the molecular energy set free by combustion of a molecule exceeds that required for initial ionisation of an adjoining molecule there is true explosion, and since it is probable that all three modes of molecular motion, translation, rotation, and vibration, have part in the process of ionisation by collision, the influence of temperature, which depends only upon the translational energy, may be in certain cases of secondary importance. Official confirmation has been given to a statement that the Standard Collieries, Ynyshir, have been purchased by the United National Collieries Limited. THE WILLIAMS METHANOMETER. An interesting device for indicating the quantity of firedamp present in mines has just been brought to our notice. The merits of this indicator, or “ methano- meter,” which is the invention of Mr. Alfred Williams, have been investigated by Prof. Silvanus Thompson, F.R.S., and Dr. J. Erskine Murray, D.Sc., F.R.S.E., M.I.E.E., and we have been favoured with copies of. their reports. Prof. Silvanus Thompson, in his report, states that the indicator is a simple scientific instrument which indicates the percentage of the methane present. It is based on the principle that platinum black, if placed in an atmosphere containing even a small percentage of methane, experiences a rise in temperature, such rise increasing with the percentage of methane present, but the attempts made hitherto to utilise this phenomenon in a practical way have been nullified by two circum- stances :—'(1) The difficulty - of making a reliable indi- cator of this rise of temperature which will tell the per- centage of gas present; (2) the fact that platinum black when cold is very insensitive to the presence of small percentages. In the invention of Mr. Williams, the first of these difficulties has been overcome by the adoption of a thermo-electrical method of temperature measure- ment, and the second has been met by providing means for producing a preliminary heating of the platinum black. In the instrument arrangements are made for air from the surrounding atmosphere of the mine to pass through a tube in which are placed a pair of differential thermo-electric junctions in series in the circuit of a sensitive galvanometer of the milli-voltmeter type, the index of which moves over a graduated scale. So long as these two junctions remain at the same temperature, whether warm or cold, their actions are in balance, and no movement of the indicating hand of the instrument occurs, so that the readings of the instrument are inde- pendent of any local changes of temperature between one part of the mine and another. Surrounding each of these two junctions is a small bead or capsule of a’ porous stony material, resembling compressed pumice stone, and one of these is impregnated with platinum black. If, then, the air that passes through this tube contains methane, even in small quantities, the impreg- nated bead rises in temperature; so that the action is no longer in balance, but a thermo-electric current is generated which actuates the indicating hand, and makes it move over the scale. Around each of the beads, or imbedded within it, passes a minute loop of platinum wire in the circuit of an auxiliary battery—a single cell —which thus provides a means of preliminary heating. If no firedamp is present the two beads and the two junctions are equally heated, and the action is still balanced. But if firedamp is present the impregnated bead grows warmer than the other, and the difference between their temperatures at once causes the index to move over the scale. The scale may be graduated in milli-volts; but preferably the scale is calibrated so that the percentage of firedamp present is read directly by inspection. The tests made in the laboratory were directed to the two main points :—(1) The possibility of obtaining on the calibrated scale an adequately accurate direct reading of the percentages of gas present; (2) the efficiency with which the sensitiveness of the instrument is increased by providing for the preliminary heating of the beads. The gas employed was collected from a blower in the Cymmer Colliery at Forth, Glamorganshire, and con- veyed to London compressed in a steel cylinder. Samples of this gas diluted with varying amounts of air, so as to have various percentages, large and small, were passed through the tube of the Williams’ instrument, and the indications were observed on the scale of a milli- voltmeter (uncalibrated) of the ordinary type. At each experiment another portion of the same sample was examined in a Haldane apparatus to ascertain chemi- cally the amount of methane actually present. The auxiliary current which provided for the initial heating of the beads was maintained constant throughout this series of experiments. The following table gives the results of a series of observations. In the first column are the values of the observed deflexions, and in the second the corresponding values (each the mean of two determinations) of the percentage of methane as deter- mined by the Haldane apparatus. In the third column are given results of an independent gas analysis by Mr. A. J. Hale, B.Sc., of some samples drawn at the same time. Their close coincidence with the other values obtained demonstrates the degree of reliability of the method :— Deflexion (scale uncalibrated). Percentage Independent analysis. of methane. Percentage of methane. 3 0’65 0’9 9 1T8 1’3 15 1’50 1’5 27 2T1 2’0 35 2’65 2’5 37 3’03 2’6 38 3’21 2’7 39 3’53 3’2 It will be noted that the calibration by the Haldane apparatus makes the figures of percentage of gas pre- sent slightly higher than those of the independent analysis for all percentages over 1-5. In the tests of the second point an explosive mixture containing 2-5 per cent, of methane was used through- out. . The initial heating of the beads was varied by sending through the platinum wires currents raised by stages from one to two amperes, and the rise in thermo- electric effect was ascertained by observing with the milli-voltmeter the corresponding values, in milli-volts, of the resulting electro-motive force. The following table records the results :— Auxiliary current in heating wire. Amperes. 1-00 Thermo electro- motive force. Milli-volts. 0 1-10 0 1-20 0’07 1-30 0’31 1-35 0’68 1’4 1’44 Prof. Silvanus Thompson observes, in conclusion that the very rapid rise in the thermo-electric motive force after the auxiliary current had been raised to 1-2 amperes demonstrates the second point in Mr. Williams’ invention, that the sensitiveness of the indi- cator (using an atmosphere containing a given percen- tage of firedamp) increases with the initial temperature given to the platinum black bead. It will therefore be necessary, in the definitive patterns of instrument when put on the market, to provide means for securing con- stancy in the auxiliary current employed to produce this initial rise of temperature, a condition the fulfilment of which presents no difficulty to the instrument con- structor, and which in no way affects its portability, or its complete independence of changes in the temperature or pressure of the atmosphere in the mine. Constructed as Mr. Williams’ instrument is, it is strong, not liable to derangement, produces within itself neither flame nor spark, and is incapable of originating any explosion when used in the most highly explosive mixtures. As its scale is calibrated by use of actual colliery gases, and not from artificial mixtures of coal gas, the indications may be relied upon. Its readings are not affected by a rise or fall in the barometer. Dr. Erskine-Murray’s conclusions are summarised below :— As regards the sensitiveness of the apparatus to various mixtures of methane, the relation between per- centage of- methane and milli-volts generated is shown by the curve, to obtain which the heating current throughout was kept constant at a suitable value. For mixtures of methane above 3 per cent, the e.m.f. rapidly increases, which no doubt is due to the much greater rate of oxidation on the surface of the platinum black at the higher temperature. As regards the effect of a draught of air or of the mixture upon the readings, it was found that the thermo junctions in an unprotected state are sensitive to currents of air. This renders it necessary to provide for the breaking up of any air cur- rents that may be encountered, so that the heaters may only be in contact with slowly diffusing gases. An arrangement of screening, which was tried experiment- ally, did away with the trouble. The zero keeping properties of the apparatus tested are sufficient for the accuracy required. As the apparatus depends for its action on oxidation, it is obvious that it will indicate the presence of inflam- mable spirit vapours in the surrounding atmosphere. The effect observed in the case of each of the following, viz., commercial petrol, benzine, ether, methylated spirit, chloroform, and paraffin, is shown in the follow- ing table :— Percentage Substance tested. by weight in mixture. Petrol ................ 2*8 Benzine ............... 2’8 Ether ................. 2*3 Methylated spirit... 2*9 Methane................ 2*7 Deflection indicated. 0’40 0’28 0’20 0T4 1’80 The torch supplied was tested for electrical resistance and insulation, thus :—Resistance of thermo couples, 0-18 ohm; insulation resistance of thermo couples to brass head, 126 megs.; insulation resistance of heaters to brass head, 109 megs.; insulation resistance of thermo couples to heaters, 41 megs. Throughout the experimental work attention was directed towards determining the reliability of the apparatus and the accuracy of the indications obtained. There is no doubt that an accurate relation can be estab- lished between percentage of methane and deflection indicated provided that the heating current is main- tained constant, and that the detector is efficiently screened from draughts. The Apparatus Demonstrated. A highly successful demonstration of the appliance was given on Friday afternoon at the Hotel Cecil, in the presence of a large number of persons interested in the coal mining industry. The presence of the firedamp was at once indicated by the ringing of a bell, -which con- tinued to ring until the supply of the gas was shut off. At the same time the percentage of firedamp was recorded on the calibrated scale of the instrument. Lord Ninian Crichton Stuart, M.P., presided, and short addresses were given by Prof. Silvanus Thompson and Dr. J. Erskine-Murray. The former spoke of the need for an accurate firedamp detector, which would record the percentage of methane present in the mine, from hour to hour, and which could be easily carried about from one place to another. The Williams methanometer, he thought, was such an instrument, as it was portable, and gave direct and reliable readings. He further remarked that an automatic record of the varying percentages could be made by the apparatus, and that such a record compared with the barometric pressures would probably assist the ventilation problem, and also enable them to know something more about out- bursts of firedamp in mines. Dr. Murray also referred to the value of the instrument in this respect. Other speakers were Mr. Brace, M.P., and Mr. Smillie (presi- dent of the Miners’ Federation of Great Britain).