January 14, 1916. THE COLLIERY GUARDIAN. ________________________________________________________________________________ maximum, and increasing the spring tension to the maximum at which the bell could be actuated with a pressure of 4 volts, the relationship between volts and amperes is given' by the curve Mg.Mt., in which the current rises from 0-1 ampere at 4 volts to 0-3 ampere at 12 volts. Still keeping the maximum tension, but adjusting the gap to the minimum, there was obtained the curve mg.Mt., which varies from 0-2 ampere at 4 volts to 0-4 ampere at 12 volts. A much higher 1 current could be passed through the system by largely increasing the spring tension, but the bell could not then be actuated at a pressure below nine volts. The result is shown in the short upper curve which rises from 0-75 ampbre at 9 volts to 0-9 ampere at 12 volts. Repeating the above experiments in order to deter- mine the conditions necessary for ignition of a coal gas mixture, it was found that in the tests corresponding to the first three curves ignition could be obtained at six volts, but not lower; whereas when the spring tension was increased and the gap diminished as above stated, ignition could be obtained at four volts. , It is neces- sary to note that when the gas could be ignited at the trembler spark, it could also be ignited (although some- times only after a number of trials) at the single break spark. In these tests, therefore, both sparks were equally dangerous. It is also necessary to note that the system which could be made to operate safely below six volts could be rendered dangerous by altering the trembler spring. The above remarks are only true for coal gas. It may be rather severe to test a mining installation in coal gas, but if it can stand the coal gas tests it is abundantly safe in methane. An ignition test made by Dr. Wheeler on the above bell showed that with the armature fixed and the trembler contacts touching (a condition which gives the largest break flash), the least current required to produce an igniting break flash in an 8 per cent, mixture of methane and air was 0-26 ampere at 25 volts. An oscillogram of the current taken by the bell while ring- ing has been kindly supplied by Dr. Wheeler, and is shown in fig.' 3. To,ascertain whether a large reduction in the strength of the magnet coils would reduce the capability of the trembler and break sparks to ignite coal gas, a pair, of Fig 5. weaker coils was arranged in conjunction with a vibrating armature contained within a small explosion chamber, the armature being controlled by a spiral spring instead of the flat or blade spring usually employed in bell con- struction. The results of tests are shown in fig. 4. In all cases the current values are higher and the sequence of the curves is slightly different from that of the bell tests. With minimum gap and minimum tension, igni- tion was first obtained both at the trembler and the break at six volts. With the other adjustments ignition was obtained at four volts. It follows, apparently, that it is useless to reduce the inductance of the circuit to avoid dangerous sparking unless the value attainable by the current is also reduced. In his report, Dr. Wheeler remarks upon the desirability of making the resistance as high as possible, and the above experiments support this conclusion. • An obvious deduction from the foregoing is that for safe working the maximum voltage should be kept as low as possible. In the author’s opinion, the value permitted by the Home Office Regulations, namely, 25 volts, is much too high. Also the internal resistance of the batteries should be high in order to avoid a rela- tively large current in the event of an accidental short circuit. Further, some additional precaution should be taken to abolish the spark, or to reduce it to negligible dimensions. A variety of expedients have been devised. One, well known to telegraphic engineers, consists in the arrangement of a non-inductive shunt of comparatively high resistance across the ends of the magnet windings. This diminishes the trembler spark, and probably reduces the break spark. A condenser across the trembler gap improves the conditions at the trembler, but does not remove the danger from the external cir- cuit. Another device consists of a short-circuited wind- ing or layers of tin-foil between the magnet windings, for the purpose of reducing the energy available for sparking at the gap or break. Layers of tin-foil were wound in the magnet windings of the coils used for the tests recorded in fig. 4, and it was found that whilst they increased the voltage required for ignition at the trembler from 4 to 6 in the Mg.mt. curve, no improve- ment was found at the break spark of the external circuit. An arrangement which has been found to give good results at both the trembler and the break is shown in fig. 5. The action at the trerhbler is the reverse of the ordinary action. Instead of interrupting the circuit, the trembler short-circuits the magnet. The magnet windings are indicated by a, and the spring-controlled armature by b. One end of the armature is connected to one end of the magnet coils, and the fixed contact c is connected to the other end. When the external cir- cuit is closed, as indicated diagrammatically at d, the magnet is excited, and the armature is attracted by the same. Near the end of the movement a spring blade e on the armature touches the fixed contact c and short- circuits the magnet. By means of its spring the arma- ture is returned, and the parts c and e are separated. The action is then repeated, and a vibratory motion of the armature is obtained. Only a very minute spark is produced at the trembler, and this is entirely negligible. The intensity of the single break spark at d is not reduced, however, to the same extent, and although it is much less active than when the ordinary trembler construction is used in the bell, it is possible to produce ignition of an explosive coal gas mixture. The mining bell above refered to was converted, with- out altering the magnet coils, to correspond to fig. 5, and the result of a test is shown by th& top line in fig. 2. It will be observed that the current taken by the bell was very considerably larger than that by the normal bell. At four volts the current was 1 ampere, and at 12 volts it was 3 amperes. Notwithstanding the relatively large amount of current, no ignition of a coal gas mixture could be obtained with any voltage at the trembler, but ignition could be obtained with six volts at the external break. By suitably proportioning the magnet wind- ings, and inserting a resistance between the battery and the bell, the current can be reduced to something of the same order as that required for an ordinary bell, and in that case the external circuit would be rendered quite safe. Notwithstanding the improvements obtainable by devices such as those above described, the complete solution lies, in the author’s opinion, in a suitable relay system. By using a small relay arranged to be actuated by a 4-volt battery, he obtained perfect operation of the bell with a current of 0*1 ampere in the relay circuit. Sparking in the external circuit of the relay was quite insignificant, and no ignition of coal gas could be obtained. It was necessary to increase the relay cur- rent to 0-6 ampere before ignition could be produced’. At the relay contact the condition was the same as in the external circuit of an ordinary bell; but it is obvious that this could be avoided by the use of a condenser or non-inductive resistance at the relay contact, and the employment of a short-circuiting trembler at the bell. A similar experiment to the foregoing was performed by Mr. Watts,* with similar results. Under, laboratory conditions, there is no difficulty in rendering a bell system safe on the lines above indicated, and there appears to be no reason why the same conditions cannot be realised in a practical apparatus suited for mining conditions. Conclusion. The opinion has already been expressed above that the Home Office Regulations limiting the highest permis- sible voltage in bell signalling systems to 25 cannot secure safety, as the figure is obviously too high. If it is desirable to fix a limit, the figure should be reduced to six. With the adoption of a relay system, this lower figure is practicable. The fixing of a voltage limit is not, however, sufficient in itself to ensure safety. There should be added the condition that sparks produced in the system should not be capable of igniting a specified mixture of methane and air, as this result is easily obtained with a relay. _______________ * T. G. Watts. “ Investigations into a System of Electric Bell Signalling for Use in Collieries.” Proceedings of the South Wales institute of Engineers, vol. 30, p. 529, 1914. (See Colliery Guardian, December 4, 1914, p. 1170). _______________________ EMPLOYMENT OF BOYS AT MINES. Statistics are given in Part II. of the General Report on Mines and Quarries, which show that the occupation of boys underground is slightly less dangerous than that of persons over 16; the average death rate for 10 years per 1,000 persons employed underground being 1’41 in the case of the lads under 16, and 1-49 in the case of persons above that age. The following table gives the death rate per 1,000 persons employed underground, under and -above 16 years of age respectively, at all mines, for the years 1905-14 :— Age. ______ Year. >---------------A---------Total. Under ’6. Above 16. 1905 .............. 1’76 ... 1’5" ... B52 1906 ____ ............. 1’26 ... 1’44 ... 1’42 1907 .............. 1’33 ... 1’4*; ... 1’46 1903 .............. 1’52 ... 1’46 ... 1’47 ’909 _______............ 1’59 ... 1 62 ... l’C2 1910 __.............. 1’77 ... 1’93 ... 1’92 1911 .............. 1’26 ... 1’31 ... 1’31 1912 .............. 1’09 ... 1-28 ... 1’27 1913 ______________ 1’37 .. 1 76 1’74 1914 .............. 1’34 ... 1’18 ... 1’19 _____________________________ Shipments of Bunker Coals.—During the completed year ended December 31, the quantity of coal, etc., shipped for the use of steamers engaged in the foreign trade amounted to 13,630,964 tons, as compared with 18,535,616 tons in 1914, and 21,031,550 tons in 1913. The quantity so shipped in December was 953,878 tons, as against 1,248,215 tons in December 1914, and 1,842,006 tons in December 1913. Statistics of Rainfall. — Messrs. John Davis and Son (Derby) Limited report as follows on the rainfall of Derby and Duffield during 1915: — Derby. Duffield. A ( In. No. of" rainy days. ( In. No of fOTDerby rainy I88.4-1?04 days: lneluslve- January ... 2’97 . ... 21 ... . 3’18 ... 18 .. . 2’13 February ... ... 2’41 . ... 16 ... . 3’36 ... 18 .. . 1’57 March ... 1’00 ... 14 .. . 0’91 ... 12 .. . 1’67 April ... 0’89 . ... 12 .. . 0’80 ... J3 .. . 1’78 May ... 1’69 ... 9 .. . 1’47 ... 11 .. . 2’02 June ... ... 1’62 ... 7 .. . 1’27 ... 8 .. . 2’10 July ......... ... 6’26 ... 21 .. . 5’71 ... 21 .. . 2’47 August ... 1-98 . ... 16 .. . 2’14 ... 17 .. . 2’67 September ... ... 0'78 ... 8 .. . 0’79 ... 10 .. . 1’80 < October ...... ... 1-79 ... 11 .. . 1’90 ... 13 .. . 2’60 November ... ... 2’54 ... 11 .. . 2’85 ... 11 .. . 2’06 December ... 4’92 ... 27 .. . 5’43 ... 27. .. . 2’43 Total ... ... 28’85 ... 173 .. . 29’81 ... 179 . .. 25’30 MANCHESTER GEOLOGICAL AND MINING SOCIETY. A meeting of the Manchester Geological and .Mining- Society was held on Tuesday last, Mr. Leonard R. Fletcher presiding. Mr. Robert William Cuthbertson, Laneside, Chequer- bent, Bolton, and Mr. Robert Ernest Grime, 224, Frederick:street, Oldham, were elected federated members. Experimental Fan. Mr. D. E. Thomas read a paper on “ The Value of the Experimental Fan in the Mining Laboratory,” which will be found in another column. The President, opening the discussion, said that any- thing in connection with the ventilation of mines was always interesting to those engaged in mining woik, and he was sure the members would desire to express their appreciation of Mr. Thomas’s contribution to the Transactions of the society. Mr. R. Orchard said there was evidence that great care had been taken in connection with the experiments made by Mr. Thomas, who, however, refrained from deducing any general principles from the results he had obtained. The author mentioned that mining text- books generally stated that, “ if the fan inlet be com- pletely closed, the water gauge observed will be the maximum obtainable at that speed of the fan.” He (Mr. Orchard) did not know whether that assertion referred to an actual fan or a theoretically perfect fan. Mr. Thomas asked for suggestions as to the apparent discrepancy between the accepted theory and what actu- ally took place. To the speaker’s mind, the matter seemed fairly simple, the difference merely proving that a fan was not a perfect vacuum producer. When the inlet to a fan was closed, it did not prevent air getting back from the outlet side and setting up a degree of backlash or current eddies, which had some bearing on the change in the water gauge. Mr. Noah T. Williams said that Mr. Thomas had placed before them very important figures and state- ments which must have taken a considerable time to compile. The author’s conclusions from experiments with regard to the value of the water gauge with , vary- ing orifices, and to the value of the coefficient of vena contracta, did not conform to the generally accepted figures arrived at by other workers on the subject. He noticed that Mr. Thomas had obtained most of his results by means of the Pitot tube, and, in order to get his velocity pressure, or, what was termed in the paper, the dynamic pressure, had inserted the end of the tube, facing the current, in the outlet of the fan. The figures obtained were satisfactory, but he should like to ask whether the tube recorded any pressure at that point when the fan outlet was completely closed; there should be no velocity pressure then. He had tried similar experiments at the Manchester University with a similar small fan, but obtained most astounding results, which he would not like to put before them. When he put the Pitot tube on the inlet side, however, he obtained far more satisfactory results. Mr. Thomas rightly emphasised the importance of using the improved form of Pitot tube, and he might have added that it was necessary to take a great deal of care with even the best form of Pitot tubes. Various authorities had proved that a crude form gave very inaccurate results, and that even the best tubes were apt to give, mis- leading records. That probably was one reason why the instrument had not become more popular. In view of the unsatisfactory results obtained from his fan, he went back to the water gauge and anemometer; and, in order to make the results a little more satisfactory, he replaced water by alcohol of 0-8 sp. gr., and reduced the values down to the equivalent of the water gauge. The figures he got with the ..fan running at a constant speed of 1,800 revolutions per minute were as follow :— Area of opening. Water gauge. Completely closed ... 2’28 0’4 square ft. ... 1’56 0 5 „ ... 1’12 0 6 „ ... 0’8 2 25 „ ... 0’12 Those results agreed pretty well with the accepted theories as laid down by Murgue. He (the speaker) tried the Pitot tube in the exhaust, but, owing to the oscillation, the results were too vacillating, and he reverted to the inlet side. That fact, he thought, emphasised the need for uniform methods of testing with experimental fans, as well as with large mine fans. That point was brought out very forcibly by Mr. Watson in a paper before the Scottish Institute some time ago, in which he put forward a plea for uniform methods of testing fans. So varied were the results of tests that it was important some attempt should be made at standardisation and regulation of experiments. As matters stood at present, it was almost impossible to make a fair comparison between different fans. The subject was undoubtedly worthy of further considera- tion, and he would like to suggest that one satisfactory way of testing fans for comparison, and the form of the water gauge to be used, would be to try the experi- mental test tube at the station at Eskmeals for the purpose of trying fans submitted by various makers. By that means further knowledge would be obtained with regard to design and efficiency, and also as to the best position in which to place the water gauges. Mr. Bolton Shaw remarked that lessons to be learned from the results obtained by Mr. Thomas were very useful to mining men. At the present time the average man at a colliery appeared to understand very little about the theory of fans and centrifugal pumps, but fortunately they were becoming bet .er acquainted with them, owing to the increase in the number of ventilating and pumping