168 THE COLLIERY GUARDIAN. July 23, 1915. sources of power were ever used they should be acquired by the Government and used for the benefit of the whole nation. The greatest water power we possessed was undoubtedly that of the waves. Many inventions had been devised for utilising wave power which would no doubt be effective. The chief reasons why they had not been applied were— (1) that in years gone by the cost of making the necessary apparatus had been too great; (2) the cheapness of coal; and (3) that the electric transmission of power had not been worked out. Now we were in a different position. We could make the plant cheaply, coal was much dearer, and when a simple and cheap method of storing energy had been discovered the waves would be a great source of power and coal would become an important raw material for countless chemical industries. From the chemists’ point of view the burning of coal was a waste. There were already mechanical means of separating the constituent parts of coal, and in time, he believed, we should use these constituent parts without destroying the coal by burning. Discussing the paper, Prof. Bone, of Sheffield, suggested the advisability of establishing in all large factories a “ fuel control ” department. In works where this had been done great savings had been effected. He also suggested that a systematic survey of British coals from the chemical point of view was a great necessity. Troubles and difficulties often arose because people made their arrangements as though coal were a perfectly uniform substance. Prof. Louis, of Newcastle, said he thought we should make up our minds to face a continual increase in the price of coal. Seventy-five per cent, of the first cost was labour. One effect of the war would be to render labour more powerful than ever. Other factors in the increasing cost were the rising price of pit props and unnecessary and prejudicial Government regulations. He did not think the gas engine would ever replace steam; the two methods of power production would continue side by side. As to the production of gas underground, he thought it would be very difficult to obviate the dangers of explosions caused by the accidental escape into the mine of carbonic oxide from the plant. In his opinion, Prof. Louis said, the most serious waste of coal in this country was the enormous quantities of small coal left underground. There were collieries where probably 15 per cent, of the coal output was thus wasted, and from the point of view of power generation that wasted small coal was every bit as good as that which was raised. The simple reason why this coal was left in the pits was that if it were brought to the surface the men would have to be paid for hewing it. In this way the colliery pro- prietors, for their own ends, were wasting a national asset. Nobody liked Government interference less than he did, but he thought this was a case in which the Government might very properly interfere. The question of alcohol as a cheap source of power was raised by the President, Prof. Henderson, and in his reply to the discussion, Mr. Reid said he had not included alcohol as a source of power because at the present time it was manufactured chiefly from foodstuffs. EXPLOSIVES IN COAL MINES. New Order. The Home Secretary has issued an Order under section 61 of the Coal Mines Act, 1911, amending the Explosives in Coal Mines Order oi September 1, 1913, by the addition of certain new explosives to the First Schedule. The Order may be cited as “ The Explosives in Coal Mines Order of the 3rd July, 1915.” Details of the explosives referred to are as follow:— Abbcite No. 2, consisting of the following mixture :— Parts by weight. Ingredients. Not more Not less than than N itro-glycerine 9’5 7’5 Di-nitro-toluol 2’5 0’5 Nitrate of ammonium 59’5 56’5 Chloride of sodium 24’0 22’0 Wood-meal (dried at 100 degs. C.) 9'0 7'0 Moisture 2’0 0’5 The explosive is to be used only when contained in a case of paper- thoroughly waterproofed with paraffin wax; and with a detonator or electric detonator of not less strength than that known as No. 6; the greatest weight of the explosive which may be used in any one shothole shall not exceed 18 oz.; and the explosive shall have been made at the works of Messrs. Kynoch-Arklow Limited, at Ferrybank, Arklow, in the county of Wicklow. Four ounces of Abbcite No. 2 gave a swing of 2’54 in. to the ballistic pendulum compared with a swing of 3’27 in. given by 4 oz. of gelignite containing 60 per cent, of nitro-glycerine. Mei sey Powder, consisting of the following mixture Parts by weight. Ingredients. Not more Not less than than Nitro-glycerine................ 6’5 ... 4'5 Tri-nitro-toluol .............. 7’0 ... 5’0 Nitrate of ammonium .......... 52’5 ... 49’0 Nitrate of sodium ............ 12’0 ... 10'0 Wood-meal (driedat 100 degs. C.) 4’0 ... 2’0 Chloride of ammonium.......... 24’0 ... 21’0 Moisture....................... 1'5 ... — The explosive must be used only when contained in a stout case of paper thoroughly waterproofed with a mixture of ceresine and resin ; and with a detonator Di- electric detonator of not less strength than that known as No. 6; the greatest weight of the explosive which may be used in any one shothole shall not exceed 18 oz.; the explosive must have been made at the works of the Cotton Powder Company Limited, at Uplees Marshes, near Faversham, in the county of Kent, or at their works near Melling, in the county of Lancaster. Four ounces of Mersey powder gave a swing of 2’60 in. to the ballistic pendulum compared with a swing of 3’27 in. given by 4 oz. of gelignite containing 60 per cent, of nitro-glycerine. PROTECTION AGAINST SURGES. Many accidents to electrical apparatus installed in and about mines, which in the past would have been attributed to defective insulation, are now recognised as having been due to the stresses imposed by surges. The origin of these surges cannot always be accurately deter- mined, the source in many cases being atmospheric. It is, nevertheless, important that adequate precautions should be taken against such occurrences, which may be fraught with serious consequences, and some useful information is supplied in a bulletin which has just been issued by the General Electric Company. In addition to a description of the nature of surges, the bulletin contains an account of various protective measures that have been introduced to cope with them. With the exception of condenser protective gear, every form of surge arrester at present in use involves the Fig. 1.—Moscicki Condenser. insertion of a spark gap between the device itself and earth, and this has a vital defect in that it introduces a dielectric spark lag, and has no effect upon high frequency surges of low voltage. For these and other reasons, it is claimed that the efficacy of a condenser is incomparably greater than that of other forms of surge protective gear. The condenser depends for its operation upon the fact that its impedance varies inversely as the frequency of the applied current. Its main advantages are that it is connected direct to the line; it contains no air-gap; dis- criminates adequately between the low frequency of the line and the high frequency of a surge; it can be installed without a series resistance; can effect a much greater reduction of surge pressure than any other form of arrester; it does not permit line current to flow during the time it is discharging the surge, beyond the charging current, which has a very low power factor, and flows continuously; and it has the capacity to modify the wave form of steep fronted waves. The Moscicki condenser, which is shown in fig. 1, con- sists of a glass tube composed of a special glass, mixture, the inner and outer surfaces being coated with chemi- cally deposited silver, which is afterwards plated with copper. At the neck the tube thickens considerably in order to withstand the static stress which at this point is greater than at any other part of the surface. This con- struction enables the thickness of the dielectric to be reduced to a minimum, thereby giving a high capacity, whilst there is ample security against a perforation of the dielectric. The neck of the condenser is enclosed by a high-pressure china insulator, the space between the neck and insulator being filled with an insulating compound in such a way that the edge of the coating is completely buried in this mass, and no surface discharge can occur. Each condenser is mounted in an iron tube, the space between the condenser and tube being filled with a mixture of water and glycerine, so that any local heating which might result in the perforation of the dielectric is rendered impossible, as the liquid distributes the heat throughout the whole volume. This liquid is also non-freezing at all the atmospheric temperatures usually met. Sometimes circumstances occur in practice when high pressures may be present in the circuit unaccompanied by high frequency. These pressure surges the condenser would not be able to discharge. Some apparatus, as an auxiliary to the condenser, is therefore needed in these cases to discharge such surges. The most successful device for this purpose is the multi-gap arrester; but an improvement upon this type of apparatus has been developed in the Giles valve (fig. 2), which is in effect a multi-gap arrester, with the individual gaps shunted by condensers in place of resistances, as is the practice with multi-gap arresters. It consists of a spark gap in series with an ohmic resistance R, and also of a number of fixed electrodes, the last of which is connected to earth. In normal working, one electrode is at the potential of the line, while the remainder are at earth potential through the condensers, with the exception of the last one, which is connected direct to earth. By this means the discharge is broken as the current passes through zero at every half-cycle. The Giles valve com- prises a number of units in parallel, the essential features of each unit being a main spark gap, a series resistance, and a number of small spark gaps, each shunted by a condenser. The whole of the units are mounted together on one frame, as shown in fig. 2. The main spark gap is adjustable in setting, and is enclosed in a small glass tube to prevent dust being deposited. The resistance, in the form of wire, is mounted upon a vertical insulating tube. At the bottom of this tube are situated the small gaps, consisting of heavy metal rings of high heat capacity. The bottom part of the tube is filled by a metal plug, and the condenser for each unit is formed over the dielectric of the tube between the plug and the metal of the rings. The interior of the insulating tube is completely filled by an insulating compound to prevent the entrance of moisture. When the pressure of the circuit rises to the point for which the valve has been set, one of the valves commences to discharge first. If the rate of discharge is adequate, the arc potential immediately drops. If inadequate, the line potential continues to rise, and the next unit is set into operation, and so the process continues until the whole of the valves are at work, and the surge pressure has been reduced to a safe value. Compared with the multi-gap arrester, the valve has the advantage owing to the introduction of the condenser feature that the whole pressure of the system is available in order to start successively each spark gap. For protection against high frequency surges due to lightning, or, indeed, against any form of high frequency or high voltage surge, the Moscicki condenser can be as effectively used on continuous as on alternating current circuits, while to relieve a continuous current system from low voltage surges of internal origin the electrolytic condenser is ideal. In the case of alternating current, it cannot be connected direct to the circuit, but with continuous current it can, and it affords a most satis- Fig. 2.—Giles Valve. factory protection. It is affected both by high voltage and by high frequency, in the latter ease due to its capacity. In the electrolytic condenser, the electrodes are composed of two sets of aluminium plates of spiral formation wound together round a vertical axis. One spiral forms the positive electrode and the other the negative. With this formation a large surface is pro- vided with minimum amount of edge, the edges being the weakest point in an electrolytic condenser owing to leakage. These electrodes are immersed in a special electrolyte. The current in attempting to pass through the electrolyte forms on one electrode an insulating layer composed of gas and aluminium compounds. This layer offers a high resistance to the flow of current, and, indeed, increases its resistance as the applied pressure rises, until a certain critical pressure is reached. When this pressure is put across the element the resisting film