August 9, 1918. THE COLLIERY GUARDIAN. 283 The field shapes shown in fig. 5 would correspond to such a winding as fig. 14, which is intended in actual practice to approach a sine curve in shape. Relation of Revolutions per Minute of Field to Number of Foies and Frequency. Taking a motor whose stator is wound for a four-pole field, as in figs. 5 and 7, it will be found that—by taking a given pole group and commencing at any point—by the time a complete revolution of the stator has been made, any one of the phases will have passed through two complete cycles, i.e., two positive and two negative waves. Fig. 11. 100, .8 0 C ZEM, B ZE7?0 Fig. 12. From this it follows that the revolutions per minute of the foqr-pole field will be equal to— Frequency per second x 60 Number of pairs of poles. Therefore, taking a frequency of 50 cycles per second, we have— Revolutions per minute - — 1,500. On the other hand, if the speed in revolutions per minute and the frequency of the supply are known, the number of pairs of poles which a given motor possesses is equal to— Frequency X 60 Revolutions per minute, so that a machine running at 1,500 revolutions per minute on a frequency of 50 cycles per second will have Pa^rs of poles, z.e., two north poles and two 1,500 south poles, or four poles altogether—hence, a four-pole machine. Rotor. Constructional details of the rotor need not be con- sidered. It may, however, be said that the ordinary short-circuited or squirrel-cage rotor consists of a number of copper bars fastened to end rings, the whole forming a closed circuit of bars. Fig. 13. On the other hand, the rotor may be supplied with windings, so that the ends can be brought to slip rings, to enable resistances to be inserted for controlling the speed. When the slip-ring rotor of a four-pole motor is standing, while its stator is switched onto the line, the stator field is cutting the rotor conductors at a speed corresponding to 1,500 revolutions per minute. This is equivalent to keeping the stator field at rest while the rotor is rotated at 1,500 revolutions per minute, so that, obviously when the rotor is standing under the condi- tions named, the electromotive force induced and which exists between the slip rings is a maximum. When the rotor is allowed to speed up, the electro- motor force induced in it begins to fall, until the t’ce is set up m the maximum revolutions per minute is reached, by which time the electromotive force will have fallen to such a low point, that the slip rings may be short- circuited. Only sufficient electromotive force is neces- sary to overcome the resistance of the rotor windings, and as these are always of a large section, a very low electromotive force is sufficient to cause heavy currents to circulate. Limitations of Stator Current. As the rotating field of an induction-motor stator sweeps over the windings producing it, an electromotive indings in opposition to that supplying the current to the windings. A stator winding is so designed that the opposing electromotive force shall be the primary factor in limiting the current through the windings. This will be understood, in view of the comparatively low resist- ance of stator windings in relation to the current required to magnetise them and the electromotive force of the supply, which in many cases runs into some thou- sands of volts. The current is clearly limited by some- thing other than the ohmic resistance of the windings. The stator windings simply carry sufficient current, when the rotor is on open circuit, to enable the counter electro- motive force to be generated, which, together with the ohmic resistance, balances the supply voltage at a pre- deter mined magnetising S. N S. £ Fig. 14. current. If the rotor is allowed to speed up and then a load is applied, heavy currents will circulate in the rotor which will tend to demagnetise the stator field, with the result that the back electromotive force of the stator will fall and allow a larger current to be drawn from the supply. In conclusion it may be said that only the fringe of the subject has been touched, and that in a purely prac- tical sense, but if the principles outlined are thoroughly grasped by those interested in the operation of induc- tion motors, they will at least have a practical working knowledge of a most complex problem. Midland Iron Production.—According to the bi-monthly return of the Midland Iron and Steel Wages Board (deal- ing with the records of the seventeen selected firms on whose experience the sliding scale of the Board is based), the output for May and June dropped by nearly 3,000 tons compared with the preceding two months, and by close on 5,000 tons compared with the corresponding months of 1917. For the first time for many months the production of bar iron has fallen substantially, the record for the seventeen firms being 18,978 tons, compared with 21,581 tons in the preceding two months, and 22,488 in June and July last year. Angles and‘tees have dropped by about 200 tons, plates and sheets by 130 tons, while hoops, strip, and miscellaneous remain about the same. The aggregate output for the seventeen firms is 27,578 tons. The comprehensiveness of the Government control is shown in the prices, the average for all classes being £15 15s. 2d., less than 2s. a ton in advance of March and April. Prosecution Regarding Coal Prices.—The Liverpool sti- pendiary magistrate, on Tuesday, resumed the hearing of evidence in the case in which the New Moss Colliery Company and Messrs. H. S. Higginbottom and Company, coal merchants, Liverpool, are charged with having sold and offered to sell coal at prices in excess of those speci- fied in the Coal Prices Limitation Order. The proceedings began last November, and on being referred to arbitration the defendants were found to have contravened the Order, and the matter was referred to the stipendiary for penalty. Fourteen separate summonses were said to involve excess charges totalling £1,400. The defence is that the firms reasonably believed they were justified in charging higher pi ices on account of the improved quality of the coal mined, due not only to superior quality seams having been reached, but also to better methods of treatment at the mine. These improvements were only secured after heavy outlays. Col. Garside, of Ashton, mining engineer, who has known the New Moss Colliery for thirty years, said when Mr. Higginbottom took it over it was not being well mined, and he agreed to extend the area of working and to intro- duce up-to-date mining methods. This was done, and witness claimed that in various ways Mr. Higginbottom had brought about a 15 per cent, improvement in the quality of the output. In consequence of opening cut new portions of seams, Mr. Higginbottom was able to work a particular variety of coal which was worth 3s. a ton more than the similar variety would produce in the pre-war period. The hearing was adjourned to September 17. MIDLAND INSTITUTE OF MINING, CIVIL AND MECHANICAL ENGINEERS. The annual meeting of the members of the Midland Institute of Mining, Civil and Mechanical Engineers was held at the Applied Science Department of Sheffield University on July 25, when Mr. W. D. Lloyd (Altofts) presided. The President said his first duty was the sad one of reporting that one of the members of the institute, Captain H. B. Holdsworth, had since their last meeting laid down his life for his country. He proposed that a vote of condolence be sent to his relatives. The members by standing signified their approval and sympathy. The following were elected. Members: Professor W. W. Watts and Mr. A. Scargill. Associate members : Messrs. T. H. Sargeson, P. Hopkinson, E. P. Austin, and D. Macaskill. • Resignation of Mr. G. B. Walker as Secretary. The President thought it was opportune to inform the members that the secretary of the institute, Mr. George Blake Walker, who had so ably filled the dual offices of secretary and treasurer for many years, had been nominated as president of the Institution of Mining Engineers for the ensuing year. That being so, he felt he could not at the same time continue the secretaryship of that institute, and he had there- fore very reluctantly asked the council to accept his resignation of that office. He would still continue to act as treasurer. The council had that afternoon appointed Professor Hummel, Mining Professor at Leeds University, to act as secretary. The president added that he was sure all the members of the insti- tute would wish to show their appreciation of the way in which Mr. Walker had served the institute as secre- tary, and he therefore wished to propose a very hearty vote of thanks to him for the able manner in which he had looked after the affairs of the institute during the many years he had acted as secretary. As they all knew, Mr. Walker had been really the life and inspiration of the institute, and it was largely owing to his great abilities, which he had ungrudgingly given to the institute, that it had attained and maintained its present proud position and had carried on such an active part in the reading and discussion of papers on mining subjects. He had therefore very much pleasure in proposing that a hearty vote of thanks be accorded Mr. Blake Walker for the able way in which he had discharged the duties of secretary, and ex- pressing regret at the loss of those services, coupled with congratulation upon the additional honour that had now been offered to him, which deprived that institute of his services. At the same time he had no doubt that Mr. Walker would continue to take a very active interest in the affairs of the institute. Professor L. T. O’Shea (Sheffield University) said it gave him the greatest pleasure possible to second the vote of thanks. Having preceded Mr. Blake Walker in the office of secretary of the institute, he knew exactly how arduous the duties were, and what amount of careful attention they claimed. The manner in which the affairs of the institute had been conducted during the nine years Mr. Walker had held the secretaryship was evidence of the very care- ful attention that had been devoted to it. He felt he could safely say that among the institutes which constitute the Institution of Mining Engineers there was none that held a higher position than the Mid- land Institute, and he thought it was due entirely to Mr. Walker’s services, and the number of papers that he had secured, and the energy he had thrown into the office, that this result was due. Not only the number, but the quality, of the papers had been very high indeed, as would be expected from one of Mr. Walker’s ability. It gave him great pleasure to second the vote of thanks on personal grounds. He had known Mr. Walker since 1882, and he welcomed that opportunity to acknowledge his indebtedness to Mr. Walker for his unvarying kindness and assistance. The resolution was then adopted with acclamation. Peake Medal. The President announced that the council had decided, to award the Peake Medal, which was insti- tuted about a couple of years ago, to Sir William Garforth for the present year. The presentation, he believed, would be made at the next meeting. He did not think it needed any words of his to express the gratification of all the members of the council and of that institute at this decision, for they all had the highest appreciation of the work that Sir William Garforth had done for the institute and for the pro- fession of mining generally. (Applause.) Election of Officers. The ballot for officers for the ensuing year was then announced as follows. President: Mr. W. D. Lloyd; vice-presidents: Prof. L. T. O’Shea and Messrs. G. H. Peake and H. F. Smithson; council: Messrs. T. Beach, E. W. Thirkell, J. Gill, J. E. Chambers, H. Rhodes, R. Clive, and A. Thompson; representa- tives on council of the Institution of Mining Engi-