of such reading is to provide plenty of light. Better a superabundance than a lack. It is, then, not so much en account of the less danger when an accident occurs that we advocate the use of electric train-lighting, because we believe the great object is primarily to prevent the accident; but we would have such lighting become general in order to prevent the evils arising from the present system, which consists simply in making darkness visible. What is there to prevent electrical train lighting? It is by no means a difficult problem, and we doubt if it is a more costly one than some of the systems at present in use of lighting by gas. TRAMWAYS. It may be remembered that exact figures as to the electrical working of tramways was in a measure promised by the chairman of the Birmingham Central Tramways. This promise has been fulfilled in the tables appended to the report of the directors, given elsewhere. Even these figures are delusive, and the electrical work does not show as well as it will in the future. Difficulties were encountered and had EDISON DYNAMO AND MOTOR.* It is one thing to make a dynamo or motor from explicit instructions and quite another thing to design a machine adapted to generate or be operated by a particular current. The former is purely mechanical and within the range of most machinists and amateurs, while the latter is entirely within the province of the electrical engineer or electrician. When the work of machine building proceeds simultaneously with the study of fundamental principles, real progress is made. For the benefit of those who proceed in this way, and in answer to many enquirers, we give a detailed description of an Edison 25-kilowatt machine, designed for use as a dynamo for supplying a current for five Edison standard lamps, or for use on the Edison circuit as a 1-h.p. motor. Before beginning the description of the machine, it is but fair to say that it is thoroughly well made in every particular. The insulation in every part is very perfect, and the whole is so well made that any single machine built by a mechanic or amateur could but suffer by comparison with it; and, furthermore, we doubt if any maker of a single machine could even purchase the materials required for the price asked for the machine by the regular manufacturers. Therefore, if the machine is wanted, we advise a purchase. If experience is wanted, the making of the machine comes first in order, with a probable purchase to follow. The engravings are one-sixth the actual size, linear measurement. It is 14in. long, 73in. wide, 1 in. deep at the ends, with two 1in. elevations at the middle for receiving the castiron pole-pieces of the field magnet, which are each secured to the base by two small tap bolts extending upwardly through the base and into the pole-pieces. The base, which is of brass, is made hollow, as shown. to be overcome, which increases the cost, but it is satisfactory to learn from the report that the profit earned is steadily improving. Care must be taken not to make invidious comparisons between tram lines. It cannot be taken that because a line at The upper surfaces of the pole-pieces are truly faced one place is satisfactorily worked, a similar line for receiving the cylindrical field magnet cores, which at another place would show similar results. The are made of Swedish iron 2ĝin. in diameter and 4in. amount of possible traffic varies in different localities. long. These magnet cores are each held in position by a threaded stud screwed into the pole-piece and entering However, the important point to decide is to find magnet core. Each core is provided with a vulcanised fibre the minimum cost of working under given conditions. collar at each end, which is in. thick and fin. wide. Upon The tables referred to analyse the expenses showing each core, and between the fibre collars, is wound 51lb. of the cost per mile run. No. 24 silk-covered copper wire, with a wrapping of thin The totals show cost per varnished paper between the layers. The cores, before mile with steam to be 10.99d.; with horses, 9.79d.; | winding, are thoroughly insulated with the same with cables, 6.33d.; and with electricity, 9.90d. material. The fibre collars are each held in place by Here cable traction shows best, that of electricity three conical-headed screws entering the end of the and horses being almost the same, while steam is the of the core. core, with their heads projecting beyond the body To the inner and outer ends of the most costly. winding of each arm of the magnet are attached pieces of larger wire to avoid breakage, and the inner ends are led out through grooves in the fibre collars. The yoke, of Swedish iron, is 2ĝin. wide, 24in. thick, and 7in. long. It is held in position on the cores by two in. bronze studs, each threaded at the upper and lower ends, and furnished with a collar which fits into the counterbored part of the hole in the yoke. The studs are squared at the upper end to receive a wrench, and a nut is placed on each stud above the yoke for clamping it securely after adjustment. The machine is regulated or adapted to any work requiring less than its full power by raising the yoke more or less. The yoke is provided with an eye, by means of which the machine be lifted. CORRESPONDENCE. "One man's word is no man's word, Justice needs that both be heard.' TOWNLEY'S DUPLEX ALARM. SIR,-In answer to Mr. Townley's letter of to-day's issue re his duplex alarm, I have taken his advice and again taken the trouble to read carefully the description of his alarm given in your paper, and fail to see where the freedom from switches and complicated parts comes in. If Mr. Townley will now do me the favour to also carefully read both his articles and my own, he will see that his arrangement comprises a bell with five moving parts. In mine there is one. His contacts, P and P1, are liable to oxidise. With mercury cups bad contact is impossible. He trusts to plain contact in clock. In mine there is a switch, with a knife edge lever, which drops into a split pillar, actuated by a spiral spring with a rotary pressure of 2lb.; thus securing a powerful rubbing contact. He has had his clock and alarm in use for 12 months; I have had mine in use full five years, therefore five recommendations. = In conclusion, I still fail to see the novelty attached to may Front and rear boards of mahogany are arranged on opposite sides of the yoke, and held in place by brass plates at the ends. The outside ends of the field magnet coils are connected with binding posts on the rear board. A variable resistance of 10 or 15 ohms is inserted between these posts when the machine is used as a dynamo. In the front board, at the right-hand side, is secured a bronze terminal. This is adapted to receive the line wire, also one casting, known as the right-hand motor head field magnet of the leads, the upper end of which is screwed to the casting. The lower end of the lead is secured to a lead terminal attached to a block of wood secured to the right*From the Scientific American. hand pole-piece. At the right-hand side of the machine a similar arrangement of the lead is found, but the upper lead terminal is made in two separate parts, one attached to the lead, the other being connected with the line; both being furnished with copper switch tongues. The switch arm turns on a stud projecting from the front board, and carries a loose triangular switch-plate of copper, having a knifeedge which readily enters between the switch tongues. The switch has a T handle of hard rubber, by means of which it is turned. A stop pin projecting from the front board limits the rearward movement of the switch arm. into the hollow standard. A screw plug in the lower portion of the standard allows of the renewal of the oil. The bearings at opposite ends of the machine are alike Edison Switch. except that the cast-iron support of the bronze journal box, at the commutator end of the armature, is turned on its inner end to receive the brush yoke. (To be continued.) Small Edison Dynamo or Motor. The inside end of the right magnet coil is connected with the right-hand lead, and the inside end of the left-hand magnet coil is connected with the lower half of the lefthand lead terminal. At opposite ends of the base there are plane surfaces, to which are secured the self-oiling bearings of the armature shaft. Each bearing has a hollow standard furnished with a cap which, together with a cross-piece in the hollow standard, forms a support for the spherical central portion of the bronze sleeve forming the journal box proper. Side View of Field Magnet, Partly in Section. This sleeve is shorter than the outer portion of the bearing, and is slotted across the top to allow two brass rings to ride upon the armature shaft. These rings dip in the oil in the hollow standard, and as they revolve carry oil to the shaft in quantities more than sufficient for the purpose of lubrication. The oil is distributed throughout the bearing by means of spiral grooves formed in the inner surface of the journal box. The surplus oil drops back THE ELECTRIC TRANSMISSION OF POWER.* BY GISBERT KAPP. LECTURE III. As an example of a large modern transmission plant, I select, for illustration, that erected a few months ago for the Schaffhausen Spinning Mills. This example is not only interesting on account of its magnitude, but because it has been planted, so to say, into the very stronghold of rope transmissionnamely, at the Falls of the Rhine, where the last generation of Swiss engineers carried out such admirable work in teledynamic transmission that the present generation can only copy, but cannot improve upon it. And the grand example set by Redtenbacher, Amsler, and others, on the Rhine has, as a matter of fact, been largely copied at other places. There is hardly a large engineering works in Switzerland or the South of Germany where rope transmission in some form or other will not be found, but the best days of this system are passed. Till recently, rope transmission held the field absolutely, not because it was perfect, but because there was nothing better. Now, however, we have something better in electric transmission, and the flying ropes are being steadily replaced by the electric conductors. In the first place, the capacity of teledynamic transmission to deal with large powers is limited. During last year the Niagara Commission inspected a large number of plants in Europe, and came to the conclusion that 330 h.p. is the very utmost which can be dealt with by a single rope, so that above this power we must employ more ropes with a corresponding complication in the gear. I need hardly say that no such limit exists in electric transmission. But there are other difficulties in connection with ropes. They wear out very fast, their support at the translating stations on the line requires the erection of very heavy and costly structures, and they are largely influenced by climatic changes, causing excessive strains at some times, and slipping at others. These considerations have induced the managers of the Schaffhausen Spinning Mills to adopt electric transmission in the very spot where rope transmission, in years gone by, has received its most perfect development possible. The situation of the works is shown on the diagram, Fig. 1. The spinning mills are on one side, and the generating station is on the other side of the river, the distance between the two being about 750 yards. In the generating station there is room for five 350-h. p. turbines, of which four are now in place, but of these only two are as yet used in connection with the electric power transmission I am about to describe. The power of these turbines is sold to the Spinning Company at the rate of £2. 16s. per annual horse-power taken off the rope pulleys, Fig. 2. The turbines are horizontal wheels, and their vertical axes are geared by bevel wheels with the rope pulleys, by which motion is conveyed through cotton ropes to the two generating dynamos. The latter are six-pole machines, each designed for an output of 330 amperes at 624 volts, and in regular work these machines are coupled parallel. The machines, and, in fact, the whole installation, with the exception of the hydraulic works, has been designed by Mr. Brown, to whom I am indebted for the particulars I now bring before you. The electrical part of the plant was made and erected by the Oerlikon Engineering Works. The line consists of four cables (each having an area of 437 of a square inch), and is sup ported at four intermediate points, besides the supports at the terminuses. One of the intermediate supports is the old turbine-house, which in former times was used in connection with the wire rope transmission; the others are towers of iron * Cantor lectures delivered before the Society of Arts. and prevent sparking at the commutator, I was obliged to insert into the armature circuit a variable resistance, which was withdrawn after the motor had gathered enough speed to make this safe. There is no inconvenience in using such a resistance when we are dealing with small currents; but when it is a question of several hundred amperes and the absorption of as many horseappliance. To get over this difficulty, Mr. Brown has devised a very ingenious method of coupling between the line and machines, the essential features of which are shown in Fig. 6. As I have already mentioned, there are four main cables-two positive and two negative. Three of these cables contain no switches which need be used for starting, although, of course, they contain the switches and fuses which may be required for testing purposes and as safety devices, but these not being essential to the explanation of the starting arrangements, I have not shown in the diagram. Call the two outer cables the electric line. But lightning flashes are sometimes very erratic, as was shown experimentally in this very room, in the admirable "Mann Lectures" which Prof. Lodge delivered before this society in 1888. It is, therefore, also necessary to make provision for flashes which will, for some reason or other, stray away from the direct path provided for them. And this has been done in the Schaffhausen installation by the employ-power, the resistance becomes a very cumbersome and unwieldy ment of lightning arresters at both terminal stations. At each station there are four lightning arresters, one for each cable. They consist of a pair of toothed plates, of which, however, only one is fixed, the other being movable. When a flash strikes one cable only, it goes to earth by the corresponding plates, and no further damage is done. Should, however, both a positive and a negative cable be struck at the same time, then the arc set up between the plates by the passage of the lightning flash provides an easy path for the passage of the power current also; in other words, the generator will be short-circuited. The object of making one of the plates movable is to cut off the short-circuit current before any harm is done to the machinery. The movable plate of the lightning protector is connected to the core of a solenoid, through which the short-circuited current must flow. Immediately this current is started, the core is sucked in, and the movable plate falls away from the fixed plate, thus acting the part of an automatic switch. SM, SM, FIG. 5. Returning now to the Schaffhausen plant, the generating station contains two 300-h.p. dynamos, which are over-compounded, so as to produce a constant pressure of 600 volts at the motor station, the loss in the line being with full current 24 volts. These machines have series-wound drum armatures, running at 200 revolutions per minute. Their more important electrical data, as well as those referring to the motors, are given in the following table: SCHAFFHAUSEN TRANSMISSION PLANT. Loss in shunt excitation per cent. 1:35 6 3 Type of armature...... Drum 7,600 1.68 4 2 Small motors. 2 60 2 positive, and the two inner cables negative. The positive cables are looped at both terminuses, and the inner cables are also looped in this way, but a switch is inserted in the righthand cable at the motor station. Now imagine all the machines at rest, and this switch to be open. To start the 350 plant, the turbine-driven generator, G1, is set in motion, and 600 the speed run up till this machine excites itself by its own 81 shunt. If you follow the connections you will find that the 234 shunts of the other three machines will at the same time also 22/2 become excited. The motors have now made their fields, and if we start the second generator, G2, slowly, a power current of 4 gradually increasing strength will be sent through both motors, and the latter will gradually start. As they gather speed, their counter E. M.F., which is indicated by a voltmeter at the motor station, gradually rises; and if it has become equal to the E.M.F. indicated by a second voltmeter in connection with the 2.7 current from the first generator, G1, the attendant closes the switch, and the operation of starting is completed. It should be noted that on closing this switch there is no sudden rush of current, since the pressure on both sides of the switch is approximately equal. *0287 540 15,800 Drum Cylinder Fig. 3 shows the twin motor, which receives the bulk of the power at the spinning mills, whilst the remainder is taken up by a couple of two-pole motors, placed in other parts of the mills. These are not shown on the diagrams, as they are of the ordinary design, with which you are already familar. The twin motor is rated at 380, and each of the single motors at 60 h.p., making in all 500 net brake horse-power delivered to the mill shafting. The coupling between the motors and the mill shafting is by cotton ropes, as shown in Fig. 3, the arrangement chosen having the advantage that very little side strain is thrown upon the motor bearings, owing to the ropes pulling opposite ways. An interesting and novel feature of the plant is the arrangement adopted for starting gradually, and yet without the use of resistance. In my experiments last week I used current delivered at constant pressure; and to start the motor gradually Originally, the motors were intended to be pure shunt machines; but it was soon found that, owing to the very small armature resistance and armature reaction, it was very difficult to get the load equally divided between them. Mr. Brown, to overcome this difficulty, hit upon the ingenious device of making the machines mutually control each other by putting on demagnetising main coils, and crossing the connections between armatures and fields, so that the machine which might at any moment develop a tendency to take more than its fair share of current would have its field strengthened by the deficiency of current passing through its main turns to the other armature, and would thus immediately raise its counter E. M. F., and check the excess of current, whilst the other machine which was not taking enough current would have its field weakened, and would thus be forced to take more current. It is clear that by this cross-connection even a neglect on the part of the attendant to set the brushes properly cannot materially influence the even division of current and load between the two machines. At the same time the demagnetising influence of the main coils has the same effect as if the armature reaction were increased, and ensures thus constancy of speed, as I have shown you experimentally last week. In the diagram, Fig. 6, the machines are represented as if they had only two poles each. This I have done to make the diagram as simple as possible, and for the same reason I have shown the shunt and main coils on separate magnet limbs, but you will have no difficulty in translating in your own mind this principle of circuit connections to multipolar machines. It may interest you to have a few details of a commercial nature regarding this transmission plant. The manufacturers have guaranteed a commercial efficiency at ordinary full load of 78 per cent., also that the machines must be capable of transmitting an excess of 20 per cent. over their normal power for one hour and a half without damage. The wear of one set of brushes to be not less than 2,000 hours, and the life of a commutator not less than 20,000 hours. The variation of speed of the motors between running idle and under full load not to exceed 3 per cent. The total cost of the electrical part of the plant, including cable towers and erection, was £6,800, or £13. 12s. per net horse-power delivered. I have occupied some time in putting before you this transmission plant, because exact information about successful engineering work is of great value to practical men; and the Schaffhausen plant is certainly one of the best and most successful examples I could have chosen. The power transmitted is certainly large, according to our present ideas, but there is good reason to believe that-in point of magnitude, at any rate this transmission will very soon be eclipsed by other work of this kind. There are projects afloat for utilising the power of the Rhine, near Bâle, to the tune of tens of thousands of horsepower, and at Niagara, as you all know, a total of 125,000 h.p., or a little over 3 per cent. of the total power of Niagara, is to be taken from the Falls and transmitted to various distances, the longest distance being some 20 miles. I am not in a position to give you details of any of the schemes which have been submitted to the Niagara Commission, since these are the property of the Cataract Company, but by the courtesy of several members of the Commission, notably Dr. Coleman Sellers, I am able to give you a general outline of the schemes. My object in applying to the Niagara Commission for information of this kind was to obtain some indication of the opinions which leading modern engineers entertain of electric power transmission, and to put the result of my enquiry before you. Lest the general condition of the Niagara scheme may not be quite familiar to all of you, I shall now throw upon the screen a picture of the Falls, and give you very briefly an outline of the objects for which the Cataract Company has been established. Of the immense power represented by the descent of the river from its upper to its lower level over the Falls (about 31⁄2 million horse-power), there is utilised at present an aggregate of only about 5,000 h.p. in the mills you see on the left of the picture. The water is brought to these mills by a surface canal from the upper reaches of the river, and, after passing through turbines, is discharged into the open air about half-way between the level of the ground and the level of the river below the tail races, forming a number of miniature waterfalls. Only about half the available head is therefore utilised. If the system adopted hitherto could be followed in future, there would be little difficulty in establishing a station for the generation of any amount of power in this locality, but there is a strong tide of public opinion against the establishment of any more hydraulic works on the river bank, to say nothing of the difficulty of finding room for them and the open-air canal which would be required. The Cataract Company have therefore resolved to carry out their operations, to a great extent, underground; and at the present moment are driving a tunnel 30ft. high by 20ft. wide, and about 6,700ft. long, which is to serve as a tail race for the water coming away from their power station. This tunnel is shown on the picture by two dotted lines, and its mouth is partly submerged under the level of the lower river. The total fall between the upper and lower river is 200ft., and the net fall available for the turbines is 140ft. The fact that the tail race is a tunnel, necessitates the turbines being placed at least 110ft. underground, since the suction tube of a turbine cannot be made longer than the column of water which can be balanced by atmospheric pressure, and this increases very materially the engineering difficulties of the work. Last summer the Cataract Company invited a limited number of engineers to send in projects for the creation and transmission of power, and instituted a commission, under the presidency of Sir William Thomson, to investigate and report on the projects. There were in all 20 competitors, but of these only 14 complied with the programme drawn up by the commission, and were therefore held to be qualified to have their projects examined. Of these 14, eight competitors sent in combined projects for the creation and transmission of power, four referred only to the creation, and two only to the transmission of the power. The point of interest to us is what methods were suggested by the 10 qualified competitors in transmission. The question is somewhat complicated by the fact that some competitors have suggested mixed systems of transmission, and that in classifying the schemes into electrical, pneumatic, and hydraulic, we must count some competitors twice over. On this basis I find that the following represents the transmission projects: Electrical seven, pneumatic six, hydraulic two. It is certainly remarkable that the balance in favour of electric transmission should be so small. And it is equally remarkable that there should have been as many as six competitors who either wholly or partly advocated pneumatic transmission. The experience of colliery managers goes to show that even over the comparatively short distances over which they use pneumatic transmission, the total efficiency lies generally between 20 and 30 per cent., and does certainly not exceed 40 per cent. We cannot suppose that engineers who have sent in pneumatic projects are ignorant of this fact, or at any rate we must suppose that the majority of them are quite aware that high efficiency cannot be expected from compressed air transmission. If, nevertheless, they have adopted compressed air in preference to electricity, it must be for one of two reasons. Either they have no confidence in the capabilities of electric transmission, or they consider the cost so high that the interest on the extra capital and the greater depreciation of the plant will more than counterbalance the advantage of high efficiency. It cannot be denied that in the present state of our knowledge of electric transmission, there is some ground for both these views. The Niagara problem is unique both in magnitude and distance, and I am bound to confess that we electrical engineers are at the present moment not quite prepared to face it. At the same time I must say that I feel convinced that in a few years from now there will be not one, but a dozen men ready to face this problem with a very good chance of successfully solving it. As a matter of fact, we are at present on the threshold of a new system of electric power transmission. The old system of using continuous currents and ordinary dynamos has been perfected to a point which leaves little to be desired, but it has its limits, and, unfortunately, the Niagara problem, or at least a part of it, is just a little beyond these limits. Hence we find that only about half of the competitors have had the courage to propose electric transmission. Of these, only two suggested the use of alternating currents at voltages of 5,000 and 10,000 respectively; the others followed the old lines of continuouscurrent transmission at voltages varying between 1,600 and 4,500 volts. This brings me to the consideration of a subject which is of great importance not only in regard to the Niagara problem, but to long-distance transmission generally-namely, the limits of distance up to which the usual system of transmission is practicable. If you will refer to the table giving the cost of transmission plants, given in my last lecture, you will find that, for large powers at any rate, an increase of distance up to four or five miles does not make the cost prohibitive, and you will conclude from these figures that, within a five-mile limit, the old system of electric transmission is certainly feasible. How much farther you might go is a matter for theoretical consideration; the table does not help you much, as the only example of a very long-distance transmission is one where the power is small, and is therefore, in a certain sense, misleading. I have given you a formula by which you can calculate the most economical voltage for any distance; and if you do this for many cases, taking, for instance, 500 h.p. as your unit of power, you will find that as the distance increases beyond about five miles, the economical voltage begins to grow beyond the limit which might be considered practicable for one machine. It is quite impossible to lay down hard and fast rules. Under certain conditions, especially if you have to transmit cheap water power, you may possibly reach a distance of 10 miles before getting to the limit of voltage; but whatever may be the special conditions of the problem, there is a limit of distance beyond which a single machine will not reach. "Very well, then," you might say, "if a single machine cannot be made to give the required pressure, let us put two or three machines in series. Το correctly appreciate such a suggestion, let us first of all see what limits the voltage of a machine. Two things limit it, the commutator and the general insulation. Practical dynamo makers will tell you that in large machines they are quite prepared to put 1,000 volts on the usual Pacinotti commutator-if necessary, they will go to 2,000 volts, but with some misgiving ; and if you ask them to make a machine for 3,000 volts, they will, as likely as not, refuse. I do not refer to the ThomsonHouston or Brush machines, which have special commutators, but to large machines giving an even current and a high efficiency, such as we require in the transmission of power. may thus conclude that 2,000, or, at the outside, 3,000 volts, is the limit of voltage to be obtained from a single commutator. But the general insulation of the machine must also stand this pressure, and where, as in dynamos and motors, the insulation consists of cotton, paper, fibre, varnish, and like materials, which are subjected not only to electrical, but also to mechanical strains, 3,000 volts is quite high enough for safe working. The commutator difficulty can of course be got We |