FIG. 1.-Oerlikon Three-Phase Rotary Current Dynamo, General View. The electric transmission of power by means of electricity has now become an almost every day occurrence, but owing to the great distances by which the waterfalls-the usual sources of natural power-are separated from the centres of distribution, high pressures are necessary to A general view of the 300-h.p. multiphase dynamo is seen in Fig. 1. This machine runs at 150 revolutions a minute. The armature circuits are arranged in the way we have already explained to give three separate alternate circuits, whose phases lag behind each other to the extent FIG. 2.-Oerlikon Rotary Current Motor, with Field Magnets Withdrawn. ensure economy in the conductors. The recent invention of the rotary or multiphase system of transmission has enabled this to be done with the greatest degree of economy. The Oerlikon Works, which, as our readers know, have of 120deg. It must be prevised that the dynamo itself is a low-pressure machine, and the enormous pressures used in transmission are produced from transformation at a special transformer immersed in oil. Each of the three circuits mentioned is wound for a metallic bands, each passing round a grooved ring on the dynamo spindle, round a pulley connected to a terminal, as shown in Fig. 1. The armature has bearings on one side only, the huge spindle being carried on a double bracket bolted, as shown, to the bed-plate. A dynamo of this type can equally well work as a synchronising motor, but it differs from the synchronising alternate-current motors already devised, inasmuch as it starts without difficulty under load. Three dynamos with vertical driving axles for connection direct to turbines, and two motors with vertical axles, are also being constructed for the supply of the whole power to their own works at Oerlikon from a waterfall at a distance of about 12 miles. FLOATING CHARGING STATION. The following figures will be read with interest. The The number of electric launches upon the upper reaches total weight of copper on the field magnets is 300 kilo- of our pretty river is constantly on the increase. The FIG. 5.-Side Elevation of Oerlikon Dynamo. grammes (660lb.), which is only a small proportion of the weight of copper usually required on machines of this size. The exciting current required to give 50 volts on open circuit is only 100 watts-that is, 05 per cent. of the output. At full load, by reason of the reaction of the armature, this amount is slightly increased, but it never has been found to be more than a fraction of 1 per cent. When running at full speed and at normal pressure, the losses by friction amount to 3,600 watts-about 16 or 1.7 per cent. of the maximum output. sight of these craft with their easy motion and noiseless machinery, apparently cutting their way through the water without any effort, and almost without any attention, is now fairly familiar to boating-men. They are not slow to perceive the many advantages of this new mode of propulsion, and we are not surprised to hear that their manufacture and equipment is taking its place among our riverside industries Messrs. Woodhouse and Rawson United, Limited, of 88, Queen Victoria-street, E.C., and Strand Works, Chiswick, The heating loss (C2 R) in the armature at full load amounts to 3,500 watts. Adding these losses, a commercial efficiency of quite 96 per cent. is obtained, and the losses being so small, the heating is negligible. The total weight without bed-plate is 9,600 kilogrammes (21,1201b., or nearly 10 tons). Besides the Frankfort transmission plant, the Oerlikon Company have several dynamos of this type under construction. Dynamos are being built for a transmission of power over a distance of six miles at Heilbronn and also at Zurich. have devoted much attention to the development of this branch of electrical enterprise. Besides building a number of these launches they have established charging stations at Kew, Chertsey, Windsor, and Abingdon, and have recently equipped a floating charging station for attendance at the ever-popular river regattas. In appearance very similar to a houseboat, the charging station consists of a river barge 80ft. long and 14ft. beam. The machinery is placed in a compartment at one end, and consists of a semi-portable steam engine plant and dynamo of sufficient output to charge the accumulators on six launches simultaneously. The remaining portion of the boat contains a storeroom, an office, sleeping apartments for the attendants, and an engineer's room, wherein a lathe is fixed, and attendants are kept constantly in readiness to effect any repairs to launches which may be required—a convenience boatowners know how to appreciate. The charging station also fulfils the duty of supplying any current which may be required for illuminating with the electric light, the numerous houseboats on the river, and at the late Henley Regatta she partly illuminated the course with her arc light. NEW (CARBONATE OF IRON) FAURE BATTERY. The following particulars of the new carbonate of iron battery of M. Camille A. Faure are given in the Bulletin de l'Electricité for August 20: The battery is composed of wooden troughs, say, about 27ft. long by 3ft. 9in. high and 6ft. 6in. wide, enclosing some hundred or so double electrodes 6ft. 6in. wide. These electrodes are constituted of an agglomerate of carbon obtained by grinding up in a mill, drying, and then carbonising at 1,400deg. C. a paste composed of quarter by weight of oats, quarter of bituminous coal, and half of very porous clayey earth. An agglomerate is thus obtained which is extremely porous, with which one side of the electrodes only is covered, the other receiving a coating of tar, rendered entirely impermeable by rebaking. The porous side of the electrode is covered with a large piece of netting or coarse sailcloth. The space between these double electrodes is filled in with granulated iron. The liquid used is salt water led in by tubes. The current is taken from two iron plates at the two extremities; the space between one of the plates and the last electrode is filled in with coke or copper turnings of a sufficient conductivity to carry the current of 1,000 amperes, generated by this battery at a tension of about 1.15 volts. The elements of iron and carbon immersed in salt water (NaCl) produce chlorate of iron, caustic soda, and hydrogen, with an E.M.F. of about 30 volt; the hydrogen recom bining with the oxygen increases this by 40 volt, and the carbonisation of the soda by the carbonate of iron adds another 30 volt, which is again increased 15 volt by the use of reduced porous iron instead of solid iron. Thus the total E. M.F., according to M. Faure, is: 30+40 +30 +15=1·15 volts. In the electrolytic reaction the carbonate of iron and the chlorate of iron form carbonate of iron and chlorate of sodium, which is thus regenerated: the battery only uses the iron transformed into carbonate, which, as will be seen, is easily regenerated, as well as the carbonic acid used for this regeneration. The cloth-covered porous surfaces constitute the positive faces of the electrodes; they are depolarised by the gases, which penetrate between the faces of each double electrode, which are arranged in cone shape, by holes placed in the bases of this cone, and of which the inert part-the nitrogen-escapes by the porous face, at the same time agitating the liquid. The reduction of the carbonate of iron takes place in a retort charged with carbonate of iron, and traversed from top to bottom by gas reducers-CO, H, etc.—coming from a gasogene. Passing from the retort, these gases take fire at the contact of air, forming carbonic acid, which passes around the retort to the chimney, whence a portion of the gas is drawn by a pump, which forces it, after washing, into the battery. The air necessary for the combustion of the gasogene arrives, already heated, by a chimney, and the reduced spongy iron passes away cold by a channel placed at the base. The installation of a Faure primary battery comprises, therefore, besides the battery properly so called-(1,000 elements)-a pump capable of forcing 1,000 cubic metres. of carbonic acid per hour, a machine to agglomerate the carbonate of iron passed out of the channels, into bricks, and the reducing chamber, According to M. Faure, the consumption of fuel in the retort is not more than 0.3lb. of coal per pound of iron used in the battery, or per horse-power hour at the battery terminals. ON THE ELECTRIFICATION OF STEEL NEEDLE- BY A. P. CHATTOCK. § 1. As Faraday long ago put it, the discharge of electricity from a point into a gas may be looked upon as a particular case of sparking between a conductor, the point, and a non-conductor, the surrounding gas. It is, moreover, a particularly interesting form of discharge, as compared with that taking place between two conductors, from the fact that it seems more likely to throw light on the unsymmetrical behaviour of positive and negative electricity. For there is evidence which points to the surface of the electrode as the origin of that want of symmetry; and by experi menting with one electrode only, one is able, to a certain extent, to separate its effect on positive electricity from that on negativesuch separation being impossible in the case of sparks between two conductors, as both effects are there necessarily present together. The phenomena connected with discharge from points group themselves naturally under two heads: Those occurring before or at the beginning of discharge, and those occurring during the passage of electricity from the point. What follows refers to the first of these divisions only. end facing the centre of an insulated metal plate, and the latter § 2. When an earth-connected sewing-needle is placed with its In Table I. the value of the square root of the pull, P, in dynes on the point of a finet sewing-needle at the instant of discharge are given for various distances, d, in centimetres between its point and a metal plate. The constancy of P speaks for itself. It is true that an ordinary sewing-needle is not a perfect cylinder, but tapers gradually to its point, so that part of P must be due to lines of force on its sides; but this must be very small, for the density of charge on the sides is small compared with that at the point, and the force per square centimetre is proportional to the square of the density, and in addition to this the force on the sides has to be resolved into a direction almost at right angles to itself before it can affect P. Indeed, the very constancy of P, when d is varied, may be regarded as evidence that this part of P is negligible. (Other reasons are given in § 3.) The two needles, A and B, on which the measurements were made were numbered alike by the makers, and the agreement in the values of P for the two is fairly close. This is the more satisfactory as the readings were taken under somewhat different conditions. The needle A was suspended in the larger of the two instruments described in § 8. It discharged on to a disc of tin 13 centimetres in diameter, with its edge protected by a ring of thick wire. The disc and the inside of the instrument were covered with a thin film of vaseline. The readings on B were taken with the small instrument. The disc in this case was a penny with its surface ground and polished. No vaseline was used. The positive value of P for the two needles differ by about 3 per cent., whereas the negative values differ by 10 per cent. This is worth pointing out, as it is in accordance with what appears to be a general rule-viz., that the positive discharge is more constant and stable than the negative, and far less independent on the condition of the discharging point. § 3. One may, however, go further than simply showing the constancy of the discharge field at the point. In terms of the attraction of the needle and the radius of curvature of the point, it is possible to calculate its value. Let da represent an element of area of the point's surface, and the angle between the direction of the lines of force at da and where r is the radius of curvature of the point. The average value of P for positive discharge from needle B (Table I.) is 1.59. That for negative is 1·16. The value of r for so fine a needle is rather uncertain, as it tapers towards the point. Measured under a microscope it seems to lie between 1.9 × 10-3 and 2.3 × 10-3 centimetres. Taking the mean of these numbers one obtains the value of the field of force, f, close in front of the point f= √8 P =2,140 E.S. units for + discharge. The highest measured value of ƒ for discharge in air is, I believe, that obtained by Dr. Liebig for sparks between plates (Phil. Mag., July, 1887, p. 106)-viz., 400 E.S. units for a sparklength of 0.0066 centimetre. This is much less than either of the above numbers, but the length of discharge at the point is probably much less than 0.0066 centimetre. The results may, moreover, be verified by calculation from potential data. Although the value of f is so large at the surface of the needlepoint, the lines of force diverge rapidly, and at a very short distance from the point the field becomes inappreciable. In other words, there occurs within this short distance a step of potential (v) which, if the point is hemispherical and small, may be taken as equal to 2, where Q is twice the charge on the hemisphere (Q, as In order to verify the above values of f, it will be convenient and sufficient to show that these values of v calculated from P are in agreement with those calculated from potential measurements. The column headed V, in Table I., gives the difference of potential in E.S. units between the needle and the plate. Now, except for the presence of the needle itself, the field of force in which it was placed would have consisted of parallel lines; for the point of the needle was arranged in this experiment to project three or four millimetres beyond a flat screen of tinfoil through a small hole in it, and the distance from the tinfoil to the disc (a penny) on to which the needle discharged was small compared with the diameter of the disc, so long as the value of d was less, say, than a centimetre. Even with the needle in position, the field would be disturbed for a very small distance only round about the point, on account of the extreme smallness of the latter. Hence V may be regarded as consisting of two parts-one constant (viz., v) and occurring in a small space close to the point; the other proportional to d for small values of d. Taking the mean of the values of V for each value of d, we obtain the numbers plotted with d in Curves I.* The upper curve is for positive discharge, the lower for negative. Up to a distance of something less than a centimetre from point to plate, the values of V lie very fairly on the straight lines ruled through them. V, a constant, is thus proportional to d; and the values of the constants are of course to be found where the ruled lines cut the axis of V. These points are hardly distinguishable from the values (which are marked by crosses) of the step of potential at the point calculated above from P (viz., 4.5 and 3·3). They show, indeed, a slight tendency to exceed these values; but the agree ment is certainly striking, considering the complete independence of the two methods of measurement. The small amount (3 to 6 per cent.) by which the potential step calculated from V exceeds that calculated from P may be accounted for, I think, by the fact that the lines of force from the needlepoint do not diverge quite as rapidly as calculation supposed. The parallel lines of the rest of the field keep them together and so increase the real step of potential at the point without altering P in proportion. To test this point I measured P for a needle of about the same size as B, first suspending it in a parallel field, and then with its point at the centre of curvature of a spherical cup. In the first case, P was about 3 or 4 per cent. less than in the second; showing, as it seems to me in the light of what follows, that in the parallel field ƒ at the point was nearer the average value of the field through a small distance from the point-surface than in the radial field of the second case; and to about the same amount that the above discrepancy requires. This accounts, moreover, for the fact that needle B is the only one which does not agree with the results of Table II. Its constant (fx 0.8) being 15'5 at a pressure of 76 centimetres. As regards the measurement of potential in the above, the electrometer described in § 8 was used; its constant being determined by comparison with short sparks between brass knobs of 5.3 centimetres diameter. The following are the readings taken : The values of V were taken from Dr. Liebig's paper already referred to. I think, then, it may be safely concluded that the attraction of a plate on a needle-point is practically due only to the lines of force ending at this point, even when the needle is not a perfect cylinder; for a conical point could only have the effect of increasing P and so making the step of potential calculated from it bigger than that from V, whereas the reverse is the case. It is true that the density must be greater at the centre of the point than at the sides of the hemisphere. But this is what P chiefly depends upon, and it is also precisely here that discharge occurs and the value of fis required. Hence the connection established above between P and seems to be practically true, although the assumption that ƒ is uniform over the point-surface is not. In what follows, therefore, the values of ƒ have in every case been determined from P and r. Effect of Curvature of Discharging Surface. § 4. Two conclusions appear, then, to result from the foregoing. In the first place, the absence of discharge below a certain definite potential, which is constant for given conditions, implies the breaking down of some resistance before electricity can pass off at a point; and, secondly, the fact that ƒ is independent of d points to the near neighbourhood or actual surface of the point as the seat of this resistance. All this is in accordance with what is known regarding the discharge of electricity in gases, and is indeed only what one would naturally have expected. It still remains to consider whether the resistance to discharge is to be sought in the gas itself, in the surface of the point, or in both. These curves may appear when Mr. Chattock's paper is published in the Phil. Mag. They were not shown at the British Association Meeting. |