Chapter 4

But when it is understood that the iron core polarized in one direction by the primary impulse does not begin to lose its magnetism when that impulse simply weakens, but waits until an actual reversal of current has taken place, it will be seen that the secondary current, which can only be produced when magnetic lines are leaving the core and cutting the secondary coil, or when the lines are being evolved and passing into the core from the primary coil, will have a beginning at the moment the primary reverses, will continue during the flow of that impulse, and will end at substantially the same time with the primary impulse, provided the work of the secondary current is not expended in overcoming self-induction, which would introduce a further lag. Moreover, the direction of the secondary current will be opposite to that of the primary, because the magnetic circuits which are opened up by the primary current in magnetizing the core, or which are closed or collapsed by it in demagnetizing the core, will always cut the secondary coil in the direction proper for this result. Transformers of the straight core type with very soft iron in the cores and not too high rates of alternation should approximate more nearly the theoretical relation of primary and secondary waves, because the magnetic changes in the core are capable of taking place almost simultaneously with the changes of strength of the primary current. This fact also has other important practical and theoretical bearings.

Let us assume a plain iron core, Fig. 7, magnetized as indicated, so that its poles, N, S, complete their magnetic circuits by what is called free field or lines in space around it. Let a coil of wire be wound thereon as indicated. Now assume that the magnetism is to be lost or cease, either suddenly or slowly. An electric potential will be set up in the coil, and if it has a circuit, work or energy will be produced or given out in that circuit, and in any other inductively related to it. Hence the magnetic field represents work or potential energy. But to develop potential in the wire the lines must cut the wire. This they can do by collapsing or closing on themselves. The bar seems, therefore, to lose its magnetism by gaining it all, and in doing so all the external lines of force moving inward cut the wire. The magnetic circuits shorten and short-circuit themselves in the bar, perhaps as innumerable molecular magnetic circuits interior to the iron medium. To remagnetize the bar we may pass an electric current through the coil. The small closed circuits are again distended, the free field appears, and the lines moving outward cut across the wire coil opposite to the former direction and produce a counter potential in the wire, and consequent absorption of the energy represented in the free field produced. As before studied, the magnetism cannot disappear without giving out the energy it represents, even though the wire coil be on open circuit, and therefore unable to discharge that energy. The coil open-circuited is static, not dynamic. In such assumed case the lines in closing cut the core and heat it. Let us, however, laminate the core or subdivide it as far as possible, and we appear to have cut off this escape for the energy. This is not really so, however. We have simply increased the possible rate of speed of closure, or movement of the lines, and so have increased for the divided core the intensity of the actions of magnetic friction and local currents in the core, the latter still receiving the energy of the magnetic circuit. This reasoning is based on the possibility in this case of cutting off the current in the magnetizing coil and retaining the magnetic field. This is of itself probably impossible with soft iron. That the core receives the energy when the coil cannot is shown in the well known fact that in some dynamos with armatures of bobbins on iron cores, the running of the armature coils on open circuit gives rise to dangerous heating of the cores, and that under normal work the heating is less. In the former case the core accumulates the energy represented in the magnetic changes. In the latter the external circuit of the machine and its wire coils take the larger part of the energy which is expended in doing the work in the circuit. In this case, also, the current in the coils causes a retardation of the speed of change and extent of change of magnetism in the iron cores, which keeps down the intensity of the magnetic reaction. In fact, this retardation or lag and reduction of range of magnetic change may in some machines be made so great by closing the circuit of the armature coils themselves or short-circuiting them that the total heat developed in the cores is much less than under normal load.

I wish now, in closing, to refer briefly to phenomena of moving lines of force, and to the effects of speed of movement. In order to generate a given potential in a length of conductor we have choice of certain conditions. We can vary the strength of field and we can vary the velocity. We can use a strong field and slow movement of conductor, or we can use a weak field and rapid movement of the conductor. But we find also that where the conductor has large section it is liable to heat from eddy currents caused by one part of its section being in a stronger field than another at the same time. One part cuts the lines where they are dense and the other where they are not dense, with the result of difference of potential and local currents which waste energy in heat. We cannot make the conductor move in a field of uniform density, because it must pass into and out of the field. The conditions just stated are present in dynamos for heavy current work, where the speed of cutting of lines is low and the armature conductor large in section.

But we find that in a transformer secondary we can use very large section of conductor, even (as in welding machines) 12 to 15 square inches solid copper, without meeting appreciable difficulty from eddy currents in it. The magnetic lines certainly cut the heavy conductor and generate the heavy current and potential needed. What difference, if any, exists? In the transformer the currents are generated by magnetic field of very low density, in which the lines are moving across the conductor with extreme rapidity. The velocity of emanation of lines around the primary coil is probably near that of light, and each line passes across the section secondary conductor in a practically inappreciable time. There is no cause then for differences of potential at different parts of the section heavy secondary. Then to avoid eddy currents in large conductors and generate useful currents in them, we may cause the conductor to be either moved into and out of a low density field with very great speed, or better, we must cause the lines of a very low or diffused field to traverse or cut across the conductor with very high velocity.

It is a known fact that, in dynamos with large section armature conductors, there are less eddy currents produced in the conductors when they are provided with iron cores or wound upon iron cores than when the conductors are made into flat bobbins moved in front of field poles. Projections existing on the armature between which the conductors are placed have a like effect, and enable us to employ heavy bars or bundles of wire without much difficulty from local currents. The reason is simple. In the armatures with coils without iron in them, or without projections extending between the turns, the conductor moves into and out of a very dense field at comparatively low velocity, so that any differences of potential developed in the parts of the section of conductor have full effect and abundant time to act in setting up harmful local currents. In the cases in which iron projects through the coil or conductor, the real action is that the lines of the magnetic circuits move at high speeds across the conductor, and the conductor is at all times in a field of very low density. Figs. 8 and 9 will make this plain. In Fig. 8 we have shown a smooth armature surface, having a heavy conductor laid thereon, and which is at a just entering a dense field at the edge of the pole, N, and at b leaving such field. It will be seen that when in such position the conductor, if wide, is subjected to varying field strength, and moves at a low speed for the generation of the working potential as it passes through the field, thus giving rise to eddy currents in the conductor.

In Fig. 9 the conductors are set down between projections, in which case both armature and field poles are laminated or subdivided. As each projection leaves the edge of field pole, N, the lines which it had concentrated on and through it snap backward at an enormous speed, and cross the gap to the next succeeding projection on the armature, cutting the whole section of the heavy armature conductor at practically the same instant. This brisk transfer of lines goes on from each projection to the succeeding one in front of the field pole, leaving a very low density of field at any time between the projections. The best results would be obtained when the armature conductor does not project beyond or quite fill the depth of groove between the projections. Of course there are other remedies for the eddy current difficulty, notably the stranding and twisting of the conductor on the armatures so as to average the position of the parts of the compound conductor.

Perhaps the most extreme case of what may be called dilution of field by projections and by closed magnetic circuits in transformers would be that of a block of iron, B, Fig. 10, moved between poles, N and S, and having a hole through it, into and through which a conductor is carried. The path through the iron is so good that we can scarcely consider that any lines cross the hole from N to S; yet as B moves forward there is a continual snapping transfer of lines from the right forward side of the hole to the left or backward side, cutting the conductor as they fly across, and developing an electromotive force in it. I have described this action more in detail because we have in it whatever distinction in the manner of cutting the lines of the field is to be found between wire on smooth armatures and on projection armatures and modifications thereof; and also between flat, open coils passing through a field and bobbins with cores of iron. The considerations advanced also bring out the relation which exists between closed iron circuit transformers and closed iron circuit (projection) dynamos, as we may call them.

I had intended at the outset of this paper to deal to some extent with the propagation of lines of magnetism undergoing retardation in reference to alternating current motor devices, transformers with limited secondary current, or constant average current, an alternating motor working with what I may term a translation lag, etc.; but it was soon found that these matters must remain over for a continuation of this paper at some future time. My endeavor has been in the present paper to deal with the lines of force theory as though it were a symbol of the reality, but I confess that it is done with many misgivings that I may have carried it too far. Yet, if we are to use the idea at all it has seemed but right to apply it wherever it may throw any light on the subject or assist in our understanding of phenomena.

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ELECTRIC LIGHTING AT THE PARIS EXHIBITION--THE OERLIKON WORKS.

Immediately on entering the Machinery Hall by the _galerie_ leading from the central dome, and occupying a prominent position at the commencement of the Swiss section, is a very important plant of dynamos, motors, and steam engines, put down by the Oerlikon Works, of Zurich. During the time the machinery is kept running in the hall, power is supplied electrically to drive the whole of the main shafting in the Swiss section and part of that in the Belgian section, amounting in all to some 200 ft., a large number of machines of various industries deriving their power from these lines of shafting, while during the evening a portion of the upper and lower galleries adjoining this section is lit by some twenty-five arc lamps run from this exhibit. Steam is supplied from the Roser boilers in the motive power court. The whole of the generating plant is illustrated in one view, and a separate view is given of the motor employed to drive the main shafting, this latter view showing the details of connection to the same. On the extreme right hand side of the first view is a direct coupled engine and dynamo of 20 horse power, a separate cut of which is given in Fig. 3. The engine is of the vertical single cylinder type, standing 5 ft. high, and fitted, as are the other two engines exhibited, with centrifugal governor gear on the fly wheel, acting directly on the throw of the cutoff valve eccentric. The two standards, supporting the cylinder and forming the guide bars, together with the entire field magnets and pole pieces of the dynamo, and the bed plate common to both, are cast in one piece.

The machine is specially designed for ship lighting, and with the view of preventing any magnetic effect upon the ship's compass, the field is arranged so that the armature, pole pieces, and coils are entirely inclosed by iron. Any tendency to leakage of magnetic lines will therefore be within the machine, the iron acting as a shield. This build of field--shown in Fig. 3A--is also advantageous as a mechanical shield to the parts of the machine most likely to suffer from rough handling in transport, and it will be seen that the field coils are easily slipped on before the armature is mounted in its bearings.

The winding is compound, and in such a direction that the two opposite horizontal poles have the same polarity; it follows from this that there will be two consequent poles in the iron, these being opposite in name to the horizontal poles and at right angles to them, viz., above and below the armature. Opposite sections of the commutator are connected together internally as in most four-pole machines, so that only two brushes are necessary, at 90 deg. apart.

The section of iron in the field is 60 square inches and rectangular in form, and the whole machine measures 4 ft. 3 in. in length, and 2 ft. in height, without including the height of the bed plate. The armature is 17 in. in length and the same in diameter, measured over the winding, and develops at the machine terminals 70 volts and 200 amperes at 480 revolutions. The moving parts of the engine are well balanced, and run remarkably well and without noise at this high rate of speed.

This dynamo serves to develop power to run a motor in an adjoining inclosure, containing some fine specimens of lathes and machine tools constructed by the Oerlikon Works. These are driven by the motor through the medium of a countershaft, and the power and speed are controlled from the switch board seen at the left of the exhibit, and in Fig. 11. The resistance, R1, serves to vary the intensity of the shunt field of the dynamo, the volts being indicated by the voltmeter V1, and a resistance separate from the switch board is inserted in the main circuit of the two machines. The ammeter, A2, is directly connected to the dynamo, and therefore indicates the current, whatever circuit this machine is running.

A larger combined engine and dynamo, seen in the center of the stand, serves to run the lighting of the galleries. The engine is a 60 horse power compound, running at 350 revolutions, and fitted with a governor on the fly wheel, like that described above.

The dynamo is a two-pole machine, the upper pole and yoke being cast in one, and the lower pole, yoke, and combined bed plate forming a separate casting. The two vertical cores, over which the field bobbins are slipped, are of wrought iron, and are turned with a shoulder at either end, the yokes being recessed to fit them exactly. The cores are then bolted to the yokes vertically from the top and horizontally below. The field of this machine is shunt-wound, and in order to maintain the potential constant a hand-regulated resistance--R2 on the switch board--is added in circuit with the shunt field. The voltmeter, V2, immediately above this resistance, serves to indicate the difference of potential at the machine terminals. Both voltmeters are fitted with keys, so that they are only put in circuit when the readings are taken.

The main terminals of this machine are fitted on substantial insulating bases, fixed one at each end of the top yoke. These connect to the external circuit by a heavy cable--the machine being capable of developing 500 amperes--and to the shunt circuit, and regulating resistance by small wires; while the two connections to the brushes are by four covered wires in parallel on each side. This mode of connection is more flexible than a short length of heavy cable, and looks well, the wires being held neatly together by vulcanized fiber bridges. The dynamo is a low tension machine, the field being regulated to give 65 volts when running the lamp circuits.

The illustration, Fig. 10, represents the automatic re-regulator--C.E.L. Brown's patent. Motion is imparted to the cores of two electro-magnets at the ends by the pulleys, W W1. The cores have a projection opposite to the spindle, ab, which latter is screw-threaded. By a relay one or other electro-magnet is put in action, and the rotating core, which is magnetized, causes rotation of the spindle by attraction, resulting in the movement of the contact along the resistance stops. The relay is acted upon directly by the potential of the dynamo, and the variable resistance is included in the shunt field of the machine, so that changes in the potential, resulting from changes in load or speed, are compensated for.

The arrangements of the lamp circuits and the lamp itself may now be described. The lamps are all run in parallel circuit, but are divided into groups of five, each group being controlled by a separate switch on the board--Figs. 11 and 11A. These switches are not in direct communication with the dynamo, but make that connection through a large central switch, S2, which therefore carries the whole current. The returns from each group are brought to the connections seen between the two resistances, where the circuits may be disconnected if desired, and the main current then passes through the ammeter, A3, to the other terminal of the machine. One of the smaller switches at the top, Fig. 11A, is directly connected with one terminal of the 20 horse power dynamo before mentioned, and the other side of the switch to the motor in the machine tool exhibit. Also one of the switches in connection with the central switch, S2, is connected to the same motor, and therefore the latter may be run by either machine, or, in fact, any combination of machines, lamps, and motor be made as required.

The form of switch made by the Oerlikon Works is illustrated in Fig. 7. Two thick semicircular bands of copper are screwed at one end to opposite sides of a square block which is turned round by the switch handle. The block has a projection at each corner, and two strong, flat, stationary springs are attached to the framework of the switch and press on opposite sides of the block. The ends of the springs engage in the projections and prevent the switch being turned round the wrong way, while the pressure of the springs on opposite sides forces the copper bands to take up a position exactly in line with the terminal contacts when the switch is closed, or at right angles to them when it is opened.

Further, each lamp has its own separate adjustable resistance, fuse, and switch. These are of special construction, combined in one, and are illustrated in Figs. 4 and 4A; the other figures, 4B and 4C, showing some of the details of the same. The wires, W W, lead from and to one lamp. The current enters at one wire, passes through the fuse, f--Figs. 4C and 4A--down the center of the cylinder to a divided contact, into which a switch arm can be shot. When this is so, a connection is made to the upright brass rod, T, which serves to grip the band, R, passing round the body of the cylinder. The current then passes through all the turns of wire above the band, and out at the other terminal. The resistance can be varied by raising or lowering the band. Fig. 4B shows the manner of tightening the band against the wires on the cylinder. The upright rod, T, is seen in section, and is fixed in one position to the frame of the apparatus. Abutting against this, and working in the block to which the two ends of the band are screwed, is a thumb screw, S, by turning which the band may be loosened for adjusting, and tightened when the right position is found. The cylinder is covered with asbestos sheet, and the wire, which is of nickel, and measures altogether from 3 to 4 ohms, is wound helically round this. The switch arm, to which the handle is attached below, does not itself make and break the circuit, but carries a spring, as shown, which, when the arm is at the end of its movement, pulls over the contact lever with a rapid action, shooting the same between the divided contact piece, and making a perfect contact. The switchboard forms one side of a closed wooden case or cupboard, with sufficient room for a man to enter and adjust the resistances or switches for each lamp. These are screwed to the inside of the case in rows, to the number of twenty-five. The greatest care has been taken in the fixing of the connections to the inside of this case, and no leading wires of different potential are allowed to cross each other.

The Oerlikon lamp, which is designed to work with constant potential, is shown partly in section in Fig. 8. There is only one solenoid, A, through which all the current passes, and whose action is to strike the arc and maintain the current constant. The soft iron core, C, is suspended from the inside of the tube, T, in which it has an up and down movement checked by an air piston in the tube. An end elevation of the brake wheels and solenoid is given in Fig. 9, where it will be seen that the spindle carrying these wheels also carries between them a pinion engaging with the rack rod, R. The top carbon attached to the rack rod falls by its own weight, and is therefore in contact with the lower carbon before the lamp is switched in circuit. When this is done the core is instantly magnetized, and attracted to the soft iron brake wheels, which it holds firmly. The air cushion in the tube prevents the core being drawn up until it has fairly gripped the sides of the wheels. The subsequent raising of the core therefore turns the wheels, raises the rack rod, and strikes the arc. The feed is operated by the weakening of the magnetic field of the coil, which causes the core to lose its grip of the wheels, and allows the top carbon to descend. The catch, L, Fig. 8, has a lateral play, and serves to engage in the teeth of the rack rod, so as to prevent its falling when being trimmed. Each carbon when in position is held against two rectangular guide bars by the pressure of a wire spring--see figure. In this way the carbon is pressed against two parallel knife edges, and is therefore always in true alignment. The action of the lamp is very simple, the working parts are few and solidly constructed, and the regulation, as exhibited by the lamps running in the galleries, is exceptionally steady.