Fig. 226.Fig. 226.
Other modifications of these methods are possible, but need not be pointed out. In all these plans, it will be observed, there is developed in one or all of these branches of a circuit from a source of alternating currents, an active (as distinguished from a dead) resistance or opposition to the currents of one sign, for the purpose of diverting the currents of that sign through the other or another path, but permitting the currents of opposite sign to pass without substantial opposition.
Whether the division of the currents or waves of current of opposite sign be effected with absolute precision or not is immaterial, since it will be sufficient if the waves are only partially diverted or directed, for in such case the preponderating influence in each branch of the circuit of the waves of one sign secures the same practical results in many if not all respects as though the current were direct and continuous.
An alternating and a direct current have been combined so that the waves of one direction or sign were partially or wholly overcome by the direct current; but by this plan only one set of alternations are utilized, whereas by the system just described the entire current is rendered available. By obvious applications of this discovery Mr. Tesla is enabled to produce a self-exciting alternating dynamo, or to operate direct current meters on alternating-current circuits or to run various devices—such as arc lamps—by direct currents in the same circuit with incandescent lamps or other devices operated by alternating currents.
It will be observed that if an intermittent counter or opposing force be developed in the branches of the circuit and of higher electromotive force than that of the generator, an alternating current will result in each branch, with the waves of one sign preponderating, while a constantly or uniformly acting opposition in the branches of higher electromotive force than the generator would produce a pulsating current, which conditions would be, under some circumstances, the equivalent of those described.
In experimenting with currents of high frequency and high potential, Mr. Tesla has found that insulating materials such as glass, mica, and in general those bodies which possess the highest specific inductive capacity, are inferior as insulators in such devices when currents of the kind described are employed compared with those possessing high insulating power, together with a smaller specific inductive capacity; and he has also found that it is very desirable to exclude all gaseous matter from the apparatus, or any access of the same to the electrified surfaces, in order to prevent heating by molecular bombardment and the loss or injury consequent thereon. He has therefore devised a method to accomplish these results and produce highly efficient and reliable condensers, by using oil as the dielectric[11]. The plan admits of a particular construction of condenser, in which the distance between the plates is adjustable, and of which he takes advantage.
Fig. 227.Fig. 227.Fig. 228.Fig. 228.
Fig. 227.Fig. 227.
Fig. 228.Fig. 228.
In the accompanying illustrations, Fig. 227 is a section of a condenser constructed in accordance with this principle and having stationary plates; and Fig. 228 is a similar view of a condenser with adjustable plates.
Any suitable box or receptacleAmay be used to contain the plates or armatures. These latter are designated byBandCand are connected, respectively, to terminalsDandE, which pass out through the sides of the case. The plates ordinarily are separated by strips of porous insulating materialF, which are used merely for the purpose of maintaining them in position. The space within the can is filled with oilG. Such a condenser will prove highly efficient and will not become heated or permanently injured.
In many cases it is desirable to vary or adjust the capacity of a condenser, and this is provided for by securing the plates to adjustable supports—as, for example, to rodsH—passing through stuffing boxesKin the sides of caseAand furnished with nutsL, the ends of the rods being threaded for engagement with the nuts.
It is well known that oils possess insulating properties, and it has been a common practice to interpose a body of oil between two conductors for purposes of insulation; but Mr. Tesla believes he has discovered peculiar properties in oils which render them very valuable in this particular form of device.
An ingenious form of electrolytic meter attributable to Mr. Tesla is one in which a conductor is immersed in a solution, so arranged that metal may be deposited from the solution or taken away in such a manner that the electrical resistance of the conductor is varied in a definite proportion to the strength of the current the energy of which is to be computed, whereby this variation in resistance serves as a measure of the energy and also may actuate registering mechanism, whenever the resistance rises above or falls below certain limits.
In carrying out this idea Mr. Tesla employs an electrolytic cell, through which extend two conductors parallel and in close proximity to each other. These conductors he connects in series through a resistance, but in such manner that there is an equal difference of potential between them throughout their entire extent. The free ends or terminals of the conductors are connected either in series in the circuit supplying the current to the lamps or other devices, or in parallel to a resistance in the circuit and in series with the current consuming devices. Under such circumstances a current passing through the conductors establishes a difference of potential between them which is proportional to the strength of the current, in consequence of which there is a leakage of current from one conductor to the other across the solution. The strength of this leakage current is proportional to the difference of potential, and, therefore, in proportion to the strength of the current passing through the conductors. Moreover, as there is a constant difference of potential between the two conductors throughout the entire extent that is exposed to the solution, the current density through such solution is the same at all corresponding points, and hence the deposit is uniform along the whole of one of the conductors, while the metal is taken away uniformly from the other. The resistance of one conductor is by this means diminished, while that of the other isincreased, both in proportion to the strength of the current passing through the conductors. From such variation in the resistance of either or both of the conductors forming the positive and negative electrodes of the cell, the current energy expended may be readily computed. Figs. 229 and 230 illustrate two forms of such a meter.
Fig. 229.Fig. 229.
In Fig. 229Gdesignates a direct-current generator.L Lare the conductors of the circuit extending therefrom.Ais a tube of glass, the ends of which are sealed, as by means of insulating plugs or capsB B.C C'are two conductors extending through the tubeA, their ends passing out through the plugsBto terminals thereon. These conductors may be corrugated or formed in other proper ways to offer the desired electrical resistance.Ris a resistance connected in series with the two conductorsC C', which by their free terminals are connected up in circuit with one of the conductorsL.
The method of using this device and computing by means thereof the energy of the current will be readily understood. First, the resistances of the two conductorsC C', respectively, are accurately measured and noted. Then a known current is passed through the instrument for a given time, and by a second measurement the increase and diminution of the resistances of the two conductors are respectively taken. From these data the constant isobtained—that is to say, for example, the increase of resistance of one conductor or the diminution of the resistance of the other per lamp hour. These two measurements evidently serve as a check, since the gain of one conductor should equal the loss of the other. A further check is afforded by measuring both wires in series with the resistance, in which case the resistance of the whole should remain constant.
Fig. 230.Fig. 230.
In Fig. 230 the conductorsC C'are connected in parallel, the current device atXpassing in one branch first through a resistanceR'and then through conductorC, while on the other branch it passes first through conductorC', and then through resistanceR''. The resistancesR' R''are equal, as also are the resistances of the conductorsC C'. It is, moreover, preferable that the respective resistances of the conductorsC C'should be a known and convenient fraction of the coils or resistancesR' R''. It will be observed that in the arrangement shown in Fig. 230 there is a constant potential difference between the two conductorsC C'throughout their entire length.
It will be seen that in both cases illustrated, the proportionality of the increase or decrease of resistance to the current strength will always be preserved, for what one conductor gains the other loses, and the resistances of the conductorsC C'being small ascompared with the resistances in series with them. It will be understood that after each measurement or registration of a given variation of resistance in one or both conductors, the direction of the current should be changed or the instrument reversed, so that the deposit will be taken from the conductor which has gained and added to that which has lost. This principle is capable of many modifications. For instance, since there is a section of the circuit—to wit, the conductorCorC'—that varies in resistance in proportion to the current strength, such variation may be utilized, as is done in many analogous cases, to effect the operation of various automatic devices, such as registers. It is better, however, for the sake of simplicity to compute the energy by measurements of resistance.
The chief advantages of this arrangement are, first, that it is possible to read off directly the amount of the energy expended by means of a properly constructed ohm-meter and without resorting to weighing the deposit; secondly it is not necessary to employ shunts, for the whole of the current to be measured may be passed through the instrument; third, the accuracy of the instrument and correctness of the indications are but slightly affected by changes in temperature. It is also said that such meters have the merit of superior economy and compactness, as well as of cheapness in construction. Electrolytic meters seem to need every auxiliary advantage to make them permanently popular and successful, no matter how much ingenuity may be shown in their design.
No electrical inventor of the present day dealing with the problems of light and power considers that he has done himself or his opportunities justice until he has attacked the subject of thermo-magnetism. As far back as the beginning of the seventeenth century it was shown by Dr. William Gilbert, the father of modern electricity, that a loadstone or iron bar when heated to redness loses its magnetism; and since that time the influence of heat on the magnetic metals has been investigated frequently, though not with any material or practical result.
For a man of Mr. Tesla's inventive ability, the problems in this field have naturally had no small fascination, and though he has but glanced at them, it is to be hoped he may find time to pursue the study deeper and further. For such as he, the investigation must undoubtedly bear fruit. Meanwhile he has worked out one or two operative devices worthy of note.[12]He obtains mechanical power by a reciprocating action resulting from the joint operations of heat, magnetism, and a spring or weight or other force—that is to say he subjects a body magnetized by induction or otherwise to the action of heat until the magnetism is sufficiently neutralized to allow a weight or spring to give motion to the body and lessen the action of the heat, so that the magnetism may be sufficiently restored to move thebody in the opposite direction, and again subject the same to the demagnetizing power of the heat.
Use is made of either an electro-magnet or a permanent magnet, and the heat is directed against a body that is magnetized by induction, rather than directly against a permanent magnet, thereby avoiding the loss of magnetism that might result in the permanent magnet by the action of heat. Mr. Tesla also provides for lessening the volume of the heat or for intercepting the same during that portion of the reciprocation in which the cooling action takes place.
In the diagrams are shown some of the numerous arrangements that may be made use of in carrying out this idea. In all of these figures the magnet-poles are markedN S, the armatureA, the Bunsen burner or other source of heatH, the axis of motionM, and the spring or the equivalent thereof—namely, a weight—is markedW.
Fig. 232, 231, 233.Fig. 232.Fig. 231.Fig. 233.
In Fig. 231 the permanent magnetNis connected with a frame,F, supporting the axisM, from which the armPhangs, and at the lower end of which the armatureAis supported. The stops 2 and 3 limit the extent of motion, and the springWtends to draw the armatureAaway from the magnetN. It will now be understood that the magnetism ofNis sufficient to overcome the springWand draw the armatureAtoward the magnetN. The heat acting upon the armatureAneutralizes its induced magnetism sufficiently for the springWto draw the armature A away from the magnetNand also from the heat atH. The armature now cools, and the attraction of the magnetNovercomes the springWand draws the armatureAback again above the burnerH, so that the same is again heated and the operations are repeated. The reciprocating movements thus obtained are employed as a source of mechanical power in any desired manner. Usually a connecting-rod to a crank upon a fly-wheel shaft would be made use of, as indicated in Fig. 240.
Fig. 234, 236, 235.Fig. 234.Fig. 236.Fig. 235.
Fig. 232 represents the same parts as before described; but an electro-magnet is illustrated in place of a permanent magnet. The operations, however, are the same.
In Fig. 233 are shown the same parts as in Figs. 231 and 232, but they are differently arranged. The armatureA, instead of swinging, is stationary and held by armP', and the coreN Sof the electro-magnet is made to swing within the helixQ, the core being suspended by the armPfrom the pivotM. A shield,R, is connected with the magnet-core and swings with it, so that after the heat has demagnetized the armatureAto such an extent that the springWdraws the coreN Saway from the armatureA, the shieldRcomes between the flameHand armatureA, thereby intercepting the action of the heat and allowing the armature to cool, so that the magnetism, again preponderating, causes the movement of the coreN Stoward the armatureAand the removal of the shieldRfrom above the flame, so that the heat again acts to lessen or neutralize the magnetism. A rotary or other movement may be obtained from this reciprocation.
Fig. 234 corresponds in every respect with Fig. 233, except that a permanent horseshoe-magnet,N Sis represented as taking the place of the electro-magnet in Fig. 233.
In Fig. 235 is shown a helix,Q, with an armature adapted to swing toward or from the helix. In this case there may be a soft-iron core in the helix, or the armature may assume the form of a solenoid core, there being no permanent core within the helix.
Fig. 237, 238, 239.Fig. 237.Fig. 238.Fig. 239.
Fig. 236 is an end view, and Fig. 237 a plan view, illustrating the method as applied to a swinging armature,A, and a stationary permanent magnet,N S. In this instance Mr. Tesla applies the heat to an auxiliary armature or keeper,T, which is adjacent to and preferably in direct contact with the magnet. This armatureT, in the form of a plate of sheet-iron, extends across from one pole to the other and is of sufficient section to practically form a keeper for the magnet, so that when the armatureTis cool nearly all the lines of force pass over the same and very little free magnetism is exhibited. Then the armatureA, which swings freely on the pivotsMin front of the polesN S, is very little attracted and the springWpulls the same way from the poles into the position indicated in the diagram. The heat is directed upon the iron plateTat some distance from the magnet, so as to allow the magnet to keep comparatively cool. This heat is applied beneath the plate by means of the burnersH, and there is a connection from the armatureAor its pivot to the gas-cock 6, or other device for regulating the heat. The heat acting upon the middle portion of the plateT, the magnetic conductivity of the heated portion is diminished or destroyed, and a great number of the lines of force are deflected over the armatureA, which is now powerfully attracted and drawn into line, or nearly so, with the polesN S. In so doing the cock 6 is nearly closed and the plateTcools, the lines of force are again deflected over the same, the attraction exerted upon the armatureAis diminished, and the springWpulls the same away from the magnet into the position shown by full lines, and the operations are repeated. The arrangement shown in Fig. 236 has the advantages that the magnet and armature are kept cool and the strength of the permanent magnet is better preserved, as the magnetic circuit is constantly closed.
In the plan view, Fig. 238, is shown a permanent magnet and keeper plate,T, similar to those in Figs. 236 and 237, with the burnersHfor the gas beneath the same; but the armature is pivoted at one end to one pole of the magnet and the other end swings toward and from the other pole of the magnet. The springWacts against a lever arm that projects from the armature, and the supply of heat has to be partly cut off by a connection to the swinging armature, so as to lessen the heat acting upon the keeper plate when the armatureAhas been attracted.
Fig. 240, 241.Fig. 240.Fig. 241.
Fig. 239 is similar to Fig. 238, except that the keeperTis not made use of and the armature itself swings into and out of the range of the intense action of the heat from the burnerH. Fig. 240 is a diagram similar to Fig. 231, except that in place of using a spring and stops, the armature is shown as connected by a link, to the crank of a fly-wheel, so that the fly-wheel will be revolved as rapidly as the armature can be heated and cooled to the necessary extent. A spring may be used in addition, as in Fig. 231. In Fig. 241 the armaturesA Aare connected by a link, so that one will be heating while the other is cooling, and the attraction exerted to move the cooled armature is availed of to draw away the heated armature instead of using a spring.
Mr. Tesla has also devoted his attention to the development of a pyromagnetic generator of electricity[13]based upon the following laws: First, that electricity or electrical energy is developed in any conducting body by subjecting such body to a varying magnetic influence; and second, that the magnetic properties of iron or other magnetic substance may be partially or entirely destroyed or caused to disappear by raising it to a certain temperature, but restored and caused to reappear by again lowering its temperature to a certain degree. These laws may be applied in the production of electrical currents in many ways, the principle of which is in all cases the same, viz., to subject a conductor to a varying magnetic influence, producing such variations by the application of heat, or, more strictly speaking, by the application or action of a varying temperature upon the source of the magnetism. This principle of operation may be illustrated by a simple experiment: Place end to end, and preferably in actual contact, a permanently magnetized steel bar and a strip or bar of soft iron. Around the end of the iron bar or plate wind a coil of insulated wire. Then apply to the iron between the coil and the steel bar a flame or other source of heat which will be capable of raising that portion of the iron to an orange red, or a temperature of about 600° centigrade. When this condition is reached, the iron somewhat suddenly loses its magnetic properties, if it be very thin, and the same effect is produced as though the iron had been moved away from the magnet or the heated section had been removed. This change of position, however, is accompanied by a shifting of the magnetic lines, or, in other words, by a variation in the magnetic influence to which the coil is exposed, and a current in the coil is the result. Then remove the flame or in any other way reduce the temperature of the iron. The lowering of its temperature is accompanied by a return of its magnetic properties, and another change of magnetic conditions occurs, accompanied by a current in an opposite direction in the coil. The same operation may berepeated indefinitely, the effect upon the coil being similar to that which would follow from moving the magnetized bar to and from the end of the iron bar or plate.
The device illustrated below is a means of obtaining this result, the features of novelty in the invention being, first, the employment of an artificial cooling device, and, second, inclosing the source of heat and that portion of the magnetic circuit exposed to the heat and artificially cooling the heated part.
These improvements are applicable generally to the generators constructed on the plan above described—that is to say, we may use an artificial cooling device in conjunction with a variable or varied or uniform source of heat.
Fig. 242, 243.Fig. 242.Fig. 243.
Fig. 242 is a central vertical longitudinal section of the complete apparatus and Fig. 243 is a cross-section of the magnetic armature-core of the generator.
LetArepresent a magnetized core or permanent magnet the poles of which are bridged by an armature-core composed of a casing or shellBinclosing a number of hollow iron tubesC. Around this core are wound the conductorsE E', to form the coils in which the currents are developed. In the circuits of these coils are current-consuming devices, asF F'.
Dis a furnace or closed fire-box, through which the central portion of the coreBextends. Above the fire is a boilerK, containing water. The flueLfrom the fire-box may extend up through the boiler.
Gis a water-supply pipe, andHis the steam-exhaust pipe, which communicates with all the tubesCin the armatureB, so that steam escaping from the boiler will pass through the tubes.
In the steam-exhaust pipeHis a valveV, to which is connected the leverI, by the movement of which the valve is opened or closed. In such a case as this the heat of the fire may be utilized for other purposes after as much of it as may be needed has been applied to heating the coreB. There are special advantages in the employment of a cooling device, in that the metal of the coreBis not so quickly oxidized. Moreover, the difference between the temperature of the applied heat and of the steam, air, or whatever gas or fluid be applied as the cooling medium, may be increased or decreased at will, whereby the rapidity of the magnetic changes or fluctuations may be regulated.
In direct current dynamos of great electromotive force—such, for instance, as those used for arc lighting—when one commutator bar or plate comes out of contact with the collecting-brush a spark is apt to appear on the commutator. This spark may be due to the break of the complete circuit, or to a shunt of low resistance formed by the brush between two or more commutator-bars. In the first case the spark is more apparent, as there is at the moment when the circuit is broken a discharge of the magnets through the field helices, producing a great spark or flash which causes an unsteady current, rapid wear of the commutator bars and brushes, and waste of power. The sparking may be reduced by various devices, such as providing a path for the current at the moment when the commutator segment or bar leaves the brush, by short-circuiting the field-helices, by increasing the number of the commutator-bars, or by other similar means; but all these devices are expensive or not fully available, and seldom attain the object desired.
To prevent this sparking in a simple manner, Mr. Tesla some years ago employed with the commutator-bars and intervening insulating material, mica, asbestos paper or other insulating and incombustible material, arranged to bear on the surface of the commutator, near to and behind the brush.
In the drawings, Fig. 244 is a section of a commutator with an asbestos insulating device; and Fig. 245 is a similar view, representing two plates of mica upon the back of the brush.
In 244,Crepresents the commutator and intervening insulating material;B B, the brushes.d dare sheets of asbestos paper or other suitable non-conducting material.f fare springs, the pressure of which may be adjusted by means of the screwsg g.
In Fig. 245 a simple arrangement is shown with two plates of mica or other material. It will be seen that whenever one commutator segment passes out of contact with the brush, the formation of the arc will be prevented by the intervening insulating material coming in contact with the insulating material on the brush.
Fig. 244, 245.Fig. 244.Fig. 245.
Asbestos paper or cloth impregnated with zinc-oxide, magnesia, zirconia, or other suitable material, may be used, as the paper and cloth are soft, and serve at the same time to wipe and polish the commutator; but mica or any other suitable material can be employed, provided the material be an insulator or a bad conductor of electricity.
A few years later Mr. Tesla turned his attention again to the same subject, as, perhaps, was very natural in view of the fact that the commutator had always been prominent in his thoughts, and that so much of his work was even aimed at dispensing with it entirely as an objectionable and unnecessary part of dynamos and motors. In these later efforts to remedy commutator troubles, Mr. Tesla constructs a commutator and the collectors therefor in two parts mutually adapted to one another, and, so far as the essential features are concerned, alike in mechanical structure. Selecting as an illustration a commutator of two segments adapted for use with an armature the coils or coil of which have but two free ends, connected respectively to the segments, the bearing-surface is the face of a disc, and is formed of two metallic quadrant segments and two insulating segments of the same dimensions, and the face of the disc is smoothed off, so that the metal and insulating segments are flush. The part which takes the place of the usual brushes, or the "collector," is a disc of the same character as the commutator and has a surface similarly formed with two insulating and two metallic segments. These two parts are mounted with their faces in contact and in such manner that the rotation of the armature causes the commutator to turn upon the collector, whereby the currents induced in thecoils are taken off by the collector segments and thence conveyed off by suitable conductors leading from the collector segments. This is the general plan of the construction adopted. Aside from certain adjuncts, the nature and functions of which are set forth later, this means of commutation will be seen to possess many important advantages. In the first place the short-circuiting and the breaking of the armature coil connected to the commutator-segments occur at the same instant, and from the nature of the construction this will be done with the greatest precision; secondly, the duration of both the break and of the short circuit will be reduced to a minimum. The first results in a reduction which amounts practically to a suppression of the spark, since the break and the short circuit produce opposite effects in the armature-coil. The second has the effect of diminishing the destructive effect of a spark, since this would be in a measure proportional to the duration of the spark; while lessening the duration of the short circuit obviously increases the efficiency of the machine.
Fig. 246, 247.Fig. 246.Fig. 247.
The mechanical advantages will be better understood by referring to the accompanying diagrams, in which Fig. 246 is a central longitudinal section of the end of a shaft with the improved commutator carried thereon. Fig. 247 is a view of the inner or bearing face of the collector. Fig. 248 is an end view from the armature side of a modified form of commutator. Figs.249 and 250 are views of details of Fig. 248. Fig. 251 is a longitudinal central section of another modification, and Fig. 252 is a sectional view of the same.Ais the end of the armature-shaft of a dynamo-electric machine or motor.A'is a sleeve of insulating material around the shaft, secured in place by a screw,a'.
Fig. 248, 249, 250.Fig. 248.Fig. 249. Fig. 250.
The commutator proper is in the form of a disc which is made up of four segmentsD D'G G', similar to those shown in Fig. 248. Two of these segments, asD D', are of metal and are in electrical connection with the ends of the coils on the armature. The other two segments are of insulating material. The segments are held in place by a band,B, of insulating material. The disc is held in place by friction or by screws,g' g', Fig. 248, which secure the disc firmly to the sleeveA'.
The collector is made in the same form as the commutator. It is composed of the two metallic segmentsE E'and the two insulating segmentsF F', bound together by a band,C. The metallic segmentsE E'are of the same or practically the same width or extent as the insulating segments or spaces of the commutator. The collector is secured to a sleeve,B', by screwsg g, and the sleeve is arranged to turn freely on the shaftA. The end of the sleeveB'is closed by a plate,f, upon which presses a pivot-pointed screw,h, adjustable in a spring,H, which acts to maintain the collector in close contact with the commutator and to compensate for the play of the shaft. The collector is so fixed that it cannot turn with the shaft. For example, the diagram shows a slotted plate,K, which is designed to be attached to a stationary support, and an arm extending from the collector and carrying a clamping screw,L, by which the collector may be adjusted and set to the desired position.
Mr. Tesla prefers the form shown in Figs. 246 and 247 to fitthe insulating segments of both commutator and collector loosely and to provide some means—as, for example, light springs,e e, secured to the bandsA' B', respectively, and bearing against the segments—to exert a light pressure upon them and keep them in close contact and to compensate for wear. The metal segments of the commutator may be moved forward by loosening the screwa'.
The line wires are fed from the metal segments of the collector, being secured thereto in any convenient manner, the plan of connections being shown as applied to a modified form of the commutator in Fig. 251. The commutator and the collector in thus presenting two flat and smooth bearing surfaces prevent most effectually by mechanical action the occurrence of sparks.
The insulating segments are made of some hard material capable of being polished and formed with sharp edges. Such materials as glass, marble, or soapstone may be advantageously used. The metal segments are preferably of copper or brass; but they may have a facing or edge of durable material—such as platinum or the like—where the sparks are liable to occur.
Fig. 251, 252.Fig. 251.Fig. 252.
In Fig. 248 a somewhat modified form of the invention is shown, a form designed to facilitate the construction and replacing of the parts. In this modification the commutator and collector are made in substantially the same manner as previously described, except that the bandsB Care omitted. The four segments of each part, however, are secured to their respective sleeves by screwsg' g', and one edge of each segment is cut away, so that small platesa bmay be slipped into the spaces thus formed. Ofthese platesa aare of metal, and are in contact with the metal segmentsD D', respectively. The other two,b b, are of glass or marble, and they are all better square, as shown in Figs. 249 and 250, so that they may be turned to present new edges should any edge become worn by use. Light springsdbear upon these plates and press those in the commutator toward those in the collector, and insulating stripsc care secured to the periphery of the discs to prevent the blocks from being thrown out by centrifugal action. These plates are, of course, useful at those edges of the segments only where sparks are liable to occur, and, as they are easily replaced, they are of great advantage. It is considered best to coat them with platinum or silver.
In Figs. 251 and 252 is shown a construction where, instead of solid segments, a fluid is employed. In this case the commutator and collector are made of two insulating discs,S T, and in lieu of the metal segments a space is cut out of each part, as atR R', corresponding in shape and size to a metal segment. The two parts are fitted smoothly and the collectorTheld by the screwhand springHagainst the commutatorS. As in the other cases, the commutator revolves while the collector remains stationary. The ends of the coils are connected to binding-postss s, which are in electrical connection with metal platest twithin the recesses in the two partsS T. These chambers or recesses are filled with mercury, and in the collector part are tubesW W, with screwsw w, carrying springsXand pistonsX', which compensate for the expansion and contraction of the mercury under varying temperatures, but which are sufficiently strong not to yield to the pressure of the fluid due to centrifugal action, and which serve as binding-posts.
In all the above cases the commutators are adapted for a single coil, and the device is particularly suited to such purposes. The number of segments may be increased, however, or more than one commutator used with a single armature. Although the bearing-surfaces are shown as planes at right angles to the shaft or axis, it is evident that in this particular the construction may be very greatly modified.
An interesting method devised by Mr. Tesla for the regulation of direct current dynamos, is that which has come to be known as the "third brush" method. In machines of this type, devised by him as far back as 1885, he makes use of two main brushes to which the ends of the field magnet coils are connected, an auxiliary brush, and a branch or shunt connection from an intermediate point of the field wire to the auxiliary brush.[14]
The relative positions of the respective brushes are varied, either automatically or by hand, so that the shunt becomes inoperative when the auxiliary brush has a certain position upon the commutator; but when the auxiliary brush is moved in its relation to the main brushes, or the latter are moved in their relation to the auxiliary brush, the electric condition is disturbed and more or less of the current through the field-helices is diverted through the shunt or a current is passed over the shunt to the field-helices. By varying the relative position upon the commutator of the respective brushes automatically in proportion to the varying electrical conditions of the working-circuit, the current developed can be regulated in proportion to the demands in the working-circuit.
Fig. 253 is a diagram illustrating the invention, showing one core of the field-magnets with one helix wound in the same direction throughout. Figs. 254 and 255 are diagrams showing one core of the field-magnets with a portion of the helices wound in opposite directions. Figs. 256 and 257 are diagrams illustratingthe electric devices that may be employed for automatically adjusting the brushes, and Fig. 258 is a diagram illustrating the positions of the brushes when the machine is being energized at the start.
aandbare the positive and negative brushes of the main or working-circuit, andcthe auxiliary brush. The working-circuitDextends from the brushesaandb, as usual, and contains electric lamps or other devices,D', either in series or in multiple arc.
M M'represent the field-helices, the ends of which are connected to the main brushesaandb. The branch or shunt wirec'extends from the auxiliary brushcto the circuit of the field-helices, and is connected to the same at an intermediate point,x.
Fig. 253.Fig. 253.
Hrepresents the commutator, with the plates of ordinary construction. When the auxiliary brushcoccupies such a position upon the commutator that the electro-motive force between the brushesaandcis to the electro-motive force between the brushescandbas the resistance of the circuitaMc' cAis to the resistance of the circuitbM'c' cB, the potentials of the pointsxandYwill be equal, and no current will flow over the auxiliary brush; but when the brushcoccupies a different position the potentials of the pointsxandYwill be different, and a current will flow over the auxiliary brush to and from the commutator, according to the relative position of the brushes. If, for instance, the commutator-space between the brushesaandc, when the latter is at the neutral point, is diminished, a current will flow from the pointYover the shuntcto the brushb, thus strengthening the current in the partM', and partly neutralizing the current in partM; but if the space between the brushesaandcis increased, the current will flow over the auxiliary brush in an opposite direction, and the current inMwill be strengthened, and inM', partly neutralized.
By combining with the brushesa,b, andcany usual automatic regulating mechanism, the current developed can be regulated in proportion to the demands in the working circuit. The partsMandM'of the field wire may be wound in the same direction. In this case they are arranged as shown in Fig. 253; or the partMmay be wound in the opposite direction, as shown in Figs. 254 and 255.