Fig. 352.—Laminated armature core with through retaining bolts. In the larger sizes, these bolts are used instead of a nut threaded on the shaft on account of the large size of the latter.
Ques. What is the construction of the core end plates, and why?
Ans. The rims are beveled quite thin to avoid eddy currents.
Ques. How is the core connected to the shaft?
Ans. Since the core has the full torque exerted upon it by the drag of the inductors, it must be firmly connected to the shaft by means of a key, as shown, so that it may be positively driven.
Core discs are stamped in one piece up to about 30 inches in diameter, and for larger sizes they are built up from sections as later described.
Figs. 353 and 354 show two forms of disc stamped in one piece. The first illustrates a solid disc, and the second a ventilated disc in which more or less of the metal is cut away near the center, thus providing passages for the circulation of air which carries away some of the heat generated in the armature.
Figs. 353 and 354.—Solid and ventilated core discs. In fig. 353, the metal cut away near the center reduces the weight and provides passages for air circulation. In some instances a forced circulation is secured by means of a fan attached to the armature, as shown in fig. 366.
Insulation of Core Discs.—When the discs are stamped from very thin metal, the mere existence of a film of oxide is sufficient insulation. It is usual, however, to apply a quick drying varnish that will give a hard tough coat and not soften with heat or become brittle and crumble under vibration. The varnish may be applied either by dipping or with a japanning machine; it must be very thin, and the solvent employed should be a very volatile spirit.
Forms of Armature Teeth.—The teeth stamped in the core discs are made in various shapes, depending largely on the method of securing the inductors in the slots against electromagnetic drag and centrifugal force. The teeth may be cut with their sides:
1. Inclined;
2. Projecting;
3. Notched.
Ques. What may be said of teeth with inclined sides?
Ans. A tooth of this type is shown in fig. 356, being slightly narrower at the root than at the top, the resulting slot having parallel sides.
Fig. 355.—Western Electric slotted armature core. The laminations are of sheet steel, annealed and japanned. They are mounted directly on the shaft, (except in the large sizes) and held in place by substantial end plates.
Ques. What are the features of the projecting type of tooth?
Ans. The projecting type is shown in figs. 357 and 358 in which the tops project; this gives a larger core area around the circumference of the armature which reduces the reluctance of the air gap, and provides projecting surfaces for retaining the inductors in the slots by the insertion of wedges.
Ques. What is the object of cutting notches in teeth?
Ans. They are provided for the insertion of retaining wedges, as in fig. 361; this results in less area at the top of the teeth.
Ques. How should teeth be proportioned to secure most efficient operation?
Ans. The width of the tooth should be about equal to the width of the slot minus twice the thickness of the slot insulation; that is, the cross sectional area of the teeth should be equal to that of the slots.
Figs. 356 to 359.—Various forms of armature teeth; fig. 356 inclined type forming a slot with parallel sides; figs. 357 and 358 projecting type which provides a support for the retaining wedges; fig. 359 enclosed type which forms "tunnels" for the inductors.
Advantages and Defects of Slotted Armatures.—The slotted armature, sometimes called the Pacinotti armature, after its inventor, has the following advantages over the smooth type:
1. The inductors are held more firmly in place to resist stresses due to electromagnetic drag and centrifugal force;
2. The inductors are protected by the teeth against mechanical injury;
3. Less reluctance of the air gap;
4. The intermittent induction due to the presence of the teeth prevents the formation of eddy currents.
5. When the teeth are saturated they oppose the shifting of the lines due to armature reaction.
Figs. 360 and 361.—Projecting and notched teeth; cross sections showing inductors and retaining wedges in place.
The disadvantages of slotted armatures compared with the smooth type are:
1. Greater hysteresis loss, caused by denser flux in the teeth;
2. Generation of eddy currents in the polar faces when the latter are not of laminated construction;
3. Greater self-induction in the armature coils;
4. Construction more expensive;
5. Leakage of magnetic lines through core, exterior to winding.
The generation of eddy currents in the polar faces may be overcome by making the air gap at least 50 per cent. of the distance between the teeth, so that the magnetic lines can spread from the corners of the teeth, and become nearly uniformly distributed over the polar faces. Magnetic leakage through the core may be reduced by making the amount of metal above the inductors very small.
Slotted Cores; Built Up Construction.—In the case of large dynamos, the core discs are built up in order to reduce the cost of construction; the following parts are used:
1. Spider;
2. Core rings split into sections.
Figs. 362 and 363.—Side and end view of built up armature core. The sheet metal ring sections containing the teeth are fastened into dovetail notches in the spider as shown. The layers of ring sections are placed so as to break joints and are held by end clamps and through bolts B. Distance pieces are inserted at intervals to provide ventilating spaces D, D, D.
Ques. What is the approved method of core construction in large armatures?
Ans. The core should be of the built up construction to avoid waste of material in the stampings.
Ques. Describe the construction of a built up core.
Ans. Ring sections stamped, from sheet metal are fastened to a central support or spider, which consists of an iron hub with radiating spokes and a rim with provision for fastening the rings. The rim of the spider is provided with dovetail notches into which fit similarly shaped internal projections on the core segments. These features are shown in figs. 362 to 364. Each layer of core sections is placed on the spider so as to break joints and the core thus formed is firmly held in place by end clamps as shown. The manner of fastening the rings to the spider is an important point, for it must be done without reducing the effective cross section of the core in order not to choke the magnetic flux.
Fig. 364.—Built up core with four spoke spider, each spoke carrying two dovetail notches In this construction a little more air space is obtained for ventilation than where a separate spoke is provided for each notch.
In order to secure a better fit and reduce the machine work, the spider hub in large machines is sometimes cored with enlarged section between the outer bearing surfaces, and it is not unusual to find these surfaces turned to two different sizes as in fig. 365, to admit of easier erecting.
To avoid any trouble that may arise by unequal expansion, the rim of the spider is not made continuous, but in several sections as shown in fig. 364. The rim here consists of four sections each of which has two dovetail notches. By thus dividing the rim into sections, its weight is somewhat reduced and the ventilating spaces between the sections increased.
Ventilation.—In the operation of a dynamo more or less heat is generated, depending on the load; hence it is desirable that provision be made to carry off some of this heat to prevent excessive rise of temperature.
Fig. 365.—Hub and shaft design on large machines to reduce the machine work and facilitate erecting.
Ques. Why do armature cores heat?
Ans. They heat from these causes: eddy currents, hysteresis, and heat generated in the inductors.
Ques. How is adequate ventilation secured?
Ans. The spider is constructed with as much open space as possible through which air currents may circulate. The core is divided into several sections with intervening air spaces D as shown in fig. 363, the discs being kept apart at these points by distance pieces. These openings between the discs are called ventilating ducts; they are usually spaced from 2 to 4 inches apart.
Fig. 366.—Western Electric barrel wound armature, having a fan attached at one end to induce a circulation of air for ventilation.
Ques. What other provision is sometimes made to secure ventilation?
Ans. In some machines a forced circulation of air is secured by means of a fan attached to one end of the armature as shown in fig. 366.
Insulation of Core.—Before the winding is assembled on the core, the latter should be thoroughly insulated. Japan or enamel insulation is not sufficient because it is liable to have bubbles or minute holes in it, or be pierced by particles of metal or by the rough edges of the core discs. Two or more layers of strong paper, fibre, canvas or mica, should be applied to the core before placing the inductors in position. The ends of the core should be insulated with thicker material, since the strain upon it is greater, especially at the edges.
Fig. 367.—Holzer Cabot partially wound barrel wound armature showing arrangement of coils. The core is built up of thin discs of soft annealed steel, which are slotted to allow the wire to sink below the surface, this being sometimes called iron clad construction. The discs are held by end plates, clamped without through bolts. The coils are machine formed of round ribbon or bar copper depending on the size and purpose of the machine, being without joint except at the commutator. They lie in insulated troughs, the upper layers being insulated from the lower layers by fibre.
Armature Windings.—The subject of windings has been fully treated from the theoretical point of view in chapter XVIII. It remains then to explain the different methods employed in the shop and the mechanical devices used to construct the scheme of winding adopted.
Ques. What is the construction of the inductors?
Ans. They are made of copper; the ordinary form consists of simple copper wire, insulated with a double or triple covering of cotton, and in some cases copper bars are used for large current machines.
Ques. What is the objection to copper bars?
Ans. They are liable to have eddy currents set up in them as illustrated in fig. 291.
Fig. 368.—Holzer Cabot iron clad band wound armature complete; view showing openings for ventilation. The advantage of the form of winding adopted, is the ease with which a coil may be replaced in case of injury and the additional cooling surface. The coils are held in place by maple wedges secured by binding wires which are soldered throughout their length.
Ques. What may be said with respect to the sizes of wire used for inductors?
Ans. Wire larger than about number 8 B and S gauge (.1285 inch diameter) is not easily handled, hence for large inductors, two or more wires may be wound together in parallel.
According to the mechanical features and manner of assembling on the core, drum windings may be divided into several classes, as follows:
1. Hand winding;
2. Evolute or butterfly winding;
3. Barrel winding;
4. Bastard winding;
5. Former winding.
Hand Winding.—The first windings were put on by hand and proved objectionable on account of the clumsy overlapping of the wires at the ends of the armature, which stops ventilation and hinders repairs, while the outer layers overlying those first wound, bring into close proximity inductors of widely varying voltage. The method is still used in special cases and for small machines. Such a winding has rarely, if ever, been made with one continuous wire.
Figs. 369 and 370.—Evolute and "straight out" connectors. In small machines the connectors must be curved as in fig. 369, but in large machines, especially where the teeth are wide, they may be straight as in fig. 370. These connectors may take either of the following forms: 1. involute or evolute connectors—An involute is the curve drawn by the extremity of a piece of string which is unwound from a cylinder; 2. spiral connectors—These consist of double spirals, the commutator being usually connected to the junction of the two spirals. These connectors are also known as "butterfly" connectors.
Evolute or Butterfly Winding.—This mode of winding, was introduced by Siemens for electroplating dynamos to overcome the objections to hand winding. It takes its name from the method of uniting the inductors by means of spiral end connectors as shown in fig. 374, also in figs. 369 and 370, which show more modern forms.
Fig. 371.—Holzer Cabot armature; rear view showing back head and coil guard. The construction of core and winding is described in fig. 367. The shaft is of crucible steel ground to gauge. The commutator segments are of drop forged copper in the smaller and hard-drawn copper in the larger sizes. The insulating material between the segments is mica. On the larger sizes, the commutator shell is fitted with a thread and mounted on a spider. This construction provides openings between the commutator and shaft for ventilation.
Ques. What are evolute connectors?
Ans. The fork shaped strips used to connect bars at different positions on the armature, as shown in fig. 369.
In large machines, especially where the teeth are wide, these connections may be straight, but in small cc machines they must be curved in the manner shown in the upper part of the figure, as the room available may diminish by as much as half, as the lowest point is reached, and the room occupied by the strip is the width of a horizontal section at various points. This width, in the case of the straight connections, is constant.
In place of the wooden block, used in early machines, for fastening the middle part of the connectors, they may be anchored to an insulated clamping device built up like a commutator and for that reason called a false commutator.
Figs. 372 and 373.—Barrel and evolute windings; end views showing placement of coils. When all the coils are wound on the former, the placing of them on the armature is a simple matter. After insulating the slots, the winder begins at any convenient slot, and inserts the coils as shown. Before he can fill all the slots, some of the first coils must be raised and the last ones inserted underneath. There is not much difference between barrel and evolute winding and one style may be used at one end of the armature and the other at the opposite end.
Ques. How are the inductors arranged in evolute winding?
Ans. In fig. 373, it will be seen that the ends of the evolute connectors lie in two planes, hence the inductors must project to different distances beyond the core. Accordingly, one long and one short bar may be conveniently placed in each slot, side by side. In large machines, especially where the teeth are wide, the connectors may be straight as in fig, 370. Evolute connectors may be used for either lap or wave windings.
Fig. 374.—Siemens' bar armature; end view. Each inductor in the form of a bar is connected to the next by means of two evolute spiral copper strips, one bending inwardly, the other outwardly, their junction being in some cases secured to a block of wood upon the shaft. Their outer ends are attached to the bars by rivets or silver solder.
Barrel Winding.—This is a form of drum winding in which the inductors are arranged in two layers and carried out obliquely on an extension of the cylindrical surface of the drum to meet and connect with radial risers.
Figs. 375 and 376.—Single layer and double layer barrel winding. Barrel winding is a method of arranging the ends of armature coils as they pass from one pole to the next, in which, instead of using involute or butterfly connections, V-shaped end connections are used which lie on a cylindrical surface, which is a continuation of the armature surface. The coil ends must of necessity be arranged in two layers, but the method may be used for either one or two coils per slot, the difference in arrangement for these is here illustrated.
Barrel winding has been very widely adopted. Although it involves an increased length of armature, this gives additional cooling surface and provides for good ventilation.
In barrel winding, the coil ends must of necessity be arranged in two layers but the method may be used for either one or two coils per slot, the difference in arrangement for these two cases being shown in figs. 375 and 376. In the single layer barrel winding, fig. 375, each slot is occupied by but one side of one coil. In the double layer barrel winding, fig. 376, the opposite sides of two separate coils occupy space in the same slot. The coils, on emerging from the slots bend in opposite directions, and if one side of a coil occupy the bottom portion of a slot, its other side usually occupies the top portion of a slot distant from the first slot by the polar pitch.
Fig. 377.—Westinghouse barrel wound armature. The coils are former wound from copper strap and are interchangeable. In the larger size machine they are of the single type. The illustration shows plainly the characteristic feature of barrel winding, namely the oblique end connectors carried out on the extended drum.
Bastard Winding.—In this type of winding, the end connectors project from the inductors in straight lines parallel to the shaft and then are bent inward. It has the effect of being somewhat shorter than the barrel winding. In order to secure better ventilation, it is usual to combine a bastard winding at the rear end of the armature with a barrel winding at the commutator end. This class of winding is used only with bar armatures.
Fig. 378.—Rear end of Westinghouse wave-barrel wound armature; view showing ventilation.
Former Winding.—This relates to a method of winding coils, and not to any particular type; that is, mechanical winding as distinguished from hand winding. While hand winding is necessary for ring armatures, a drum armature is wound better and more easily by the aid of machinery.
Ques. What is a "former" coil?
Ans. A former coil, as its name suggests, is one that is wound complete upon a former before being placed upon the armature.
Ques. What is the advantage of this method of winding coils?
Ans. By the use of formers much time is saved, thus reducing the cost, and also by their use all the coils are symmetrical which improves the appearance of the finished winding.
Figs. 379 and 380.—Diagrams illustrating lap and wave barrel windings.
Ques. How is the required shape of the template or former for winding the coils determined?
Ans. By winding one coil on the armature in order to ascertain its dimensions and shape; it is then removed from the armature and used as a pattern in constructing the former.
Types of Former Coil.—Of the numerous shapes of former coil, mention should be made of:
1. Evolute coils;
2. Straight out coils.
Ques. Describe the evolute type of former coil.
Ans. The evolute coil is wound around eight pins inserted in a board as shown in fig. 381. The required number of turns are taken around these pins and their ends G and H left projecting. The coil thus formed is now covered with tape and after removal from the board, is put into a clamp at C and F, and opened up as shown in fig. 382, which is the form required for insertion in the proper slots of the armature.
Fig. 381.—Method of winding evolute coils. In preparing the former, it is necessary to know the dimensions of the coil, hence, a pattern coil must first be made, from which the spacing of the pins can be taken so that the completed coil will fit into the slots for which it is intended. After the pins have been properly spaced on the board, the wire is wound around them as indicated, as many turns being taken as decided on for each coil. When the coil is thus completely wound, it is taken from the pins, and the lower ends, C and F, placed in a suitable clamp. The two halves of the coil are then spread apart, the coil assuming the shape illustrated in fig. 382.
Fig. 382.—Appearance of an evolute former wound coil opened out. The points A, B, C, etc., correspond to similar points in fig. 381.
Ques. What is the peculiarity of the evolute coil?
Ans. The two sides of the evolute coil have unequal dimensions. The part marked AB, in fig. 381 which is an upper layer inductor is longer than the part DE, which constitutes a lower layer inductor. The portions DC and EF act as parts of an inner layer of evolutes, and the portions AF and BC as parts of an outer layer of evolutes. These features are shown in fig. 382.
Ques. How are evolute coils placed on the core?
Ans. They are placed in position as shown in figs. 372 and 373, continuing around the core until all the slots are filled. To complete the operation it is necessary to raise some of the first laid coils and insert the last ones below them. The winding is thus completed and is symmetrical.
Fig. 383.—Westinghouse combination bastard and barrel winding. A bastard winding at the rear end is combined with a barrel winding at the commutator end, as shown in the illustration, to secure better ventilation.
Ques. Describe the method of winding the "straight out" type of former coil.
Ans. The straight out coil may be wound on a former such as shown in fig. 384. This consists of a board having four upright pins, A, B, D, E, properly spaced and two horizontal pins C, F, attached to extensions at each end of the board. A coil of the required number of turns is wound around these pins and then opened out as in fig. 385. After varnishing and baking it is ready to be placed on the armature.
Ques. For what class of winding are straight out former coils suitable?
Ans. For barrel winding.
Fig. 384.—Method of winding "straight out" coils. There are several ways of making these coils. A former may be prepared, as shown in the figure, with a board having inserted four pins, and having attached two blocks at the ends carrying horizontal pins as shown. Around the several pins, the coil is wound to the required number of turns and taped. This coil differs from the evolute coil in that the two halves are of equal size, the parts which act respectively as upper and under inductor being of equal length. The coil as shown is suitable for wave winding.
Fig. 385.—Appearance of straight out coil after being opened out. In opening out the coil, the ends C and F are put into a clamp and twisted at right angles to the plane of the coil. The letters correspond to the points indicated in fig. 384.
Ques. How are straight out coils placed on the core?
Ans. In the same manner as described for evolute coils; when in position straight out coils appear as in fig. 372.
Ques. What is the approved method of putting tape on a coil?
Ans. Considerable time is saved by the use of a machine designed for the purpose, such as shown in fig. 387.
Fig. 386.—Another and simpler method of winding a "straight out" coil. A board with only two pins is employed as shown; this plan, however, gives more trouble in the subsequent opening out of the coil.
The construction of these machines is such that a roll of tape placed on a split metal ring is revolved around the coil to be taped, the coil being gradually moved until it is entirely covered.
Coil Retaining Devices.—In the operation of a dynamo there are two forces which tend to throw the inductors out of position:
1. Armature drag;
2. Centrifugal force.
Both of these forces are present with smooth core armatures, but only centrifugal force with slotted armatures. The devices used to hold the inductors in position against these forces are:
1. Driving horns;
2. Binding ribbons;
3. Retaining wedges.
Ques. What are driving horns?
Ans. They are simply pins or strips projecting from the surface of a smooth core as shown in fig. 251.
Fig. 387.—Armature coil taping machine. Numerous machines have been invented for taping armature coils. They consist essentially of a device which revolves a roll of tape around the coil, in such a direction that the tape is unwound from the roll and rewound on the coil. The speed at which the coil is fed through the machine will determine the overlapping of the tape.
Ques. What other kinds of retainer are used on smooth core armatures?
Ans. They require several binding ribbons or brass bands placed around the winding to prevent the inductors being thrown off the core by centrifugal force.
Ques. With slotted armatures what provision must be made for retaining the inductors in position?
Ans. Retaining wedges must be inserted into the notches or between the projecting tops of the teeth.
Fig. 388.—Front view of large armature for direct connected dynamo, built by the General Electric Co.
Ques. How are the wedges made?
Ans. They are usually made of well baked hard wood, such as hornbeam, or hard white vulcanized fibre. Sometimes a springy strip of German silver is used.
An electric motor is just the reverse of a dynamo; it is a machine for converting electrical energy into mechanical energy.
The electrical energy delivered by the dynamo must be obtained from a steam engine, gas engine, or other power; the mechanical energy obtained from the motor comes from the energy of the current flowing through its armature.
Ques. What is the construction of a motor?
Ans. It is constructed in the same manner as a dynamo.
Any machine that can be used as a dynamo will, when supplied with electrical power, run as a motor, and conversely, a motor when driven by mechanical power, will supply electrical energy to the circuit connected to it. Dynamos and motors, therefore, are convertible machines, and the differences that are found in practice are largely mechanical; they arise chiefly from the conditions under which the motor must work. Hence, the study of the motor begins with a knowledge of the dynamo, and accordingly the student should understand thoroughly all the fundamental principles of the dynamo, as already given, before proceeding further with the study of the motor.
Principles of the Motor.—All the early attempts to introduce motors failed, chiefly because the law of the conservation of energy was not fully recognized. This law states that energy can neither be created nor destroyed.
Early experimenters discovered, by placing a galvanometer in a circuit with a motor and battery, that, when the motor was running, the battery was unable to force through the wires so strong a current as that which flowed when the motor was standing still. Moreover, the faster the motor ran, the weaker did the current become.
Fig. 389.—Conductor, lying in a magnetic field and carrying no current; the field is not distorted whether the conductor be at rest or in motion.
Ques. Why does less current flow when the motor is running than when standing still?
Ans. Because the motor, on account of its rotation acts as a dynamo and thus tends to set up in the circuit a reverse electromotive force, that is, an electromotive force in opposite direction to the current which is driving the motor.
Ques. What is the real driving force which causes the armature of a motor to rotate?
Ans. The propelling drag, that is, the drag which the magnetic field exerts upon the armature wires through which the current is flowing, or in the case of deeply toothed cores, upon the protruding teeth.
The Propelling Drag.—In fig. 389 is shown the condition which prevails when a conductor carrying no current is placed in a uniform magnetic field. The magnetic lines pass straight from one pole to the other. The field is not distorted whether the conductor be at rest or in motion, so long as there is no flow of current. This represents the condition in the air gap of a motor or dynamo, when no current is flowing in the armature.
Ques. What happens when a current flows in the conductor of fig. 389.
Ans. It sets up a magnetic field of its own as shown in fig. 390.
Ques. What is the effect of this magnetic field?
Ans. It distorts the original field (fig. 389) in which the conductor lies, making the magnetic lines denser on one side and less dense on the other as in fig. 390.
Ques. What is the nature of these distorted magnetic lines?
Ans. They tend to shorten themselves to their original form of straight lines.
Ques. What effect has this on the conductor?
Ans. It produces a force on the conductor tending to push it in the direction indicated by the arrow, fig. 390.
Fig. 390.—Conductor carrying a current in a magnetic field. The current flowing in the conductor sets up a magnetic field which distorts the original field as shown, making the magnetic lines denser on one side and less dense on the other. This results in a force upon the wire, which, in the case of a dynamo (fig. 391) opposes its movement, and which forms the propelling drag in the case of a motor (fig. 392).
The distorted magnetic lines may be regarded as so many rubber bands tending to straighten themselves; The result then is clearly to force the conductor in the direction indicated.
According to Lenz' law, the direction of the current in the armature of a dynamo is such as to oppose the motion producing it. When the armature of a dynamo is rotated, the bending of the lines of force of the main magnetic field due to armature reaction acts as a drag against the motion of the armature. Armature reaction increases with the increase of the armature current. Therefore, the effect of the drag increases with the increase of load and requires an additional expenditure of power to drive the armature.
In a motor, the direction of the actuating current is the reverse of that of the armature current of a dynamo, consequently, the armature reaction which constitutes a drag, acting against rotation of the armature of a dynamo, becomes a pull in the direction of rotation of the armature of a motor and constitutes its real turning effect or torque which is used at the pulley to do mechanical work. The greater the load applied to the motor, the greater will be the amount of current taken from the supply mains, and consequently, the greater the torque.
Figs. 391 and 392.—Action of the magnetic force in a dynamo and motor. In the first instance, according to Lenz' law, the direction of the current induced in the wire is such as to oppose the motion producing it. In the operation of a motor, the current supplied in flowing through the armature winding distorts the field and thus produces rotation. In the figures, the direction of the force is clearly indicated by remembering that the distorted lines of force act like rubber bands tending to straighten and shorten themselves.
Ques. What are the essential requirements of construction in a motor?
Ans. They are: 1, a magnetic field, 2, conductors placed perpendicular to the field, and 3, provision for motion, of the conductors across the field in a direction perpendicular to both themselves and the field.
The Reverse Electromotive Force.—When an electric current flows through some portion of a circuit in which there is an electromotive force, the current will there either receive or give up energy, according to whether the electromotive force acts with or against the current.
Fig. 393.—Force exerted on a current carrying conductor placed across a magnetic field. Let N, S, be the pole pieces of an ordinary electromagnet, having their faces flat and with only a narrow air gap between. In this gap is stretched the vertical copper wire A B, kept taut by a strong spring at A; current can be passed into the wire from the leads C and D. Attached to the wire in the middle of the gap is a horizontal cord passing over a pulley P and kept taut by a weight W; the pulley carries a pointer F which moves in front of a scale s s. If the electromagnet be now excited and have the polarity indicated, it will be found that on passing a strong current down the wire, the index F moves toward the right, showing a similar movement in the wire. The index returns to zero when the current in the wire ceases, and moves in the opposite direction if the current in the wire be reversed and sent up instead of down. The experiment can be further varied by reversing the magnetizing current of the electromagnet.
This is illustrated in fig. 395, which represents a circuit in which there is a dynamo and a motor. Each is rotating clockwise, and accordingly, each generates an electromotive force tending upward from the lower to the upper brush. In both cases the upper brush is positive. In the dynamo, however, where energy is being supplied to the circuit, the electromotive force is in the same direction as the current, and in the motor, where work is being done, the electromotive force is in the reverse direction to that of the dynamo.