power, and frequently a single coil of wire, wound around an iron core; a vibratory armature, pivoted at one end, is arranged to operate the clapper.

Two coils are used in the electric door-bell shown in the illustration. For a more technical description see ELECTRIC SIGNALING.

ELECTRIC BLUE-PRINT MAKING, a modern process of wholesale photographic printing by the aid of machinery, the electric light and the blue-print (q.v.). One of the best machines is continuous in its operation, and is fed by the operator with great lengths of tracings and blue paper in much the same manner as the washerwoman feeds the wet clothes into a wringing machine. The large wooden drum, around which the tracings and printing paper pass, is moved either by a connection with the shafting or by an electric motor mounted on the apparatus, the speed of the drum being regulated by a device shown on the top of the machine. A traveling apron of transparent material takes the place of the glass in the printing frame of the ordinary type, and as it is under tension at all times, it ensures an even and close contact at all points. This apron is wound on a small drum at the top and after passing along the large drum where the contact and exposure take place, it is wound up on the drum below; after the printing operation has been completed it is rewound by hand back on the upper drum. In the rear of the machine are three arc lamps with reflectors, which concentrate the light on the tracings which, with the exposed prints, drop out into the box in front. The blue paper may be kept in a roll ready for use on the upper front part of the machine, or may be fed in small sheets with the tracings where the work being done is of ordinary size.

The machines are made in two widths, 30 and 42 inches; the apron supplied with them is 70 feet long, and prints of this size can be made as readily as smaller ones where it is desired. The ability to make prints of this size greatly enlarges the sphere of usefulness of the blue-print.

ELECTRIC BREEZE, a breeze or stream of particles of air repelled from an electrified point.

ELECTRIC BURGLAR-ALARM.

ELECTRIC SIGNALING.

See

ELECTRIC CABLE. See ELECTRIC UNDERGROUND CABLES AND CONDUITS.

ELECTRIC CALL-BOX SYSTEM. See

ELECTRIC SIGNALING.

ELECTRIC CANDLE, a modification of the arc form of electric light, in which the carbon pencils are parallel and separated by a layer of plaster of Paris. It was invented in 1877 by Jablochkoff, a Russian engineer. This invention is noteworthy as having revived an interest in electric illumination. See ELECTRIC LIGHTING.

ELECTRIC CLOCKS. See CLOCK.

ELECTRIC CONDENSER, a construction for accumulating electricity, through the effect of mutual induction between conducting plates, as of tinfoil, separated by a dielectric, as of oiled paper; or some similar device as a Leyden jar. When an insulated conductor is

charged with electricity by friction, a battery or other source of electromotive force, it will excite or "induce" in any neighboring conductor a charge of electricity. If the electricity in the first body be "positive," that induced in the neighboring body will be "negative." Thus, in Fig. 1, in which A and B are metal plates separated by air, glass, mica or other insulating material, if A be charged by the positive pole of battery bit will induce a charge of negative electricity on the plate B. Such an arrangement of plates is termed an electric "condenser," and in various forms it is one of the most useful instruments employed in multiplex, printing, automatic, wireless and other systems of telegraphy. It is also indispensable in telephony and has found a field in electric light and power circuits. The electricity held or "bound" in the plates is termed static electricity. The quantity of electricity or "charge" accumulated at the plates is equal to the product of the electromotive force of the charging source by the "capacity" of the condenser. In fact, however, what the condenser holds is electrical energy, which, when discharged, is given up as work and heat. The total amount of energy (expressed in foot pounds) thus stored up by the condenser is

(KXE)+2.712, where K is the capacity of the charged condenser in farads, and E is the charged electromotive force in volts. It can be shown that the charge of a condenser rests on opposite sides of the dielectric, and that in charging the condenser as much electricity leaves plate в as enters plate A. The capacity of a condenser varies with the distance between its opposite plates, being greater the nearer they are together, and increases with the surface of the plates. The capacity also varies with the insulating material or dielectric used to separate or insulate the plates. The property of dielectrics to which this so-called inductive effect is due is termed specific inductive capacity. The property which this inductive capacity seemingly imparts to conductors is termed electrostatic capacity, or "capacity." The inductive capacity of air is taken as the standard. being unity, the specific inductive capacity of paraffin is about 2; vulcanized India-rubber, 2.94; gutta-percha, 4.5; mica, 5; flint glass, 6.5 to 10. The Leyden jar is a well-known type of condenser. The most common form of condenser is generally constructed of many sheets of tinfoil, separated by thin sheets of insulating material, such as paraffin paper, mica or glass; the alternate sheets of tinfoil are connected together metallically at their ends as indicated in Fig. 2.

Air

In making a condenser for radiography, the glass plate type is recommended as inexpensive and durable, and also much lighter than oil

99

immersed types. Photographer's negative glass, tinfoil and shellac are the materials. The foil should be cut to the required size (6 X 8 inches is convenient), and carefully smoothed to take out all wrinkles. The glass should be cleaned with alcohol and coated with shellac, then covered with the foil, and rolled or "squeegeed" so as to be perfectly smooth. In assembling the plates lugs should be placed between them. A unit may be made of 10 plates which are bound together with wire or suitable tape, and immersed for one hour in a bath of equal parts of hot melted rosin and beeswax, then allowed to drain and dry. This gives a condenser unit thoroughly moisture proof, with a capacity of .01 microfarad, which is suitable for the ordinary half-kilowatt wireless transformer for the standard 200-metre wave-length. It is good practice to make four such units, placing two multiple sets of two in each series; this reduces the strain on the condensers, without altering the capacity. They may be mounted in substantial open-side wooden boxes to protect the plates from injury.

ELECTRIC DIRECT CURRENT, as distinguished from the alternating current, is so-called because of the fact that it travels in one direction along a conductor. If this conductor joins the terminals of a source of energy, as a dynamo, the current is said to flow from the positive pole of the machine along the conductor to the negative pole.

Probably the first man to detect current electricity was Galvani about the year 1786. To Volta (q.v.), however, is certainly due the credit of first developing a practical electrochemical cell. In the year 1800 Volta exhibited a cell known as the "Voltaic Pile," consisting of a series of discs, copper and zinc, alternately separated from each other by a cloth saturated with brine; on joining wires to the end discs, quite a perceptible shock may be felt by touching with the tongue or moistened finger the two terminals simultaneously. This simple device was the starting point of all the electro-chemical batteries of the present day, With the discovery of Volta of the laws of difference of potential between different metals when placed in contact or joined by a fluid electrolyte began the development of very many varieties of cells, all on the same principle; yet even now, the two metals he chose,

zinc and copper, constitute the elements of the Daniell cell very frequently used for telegraphic purposes. The changes which would readily suggest themselves in Volta's first cell would be, increasing the amount of corroding liquid and placing the elements, zinc and copper, in a vessel which would properly contain the fluid.

The theory as given by Gore of the electrochemical cell is as follows:

"The essential cause is the stored-up and ceaseless molecular energy of the corroded metal and of the corroding element or liquid with which it unites, while contact is only a static condition; and chemical action is the process or mode by which the molecular motion of these substances is more or less transformed into heat and current."

The electromotive force of chemical generators is small, rarely exceeding two volts per cell. This necessitates a large number of cells connected in series; that is, the positive terminal of one connected with the negative terminal of the adjoining cell, the electromotive force thus produced being the product of the electromotive force of one cell by the number of cells. By connecting the two positive and the two negative terminals of two rows of cells, an increased quantity of current can be obtained, at the potential of one row. The first method is called joining battery cells for intensity, and the second for quantity. It is known that the energy generated in a chemical cell is produced by the consumption of zinc. The cost of this energy must necessarily be high, as both the zinc and the chemicals are expensive, so that the use of current electricity was quite limited until the introduction of the dynamo electric machine, which might be called the mechanical method of transforming energy from some source, such as a steam-engine, into current electricity, as contrasted with the chemical method.

In the year 1831 Faraday discovered and announced the principle of electro-magnetic induction. This opened up the field of what might be called the commercial generation of current electricity. The principle discovered by Faraday, which forms the basis of all dynamo electric machines, is that if a wire is moved in a magnetic field, so as to cut the lines of force, a current will be generated in the wire, and it is upon this principle that all dynamo electric machines depend for their action. The converse of this law he also announced, namely, that when an electric current is applied to the dynamo by some external source such as a battery or another dynamo, the machine will furnish mechanical power. Hence a dynamo electric machine may be considered either as a generator or as a motor.

All dynamos consist of two essential parts, one, the field magnet, which is usually stationary, and the other, the armature on which the copper conductors are mounted and which revolves on a shaft between the poles of the field magnet. This armature is so arranged as to cut the lines of force flowing between the magnetic poles. The lines of force are imaginary lines flowing from the north pole to the south pole of any magnet. They can easily be traced by placing a piece of paper above the magnet and sprinkling on this paper iron filings.

If the paper be covered with mucilage the filings will maintain a permanent position so that they may be studied at the leisure of the student.

The field magnets may be made of steel, magnetized, or preferably they may be electromagnets made of soft iron over which a coil of wire is wound carrying a current of electricity which induces magnetic lines in the iron. It is to be noted that if the ends of the magnet are bent in the form of a horse-shoe, the lines will be intensified by the reduction of the air space between the poles, and as the amount of current induced in the wire depends on the number of lines of force cut, the current induced will be greater, the greater the strength of the field magnets.

Considering first the ideal simple dynamo: This would consist of a single loop of wire mounted on centres, and rotating between the poles of a magnet, placed horizontally, each end of the loop being connected to a collector which in direct current machines is called a commutator, and is mounted on the shaft outside of the poles, and insulated therefrom. If the loop is placed at right angles to lines of magnet force, in a vertical position and revolved through 180 degrees, each side will pass through the whole number of lines of force flowing between the poles which will induce a current in one direction in the loop. If the rotation is maintained in the same direction during the next 180 degrees, the loop will cut the lines of force in the opposite direction, that is, the lines of force will be passing through it in the opposite direction to that in the first case. This will induce a current which will be in the opposite direction from the current induced through the first half of the revolution; so that the current will be pulsating, first in one direction and then in the other, during each revolution. If the collector or commutator be cut into two halves parallel with the shaft and the ends of the loop be connected one to each half, and if a pair of brushes be supplied to collect the current, one above and one below the commutator, then when the loop is vertical the brushes will change contact from one end of the loop to the other, and as no current is then being generated, the change is made without sparking and current flowing in the same direction continuously can be obtained from the brush terminals. During the moment of changing from the one contact to the other, the circuit is momentarily opened or interrupted. This would cause sparking at the brush or collector, were it not that the brushes are placed at a point at which the current is practically zero. This is found in practice to be slightly in advance of the theoretical neutral point on account of lines of force being dragged in the direction of rotation by the conductors.

To advance from the ideal simple dynamo: the next step is to reduce the air gap between the poles of the field magnet and concentrate the lines of force in the effective space. This is accomplished by placing an iron core on the armature which in the first place reduces the magnetic resistance of the air gap and thus increases the number of lines of force through the armature conductors, and also serves as a support for them. Other machines were built with shuttle wound armatures, the arm

ature consisting of an iron shuttle cut out with grooves longitudinally to take the conductors. These were usually wound with a number of turns of copper wire, the ends being brought out to a two-part commutator. (See ELECTRIC ALTERNATING CURRENT MACHINERY). The next step was to add to the number of coils on the armature so that, during each portion of a revolution some part of the armature conductors would be doing maximum work. Should an additional coil be added to the ideal generator, at right angles to the first coil, the capacity of the machine will be doubled. This complicates, to some extent, the collector rings and may necessitate the opening of the circuit when current is flowing so as to cause sparking and burning of the brush. A machine built on these lines would, therefore, be better adapted for generating small currents as the sparking at the brushes would be otherwise very destructive to the commutator. Machines of this type are known as open coil.

The next important step was made by Gramme and Pacinotti, which was to close the coils with themselves so as to form a continuous circuit in the armature and connect one collector section to each coil at its junction with the next one, the number of sections being the same as the number of coils. In the fourcoil armature, the current generated can either pass to the collecting brush directly, or when it moves out of position so that the contact is broken and made with the next section, the current can flow through the armature coils to the same brush if necessary, and when that coil passes from one polar position to another and is giving current of opposite polarity this current can flow directly to the other brush, and so continuous current is generated. There is also no point at which the circuit is open. There may be a slight sparking as the section moves from the brushes, but violent sparking is reduced as there is always another path for the current to flow to either brush.

The drum armature is distinct from the ring armature in that the wires are wound on the outside of the core and do not pass through it. This type is frequently called the "Siemens" armature on account of the number of successful machines built by Siemens. Of the whole number of lines of force passing between the poles and through the core, there are very few lines passing in the inside, they being diverted by an iron core so that they pass through the wires on the outside of the core; the conductors inside of the core are thus of little use, their only function being to complete the circuit and carry current between the successive turns on the outside of the core; so that by winding the wires on the outside surface only, the amount of idle wire is reduced, the only material that is not active being the cross-connecting pieces at the ends.

The Gramme ring was used very largely on early machines for the reason that it afforded means for easier mechanical construction, and machines of this type were generally successful, on account of their simplicity. Pacinotti designed a core having teeth similar to a gear wheel. In this way the air gap between the armature and pole pieces could be reduced somewhat, resulting in an increased number of lines of force. It also afforded an additional

support to the coils and added to the mechanical strength of the machine.

To be considered next are the field magnets': There are a number of constructions which may be employed. (1) The so-called permanent steel magnet which consists of a bar or bars of steel bent to the shape desired, tempered and magnetized. The method of magnetizing these magnets consisted of placing them in contact with other magnets or with an electro-magnet. The present method would be to insert the steel bar into a helix carrying a heavy current and in a short time the bar would be magnetized. The dynamos built with permanent steel magnets of this type are what is known as magneto lynamos. The chief objection to this form of magnet is that a steel magnet cannot be made as powerful as an iron magnet which is energized or, as it is commonly called, excited from a source of electricity. In the first generators permanent magnets were used, but a great step in advance in dynamo design was to arrange the magnet poles so as to be self-excited. A portion of the current generated in the armature is sent around the coils wound around the cores of these field magnets so as to excite them. At first, however, magnets were substituted consisting of soft iron upon which was wound a coil of copper wire, the current for energizing these pole pieces being first supplied from a small magneto generator or a voltaic battery. Sometimes the machine will not generate on starting up not having sufficient current to excite the magnets and it is necessary to excite them from some external source so as to give the initial strength to the magnets and allow them to build up from the current generated in the armature. It is usually found that there is sufficient residual magnetism left in the iron of the field magnets, after the machine has once been in operation, to start the current in the machine and properly build up the fields.

In regard to field windings, two distinct types are used: (1) the series winding, in which all the current generated in the armature passes around the field poles and thence out to the line or circuit; and (2) the shunt winding in which a portion only of the current is used in the field, the connection being made across the main terminals of the generator. In the first case the wire on the field windings is necessarily large so as to carry all the current for which the machine is designed and in the sec ond case it is a small wire of many turns, the product of amperes and turns being about the same in either case. In another design, both a shunt and a series winding of a few turns is employed, constituting a compound winding.

It will be seen that in the first case, that of the series winding, the field strength will depend upon the resistance of the total circuit, including the resistance of the armature, the field winding and the external circuit. In a machine of this type the voltage or pressure generated will vary in proportion to the demands. This is the standard winding for the series arc machines used for city lighting, such as the Brush and Wood types. In the case of the shunt-wound machine the current flowing in the field coil depends upon the pressure between the generator terminals, so that with an increased output and consequent loss in the

armature the voltage will fall off slightly, thus reducing the field strength. This necessitates some means of varying the field current so as to maintain a uniform pressure at the generator terminals. This is usually accomplished by means of an external resistance in the field circuit composed of German-silver or iron wire which can be varied by means of a switch-head so arranged as to cut out certain portions of this resistance step-by-step and so increase the current through the fields, thus preserving a uniform voltage.

In a combination of series and shunt windings commonly called the compound type, as the output of the generator is increased, there is a greater flow of current through the series windings and consequent increase of magnetic strength of field so that it is possible to compensate for the loss due to the resistance of the armature windings and maintain a uniform voltage at the generator terminals. The voltage as well as the output of the dynamo depends upon the strength of the field magnets, the magnetic permeability of the material and the rate at which the lines of force are cut by the armature conductors, so that the higher the speed the greater the voltage output of the dynamo. In the early machines very high speeds were common, armatures of small diameters being employed. These were objectionable for mechanical reasons so that the design was changed in order to increase the number of pole pieces. Instead of the field being composed of two poles, it was arranged so that a greater number of poles could be used, this type of machine being known as the multipolar dynamo. As each conductor would pass between a number of poles during each revolution the speed could be proportionally reduced.

The dynamo, as previously stated, is a machine for converting energy in the form of mechanical power into electrical power, or viceversa, so that a motor is a machine for converting energy in the form of electricity into mechanical power. The early types of motors were based on the principle that a magnet would attract the opposite pole of another magnet, and if one set of magnets is arranged on a wheel, and the other stationary, the movable magnets will be drawn around. To make this effective it will be necessary to interrupt the forces at what might be called the dead centres so that the wheel would have continuous motion. This is accomplished by either introducing a screen, or, more satisfactorily, by the use of electro-magnets with a movable contact so that the magnets are energized intermittently, allowing the wheel to revolve in accordance with impulses received from the magnetic poles.

When we consider the dynamo as a motor, the current supplied to the terminals may take two paths, one through the armature and the other through the field coils. The field current energizes the pole pieces, and the current trayeling in the armature is similar to another magnet inasmuch as a coil carrying the current will be attracted or repelled by a magnet according to the direction of the current through the coil, so that the wire will be forced around by attraction and repulsion. By considering the effect of the commutator the motion is seen to be continuous. When the armature starts to revolve the conditions then existing will be

similar to the armature in action as a dynamo, and an electromotive force will be generated in the armature wires, which will be in the opposite direction to the incoming current. This is what is called the counter electromotive force of the motor, and will tend to reduce the amount of current which will flow through the armature conductors. It is, therefore, evident that when a motor is started there will be a rush of current through the armature, as the resistance is very small, and as there is no counter electromotive force while the machine is not in motion to check the flow. For this reason, in the direct current motor it is necessary to introduce an external resistance into the armature circuit to hold back the current which would flow, until the machine approaches full speed. The resistance is then gradually reduced until full speed is obtained. The effect of this counter electromotive force when the resistance is cut out entirely is materially to assist the self-regulating qualities of the machine. Any load applied to the motor would tend slightly to reduce the speed, which effect, by also reducing the counter electromotive force and allowing more current to flow through the armature, tends to keep the speed from falling much below normal in the shunt motor. Motors can be built either with a plain shunt field winding or with a series and shunt winding, depending on their requirements. The direction of rotation depends on the direction of the current through the armature. To reverse the rotation, therefore, it is only necessary to reverse the current in the armature, leaving field connections as they are. If the current is changed in both field and armature, the result would naturally be that the machine will continue to revolve in the same direction as before.

To reduce the speed of the direct current motor it is only necessary to add resistance to the armature circuit so as to limit the current flowing therein, and by so doing almost any desired speed may be obtained, from 1 per cent up to full rate of speed. There are a number of other methods by which variable speeds can be obtained, one of them being by varying the field strength. Any motor, however, operating at a lower field or armature current than normal conditions would require is naturally operating at reduced power. On account of the valuable features in relation to speed control, reversibility and the automatic speed control inherent in the shunt machine, together with the large torque of the series machine, the direct current motor fulfils more nearly than any other the practical requirements in machineshops, textile mills and general manufacturing establishments.

For electric railway work, in which the direct current is employed (see TRACTION, ELECTRIC), the compound wound generator and series motor is the usual standard practice. Often this type of generator is overcompounded so as to more than overcome the drop through the armature resistance and allow higher voltage at full load than at no load, so as to overcome, in a way, the drop of potential on the feeders and preserve the uniform voltage over the system. In lighting and power work the shunt and compound dynamos are both used. (See ELECTRIC LIGHTING). And

in the business centres of our large cities where the direct current is generally used, the rotary converter fed from a high tension alternating central station is very often employed, together with storage battery.

Direct current was more generally used in the earlier installations of electric distribution, in preference to alternating current, for the reason that the direct current motor was developed before the alternating current motor; and the earlier motors possessed many advantages in their ability to be operated at any speed from slow speeds up to the maximum speed for which the motor was designed, and also permitted the use of storage batteries directly connected to the system, thus ensuring continuity and reliability of service.

The shunt and series motor each has its own field of usefulness. When a very powerful starting torque and rapid acceleration are necessary the series motor is used, as in the case of street railway, electric locomotives, electric cranes; and on steamships where the direct current alone is used, as on the Kentucky and Kearsarge, of the United States navy, not only is electricity used for lighting, but also for operating ammunition hoists, hoisting anchors, operating boat cranes, and even the steering gear of the ship itself.

In machine-shops and manufacturing establishments where a more or less constant speed may be required, and in elevator work, the compound and the shunt motor are commonly employed. The shunt motor is very well adapted for operating at any speed desired, and for machine tools it is at present without a peer for an efficient and easily regulated source of power. Unlike the series motor, where the speed varies with the load, the shunt motor is practically a constant speed machine. When thrown on the lines it rapidly comes up to normal speed, and then from no load to full load will not greatly deviate therefrom unless purposely thrown to a slower point by the controller. As a series motor would run away if left in a circuit with a load suddenly removed, the shunt motor, or sometimes the compound (which is used in order to preserve an absolutely uniform speed from no load to full load, and is necessary in a few places where absolutely constant speed is required) is the standard motor for driving textile machinery in large mills, factories and other establishments.

Direct current generally meets all of the requirements of the consumers, as it is available for motors of any size; for lighting; for chemical action, such as charging storage batteries or in electro-plating; or for electric heating.

With the large increase in the requirements of individual consumers, the advantages of direct current over alternating current are not as important to-day, for the reason that translating devices have been simplified so that alternating current may be converted, without serious difficulty, into direct current, for any special requirements.

WM. C. L. EGLIN, Second Vice-President and Chief Engineer, The Philadelphia Electric Company.

ELECTRIC DISCHARGE, the escape of electricity, whether slowly and silently, or more quickly and violently, from any receptacle or generator.