176 STANDARD ELECTRICAL DICTIONARY.Damper.(a) A copper frame on which the wire in a galvanometer is sometimescoiled, which acts to damp the oscillations of the needle.(b) A tube of brass or copper placed between the primary and secondarycoils of an induction coil. It cuts off induction and diminishes thecurrent and potential of the secondary circuit. On pulling it out, thelatter increases. It is used on medical coils to adjust their strengthof action.Damping.Preventing the indicator of an instrument from oscillating in virtue ofits own inertia or elasticity. In a galvanometer it is defined asresistance to quick vibrations of the needle, in consequence of which itis rapidly brought to rest when deflected (Ayrton). In dead-beatgalvanometers (see Galvanometer, Dead-Beat,) damping is desirable inorder to bring the needle to rest quickly; in ballistic galvanometers(see Galvanometer, Ballistic,) damping is avoided in order to maintainthe principle of the instrument. Damping may be mechanical, thefrictional resistance of air to an air-vane, or of a liquid to animmersed diaphragm or loosely fitting piston, being employed. Adash-pot, q. v., is an example of the latter. It may beelectro-magnetic. A mass of metal near a swinging magnetic needle tendsby induced currents to arrest the oscillations thereof, and is used forthis purpose in dead-beat galvanometers. This is termed, sometimes,magnetic friction. The essence of damping is to develop resistance tomovement in some ratio proportional to velocity, so that no resistanceis offered to the indicator slowly taking its true position. (SeeGalvanometer, Dead-Beat.)Dash-Pot.A cylinder and piston, the latter loosely fitting or perforated, or someequivalent means being provided to permit movement. The cylinder maycontain a liquid such as glycerine, or air only. Thus the piston isperfectly free to move, but any oscillations are damped (see Damping).In some arc lamps the carbon holder is connected to a dash-pot to checktoo sudden movements of the carbon. The attachment may be either to thepiston or to the cylinder. In the Brush lamp the top of the carbonholder forms a cylinder containing glycerine, and in it a looselyfitting piston works. This acts as a dash-pot.Dead Beat. adj.Reaching its reading quickly; applied to instruments having a movingindicator, which normally would oscillate back and forth a number oftimes before reaching its reading were it not prevented by damping. (SeeGalvanometer, Aperiodic--Damping.)Dead Earth.A fault in a telegraph line which consists in the wire being thoroughlygrounded or connected to the earth.177 STANDARD ELECTRICAL DICTIONARY.Dead Point of an Alternator.A two-phase alternator of the ordinary type connected as a motor toanother alternator cannot start itself, as it has dead points where therelations and polarity of field and armature are such that there is notorque or turning power.Dead-Turns.In the winding of an armature, a given percentage of the turns, it maybe 80 per cent., more or less, is assumed to be active; the other 20 percent. or thereabouts, is called dead-turns. This portion represents thewire on such portions of the armature as comes virtually outside of themagnetic field. They are termed dead, as not concurring to theproduction of electro-motive force.Dead Wire.(a) The percentage or portion of wire on a dynamo or motor armature thatdoes not concur in the production of electromotive force. Thedead-turns, q. v., of a drum armature or the inside wire in a Grammering armature are dead wire.(b) A disused and abandoned electric conductor, such as a telegraphwire.(c) A wire in use, but through which, at the time of speaking, nocurrent is passing.Death, Electrical.Death resulting from electricity discharged through the animal system.The exact conditions requisite for fatal results have not beendetermined. High electro-motive force is absolutely essential; achanging current, pulsatory or alternating, is most fatal, possiblybecause of the high electro-motive force of a portion of each period.Amperage probably has something to do with it, although the totalquantity in coulombs may be very small. As applied to the execution ofcriminals, the victim is seated in a chair and strapped thereto. Oneelectrode with wet padded surface is placed against his head or someadjacent part. Another electrode is placed against some of the lowerparts, and a current from an alternating dynamo passed for 15 seconds ormore. The potential difference of the electrodes is given at 1,500 to2,000 volts, but of course the maximum may be two or three times themeasured amount, owing to the character of the current.Decalescence.The converse of recalescence, q. v. When a mass of steel is being heatedas it reaches the temperature of recalescence it suddenly absorbs alarge amount of heat, apparently growing cooler.Deci.Prefix originally used in the metric system to signify one-tenth of, nowextended to general scientific units. Thus decimeter means one-tenth ofa meter; decigram, one-tenth of a gram.Declination, Angle of.The angle intercepted between the true meridian and the axis of amagnetic needle at any place. The angle is measured to east or west,starting from the true meridian as zero.178 STANDARD ELECTRICAL DICTIONARY.Declination of the Magnetic Needle.The deviation of the magnetic needle from the plane of the earth'smeridian. It is also called the variation of the compass. (See MagneticElements.)Decomposition.The reduction of a compound substance into its constituents, as inchemical analysis. The constituents may themselves be compounds orproximate constituents, or may be elemental or ultimate constituents.Decomposition, Electrolytic.The decomposition or separation of a compound liquid into itsconstituents by electrolysis. The liquid must be an electrolyte, q. v.,and the decomposition proceeds subject to the laws of electrolysis, q.v. See also Electrolytic Analysis.Decrement.When a suspension needle which has been disturbed is oscillating theswings gradually decrease in amplitude if there is any damping, as therealways is. The decrement is the ratio of the amplitude of oneoscillation to the succeeding one. This ratio is the same for anysuccessive swings.De-energize.To cut off its supply of electric energy from an electric motor, or anydevice absorbing and worked by electric energy.Deflagration.The explosive or violent volatilizing and dissipating of a substance byheat, violent oxidation and similar means. It may be applied among otherthings to the destroying of a conductor by an intense current, or thevolatilization of any material by the electric arc.Deflecting Field.The field produced in a galvanometer by the current which is beingtested, and which field deflects the needle, such deflection being themeasure of the current strength.Deflection.In magnetism the movement out of the plane of the magnetic meridian of amagnetic needle, due to disturbance by or attraction towards a mass ofiron or another magnet.Deflection Method.The method of electrical measurements in which the deflection of theindex of the measuring instrument is used as the measure of the currentor other element under examination. It is the opposite of and is to bedistinguished from the zero or null method, q. v. In the latterconditions are established which make the index point to zero and fromthe conditions necessary for this the measurement is deduced. TheWheatstone Bridge, q. v., illustrates a zero method, the sine or thetangent compass, illustrates a deflection method. The use of deflectionmethods involves calibration, q. v., and the commercial measuringinstruments, such as ammeters and volt meters, which are frequentlycalibrated galvanometers, are also examples of deflection instruments.179 STANDARD ELECTRICAL DICTIONARY.Degeneration, Reaction of.The diminished sensibility to electro-therapeutic treatment exhibited bythe human system with continuance of the treatment in question. Thegeneral lines of variation are stated in works on the subject.Deka.Prefix originally used in the metric system to signify multiplying byten, as dekameter, ten meters, dekagram, ten grams; now extended to manyscientific terms.De la Rive's Floating Battery.A small galvanic couple, immersed in a little floating cell andconnected through a coil of wire immediately above them. When theexciting battery solution is placed in the cell the whole, as it floatsin a larger vessel, turns until the coil lies at right angles to themagnetic needle. Sometimes the two plates are thrust through a cork andfloated thus in a vessel of dilute sulphuric acid.A magnet acts to attract or repel the coil in obedience to Ampére'sTheory, (See Magnetism, Ampere's Theory of.)Delaurier's Solution.A solution for batteries of the Bunsen and Grenet type. It is of thefollowing composition:Water, 2,000 parts;potassium bichromate, 184 parts;sulphuric acid, 428 parts.Demagnetization.Removal of magnetism from a paramagnetic substance. It is principallyused for watches which have become magnetized by exposure to themagnetic field surrounding dynamos or motors.The general principles of most methods are to rotate the object, as awatch, in a strong field, and while it is rotating to gradually removeit from the field, or to gradually reduce the intensity of the fielditself to zero. A conical coil of wire within which the field isproduced in which the watch is placed is sometimes used, the idea beingthat the field within such a coil is strongest at its base. Such a coilsupplied by an alternating current is found effectual (J. J. Wright).If a magnetized watch is made to turn rapidly at the end of a twistedstring and is gradually brought near to and withdrawn from the poles ofa powerful dynamo it may be considerably improved.A hollow coil of wire connected with a pole changer and dip-battery hasbeen used. The battery creates a strong field within the coil. The watchis placed there and the pole changer is worked so as to reverse thepolarity of the field very frequently. By the same action of the polechanger the plates of the battery are gradually withdrawn from thesolution so as to gradually reduce the magnetic field to zero whileconstantly reversing its polarity. (G. M. Hopkins.)Steel may be demagnetized by jarring when held out of the magneticmeridian, or by heating to redness.180 STANDARD ELECTRICAL DICTIONARY.Density, Electric Superficial.The relative quantity of electricity residing as an electric charge upona unit area of surface. It may be positive or negative.Synonyms--Density of Charge--Surface Density.Dental Mallet, Electric.A dentist's instrument for hammering the fillings as inserted intoteeth. It is a little hammer held in a suitable handle, and which ismade to strike a rapid succession of blows by electro-magnetic motormechanism.Depolarization.(a) The removal of permanent magnetism. (See Demagnetization.)(b) The prevention of the polarization of a galvanic cell. It iseffected in the Grove battery by the reduction of nitric acid; in theBunsen, by the reduction of chromic acid; in the Smee battery,mechanically, by the platinum coated or rather platinized negativeplate. Other examples will be found under the description of variouscells and batteries. A fluid which depolarizes is termed a depolarizeror depolarizing fluid or solution. (See Electropoion Fluid.)Deposit, Electrolytic.The metal or other substance precipitated by the action of a battery orother current generator.Derivation, Point of.A point where a circuit branches or divides into two or more leads. Theseparate branches then receive derived or partial currents.Desk Push.A press or push button, with small flush rim, for setting into thewoodwork of a desk.Detector.A portable galvanometer, often of simple construction, used for rough orapproximate work.Detector, Lineman's.A portable galvanometer with a high and a low resistance actuating coil,constructed for the use of linemen and telegraph constructors when inthe field, and actually putting up, repairing or testing lines.Deviation, Quadrantal.Deviation of the compass in iron or steel ships due to the magnetizationof horizontal beams by the earth's induction. The effect of thisdeviation disappears when the ship is in the plane of the electricmeridian, or at right angles thereto; its name is taken from the factthat a swing of the ship through a quadrant brings the needle from zerodeviation to a maximum and back to zero.181 STANDARD ELECTRICAL DICTIONARY.Deviation, Semicircular.Deviation of the compass in iron or steel ships due to verticalinduction. (See Induction, Vertical.) The effect of this inductiondisappears when the ship is in the electric meridian. Its name isderived from the fact that a swing of the ship through half the circlebrings the needle from zero deviation to a maximum and back to zero.Dextrotorsal. adj.Wound in the direction or sense of a right-handed screw; the reverse ofsinistrotorsal, q. v.Fig. 128. DEXTROTORSAL HELIX.Diacritical. adj.(a) The number of ampere turns, q. v., required to bring an iron core toone half its magnetic saturation, q. v., is termed the diacriticalnumber.(b) The diacritical point of magnetic saturation is proposed by SylvanusP. Thompson as a term for the coefficient of magnetic saturation whichgives a magnet core one-half its maximum magnetization.Diagnosis, Electro.A medical diagnosis of a patient's condition based on the action ofdifferent parts of the body under electric excitement.Diamagnetic. adj.Possessing a negative coefficient of magnetic susceptibility; havingpermeability inferior to that of air. Such substances placed between thepoles of a magnet are repelled; if in the form of bars, they tend toturn so as to have their long axis at right angles to the line joiningthe poles. The reason is that the lines of force always seek the easiestpath, and these bodies having higher reluctance than air, impede thelines of force, and hence are as far as possible pushed out of the way.The above is the simplest explanation of a not well understood set ofphenomena. According to Tyndall, "the diamagnetic force is a polarforce, the polarity of diamagnetic bodies being opposed to that ofparamagnetic ones under the same conditions of excitement." Bismuth isthe most strongly diamagnetic body known; phosphorus, antimony, zinc,and many others are diamagnetic. (See Paramagnetic.)182 STANDARD ELECTRICAL DICTIONARY.Diagometer.An apparatus for use in chemical analysis for testing the purity ofsubstances by the time required for a charged surface to be dischargedthrough them to earth. It is the invention of Rousseau.An electrometer is charged with a dry pile. One of its terminals isconnected with one surface of the solution or substance to be tested,and the other with the other surface. The time of discharge gives theindex of the purity of the substance.Diamagnetic Polarity.Treating diamagnetism as due to a polar force, the polarity of adiamagnetic body is the reverse of the polarity of iron or otherparamagnetic bodies. A bar-shaped diamagnetic body in a field of forcetends to place itself at right angles to the lines of force.Diamagnetism.(a) The science or study of diamagnetic substances and phenomena.(b) The magnetic property of a diamagnetic substance.Diameter of Commutation.The points on the commutator of a closed circuit ring--ordrum--armature, which the brushes touch, and whence they take thecurrent, mark the extremities of the diameter of commutation. Were itnot for the lag this would be the diameter at right angles to the lineconnecting the centers of the opposite faces of the field. It is alwaysa little to one side of this position, being displaced in the directionof rotation. In open circuit armatures the brushes are placed on thediameter at right angles to this one, and sometimes the term diameter ofcommutation is applied to it. All that has been said is on thesupposition that the armature divisions correspond not only inconnection but in position with those of the armature coils. Of course,the commutator could be twisted so as to bring the diameter ofcommutation into any position desired.Diapason, Electric.A tuning-fork or diapason kept in vibration by electricity. In generalprinciple the ends of the fork act as armatures for an electro-magnet,and in their motion by a mercury cup or other form of contact they makeand break the circuit as they vibrate. Thus the magnet alternatelyattracts and releases the leg, in exact harmony with its natural periodof vibration.Diaphragm.(a) In telephones and microphones a disc of iron thrown into motion bysound waves or by electric impulses, according to whether it acts as thediaphragm of a transmitter or receiver. It is generally a plate ofjapanned iron such as used in making ferrotype photographs. (SeeTelephone and Microphone.)(b) A porous diaphragm is often used in electric decomposition cells andin batteries. The porous cup represents the latter use.[Transcriber's note: Japanned--covered with heavy black lacquer, likeenamel paint.]183 STANDARD ELECTRICAL DICTIONARY.Dielectric.A non-conductor; a substance, the different parts of which may, after anelectric disturbance, remain, without any process of readjustment, andfor an indefinite period of time, at potentials differing to any extent(Daniell). There is no perfect dielectric. The term dielectric isgenerally only used when an insulator acts to permit induction to takeplace through it, like the glass of a Leyden jar.Dielectric Constant.The number or coefficient expressing the relative dielectric capacity ofa medium or substance. (See Capacity, Specific Inductive.)Dielectric, Energy of.In a condenser, the conducting coatings are merely to conduct thecurrent all over the surface they cover; the keeping the electricitiesseparated is the work of the dielectric, and represents potential energywhich appears in the discharge. The amount of energy is proportional tothe charge, and to the potential difference. As any electrified bodyimplies an opposite electrification somewhere, and a separatingdielectric, the existence of a condenser is always implied.[Transcriber's note: The energy stored in a capacitor (condenser) is(Q*Q)/2C = (Q*V)/2 = (C*V*V)/2The energy is proportional to the voltage SQUARED or the charge SQUARED.]Dielectric Polarization.A term due to Faraday. It expresses what he conceived to be thecondition of a dielectric when its opposite faces are oppositelyelectrified. The molecules are supposed to be arranged by theelectrification in a series of polar chains, possibly being originallyin themselves seats of opposite polarities, or having such imparted tothem by the electricities. The action is analogous to that of a magnetpole on a mass of soft iron, or on a pile of iron filings.Dielectric Strain.The strain a solid dielectric is subjected to, when its oppositesurfaces are electrified. A Leyden jar dilates under the strain, andwhen discharged gives a dull sound. The original condition is notimmediately recovered. Jarring, shaking, etc., assist the recovery fromstrain. The cause of the strain is termed Electric Stress. (See Stress,Electric.) This is identical with the phenomenon of residual charge.(See Charge, Residual.) Each loss of charge is accompanied with aproportional return of the dielectric towards its normal condition.Dielectric Resistance.The mechanical resistance a body offers to perforation or destruction bythe electric discharge.Dielectric Strength.The resistance to the disruptive discharge and depending on itsmechanical resistance largely or entirely. It is expressible in voltsper centimeter thickness. Dry air requires 40,000 volts per centimeterfor a discharge.184 STANDARD ELECTRICAL DICTIONARY.Differential Winding Working.A method of working an electro-magnet intermittently, so as to avoidsparking. The magnet is wound with two coils. One is connected straightinto the circuit, the other is connected in parallel therewith with aswitch inserted. The coils are so connected that when the switch isclosed the two are in opposition, the current going through them inopposite senses. Thus one overcomes the effect of the other and themagnet core shows no magnetism, provided the two coils are of equalresistance and equal number of convolutions or turns.Fig. 129. DIFFERENTIAL WINDING WORKINGOF ELECTRO-MAGNETIC APPARATUS.Diffusion.A term properly applied to the varying current density found inconductors of unequal cross sectional area. In electro-therapeutics itis applied to the distribution of current as it passes through the humanbody. Its density per cross-sectional area varies with the area andwith the other factors.Diffusion Creep.When electrodes of an active circuit are immersed in a solution of anelectrolyte, a current passes electrolytically if there is a sufficientpotential difference. The current passes through all parts of thesolution, spreading out of the direct prism connecting or defined by theelectrodes. To this portion of the current the above term is applied. Ifthe electrodes are small enough in proportion to the distance betweenthem the current transmission or creep outside of the line becomes theprincipal conveyor of the current so that the resistance remains thesame for all distances.Dimensions and Theory of Dimensions.The expression of the unitary value of a physical quantity in one ormore of the units of length (L), time (T) and mass (M) is termed thedimensions of such quantity. Thus the dimension or dimensions of adistance is simply L; of an angle, expressible by dividing the arc bythe radius is L/L; of a velocity, expressible by distance divided bytime--L/T; of acceleration, which is velocity acquired in a unit oftime, and is therefore expressible by velocity divided by time--L/T/T orL/T2; of momentum, which is the product of mass into velocity--M*L/T; ofkinetic energy taken as the product of mass into the square ofvelocity--M*(L2/T2); of potential energy taken as the product of massinto acceleration into space-M*(L/T2)*L reducing to M*(L2/T2). Thetheory is based on three fundamental units and embraces all electricquantities. The simple units generally taken are the gram, centimeterand second and the dimensions of the fundamental compound units areexpressed in terms of these three, forming the centimeter-gram-second orC. G. S. system of units. Unless otherwise expressed or implied theletters L, M and T, may be taken to indicate centimeter, gram and secondrespectively. It is obvious that very complicated expressions ofdimensions may be built up, and that a mathematical expression ofunnamed quantities may be arrived at. Dimensions in their application bythese symbols are subject to the laws of algebra. They were invented byFourier and were brought into prominence by J. Clerk Maxwell. Anotherexcellent definition reads as follows: "By the dimensions of a physicalquantity we mean the quantities and powers of quantities, involved inthe measurement of it." (W. T. A. Emtage.)185 STANDARD ELECTRICAL DICTIONARY.Dimmer.An adjustable choking coil used for regulating the intensity of electricincandescent lights. Some operate by the introduction and withdrawal ofan iron core as described for the choking coil (see Coil, Choking),others by a damper of copper, often a copper ring surrounding the coiland which by moving on or off the coil changes the potential of thesecondary circuit.Dip of Magnetic Needle.The inclination of the magnetic needle. (See Elements, Magnetic.)Dipping.(a) Acid or other cleaning processes applied by dipping metals incleaning or pickling solutions before plating in the electroplater'sbath.(b) Plating by dipping applies to electroplating without a battery bysimple immersion. Copper is deposited on iron from a solution of coppersulphate in this way.Synonym--Simple Immersion.Dipping Needle.A magnet mounted in horizontal bearings at its centre of gravity. Placedin the magnetic meridian it takes the direction of the magnetic lines offorce of the earth at that point. It is acted on by the verticalcomponent of the earth's magnetism, as it has no freedom of horizontalmovement. (See Magnetic Elements, and Compass, Inclination.)Directing Magnet.In a reflecting galvanometer the magnet used for controlling themagnetic needle by establishing a field. It is mounted on the spindle ofthe instrument above the coil and needle.Synonym--Controlling Magnet.186 STANDARD ELECTRICAL DICTIONARY.Direction.(a) The direction of an electric current is assumed to be from apositively charged electrode or terminal to a negatively charged one inthe outer circuit. (See Current.)(b) The direction of magnetic and electro-magnetic lines of force isassumed to be from north to south pole of a magnet in the outer circuit.It is sometimes called the positive direction. Their general course isshown in the cuts diagrammatically. The circles indicate a compass usedin tracing their course. The magnetic needle tends to place itself inthe direction of or tangential to the lines of force passing nearest it.(c) The direction of electrostatic lines of force is assumed to be outof a positively charged and to a negatively charged surface.Fig. 130. DIRECTION OF LINES OF FORCE OF A PERMANENT MAGNET.Fig. 131, DIRECTION OF LINES OF FORCE OF AN ELECTRO-MAGNET.187 STANDARD ELECTRICAL DICTIONARY.Directive Power.In magnetism the power of maintaining itself in the plane of themagnetic meridian, possessed by the magnetic needle.Discharge, Brush.The static discharge of electricity into or through the air may be ofthe brush or spark form. The brush indicates the escape of electricityin continuous flow; the spark indicates discontinuity. The conditionsnecessary to the production of one or the other refer to the nature ofthe conductor, and of other conductors in its vicinity and to theelectro-motive force or potential difference; small alterations maytransform one into the other. The brush resembles a luminous core whoseapex touches the conductor. It is accompanied by a slight hissing noise.Its luminosity is very feeble. The negative conductor gives a smallerbrush than that of the positive conductor and discharges it morereadily. When electricity issues from a conductor, remote from anoppositely excited one, it gives an absolutely silent discharge, showingat the point of escape a pale blue luminosity called electric glow, orif it escapes from points it shows a star-like centre of light. It canbe seen in the dark by placing a point on the excited conductor of astatic-electric machine.Synonyms--Silent Discharge--Glow Discharge.Discharge, Conductive.A discharge of a static charge by conduction through a conductor.Discharge, Convective.The discharge of static electricity from an excited conductor throughair or rarefied gas; it is also called the quiet or silent discharge.The luminous effect in air or gas at atmospheric pressures takes theform of a little brush from a small positive electrode; the negativeshows a star. The phenomena of Gassiot's cascade, the philosopher's eggand Geissler tubes, all of which may be referred to, are instances ofconvective discharge.Discharge, Dead Beat.A discharge that is not oscillatory in character.Discharge, Disruptive.A discharge of a static charge through a dielectric. It involvesmechanical perforation of the dielectric, and hence the mere mechanicalstrength of the latter has much to do with preventing it. A disruptivedischarge is often oscillatory in character; this is always the casewith the discharge of a Leyden jar.188 STANDARD ELECTRICAL DICTIONARY.Discharge, Duration of.The problem of determining this factor has been attacked by variousobservers. Wheatstone with his revolving mirror found it to be 1/24000second. Fedderson, by interposing resistance, prolonged it to 14/10000and again to 138/10000 second. Lucas & Cazin made it from 26 to 47millionths of a second. All these experiments were performed with Leydenjars.Discharge, Impulsive.A disruptive discharge produced between conductors by suddenly producedpotential differences. The self-induction of the conductor plays anespecially important part in discharges thus produced.Discharge, Lateral.(a) A lightning discharge, which sometimes takes place between alightning rod and the building on which it is.(b) In the discharge of a Leyden jar or condenser the discharge whichtakes the alternative path, q. v.Discharge, Oscillatory.The sudden or disruptive discharge of a static condenser, such as aLeyden jar, or of many other charged conductors, is oscillatory incharacter. The direction of the currents rapidly changes, so that thedischarge is really an alternating current of excessively short totalduration. The discharge sends electro-magnetic waves through the ether,which are exactly analogous to those of light but of too long period toaffect the eye.Synonym--Surging Discharge.[Transcriber's note: Marconi's transmission across the English channeloccurs in 1897, five years after the publication of this book.]Fig. 132. DISCHARGER.Discharger.An apparatus for discharging Leyden jars. It consists of a conductorterminating in balls, and either jointed like a tongs or bent with aspring-action, so that the balls can be set at distances adapted todifferent sized jars. It has an insulating handle or a pair of such. Inuse one ball is brought near to the coating and the other to the spindleball of the jar. When nearly or quite in contact the jar discharges.Synonyms--Discharging Rod--Discharging Tongs.189 STANDARD ELECTRICAL DICTIONARY.Discharger, Universal.An apparatus for exposing substances to the static discharge spark. Itconsists of a base with three insulating posts. The central post carriesan ivory table to support the object. The two side posts carryconducting rods, terminating in metal balls, and mounted with universaljoints. A violent shock can be given to any object placed on the table.Synonym--Henley's Universal Discharger.Discharge, Silent.This term is sometimes applied to the glow or brush discharge andsometimes to the condition of electric effluvium. (See Discharge,Brush--Effluvium, Electric.)Discharge, Spark.The discontinuous discharge of high tension electricity through adielectric or into the air produces electric sparks. These are quitestrongly luminous, of branching sinuous shape, and in long sparks theluminosity varies in different parts of the same spark. A sharp noiseaccompanies each spark. High density of charge is requisite for theformation of long sparks.Disconnection.The separation of two parts of, or opening a circuit, as by turning aswitch, unscrewing a binding screw, or the like. The term is sometimesused to indicate a class of faults in telegraph circuits. Disconnectionsmay be total, partial or intermittent, and due to many causes, such asopen or partially replaced switches, oxidized or dirty contact points,or loose joints.Displacement, Electric.A conception of the action of charging a dielectric. The charge is allon the surface. This fact being granted, the theory of displacementholds that charging a body is the displacing of electricity, forcing itfrom the interior on to the surface, or vice versa, producing a positiveor negative charge by displacement of electricity. While displacement istaking place in a dielectric there is assumed to be a movement orcurrent of electricity called a displacement current.Disruptive Tension.When the surface of a body is electrified, it tends to expand, allportions of the surface repelling each other. The film of airsurrounding such a body is electrified too, and is subjected to adisruptive tension, varying in intensity with the square of the density.Dissimulated Electricity.The electricity of a bound charge. (See Charge, Bound.)Dissociation.The separation of a chemical compound into its elements by asufficiently high degree of heat. All compounds are susceptible ofdissociation, so that it follows that combustion is impossible at hightemperatures.190 STANDARD ELECTRICAL DICTIONARY.Distance, Critical, of Alternative Path.The length of air gap in an alternative path whose resistance joined tothe impedance of the rest of the conductors of the path just balancesthe impedance of the other path.Distance, Sparking.The distance between electrodes, which a spark from a given Leyden jaror other source will pass across.Synonym--Explosive Distance.Distillation.The evaporation of a liquid by heat, and sometimes in a vacuum, followedby condensation of the vapors, which distil or drop from the end of thecondenser. It is claimed that the process is accelerated by the liquidbeing electrified.Distributing Box.In an electric conduit system, a small iron box provided for givingaccess to the cable for the purpose of making house and minorconnections.Synonym--Hand Hole.Distributing Switches.Switch systems for enabling different dynamos to supply different linesof a system as required. Spring jacks, q. v., are used for the lines,and plug switches for the dynamo leads. Thus, dynamos can be thrown inor out as desired, without putting out the lights.Distribution of Electric Energy, Systems of.The systems of electric current distribution from central stations orfrom private generating plants, mechanical or battery, the latterprimary or secondary. They include in general the alternating currentsystem and direct current systems. Again, these may be subdivided intoseries and multiple arc, multiple-series and series-multipledistribution, and the three, four, or five wire system may be applied tomultiple arc or multiple series systems. (See AlternatingCurrent--Current System--Multiple Arc--Multiple Series--SeriesMultiple--Three Wire System.)Door Opener, Electric.An apparatus for opening a door by pushing back the latch. A spring thendraws the door open, and it is closed against the force of the spring bythe person entering. Electro-magnetic mechanism actuates the latch, andis operated by a switch or press-button. Thus a person on the upperfloor can open the hall door without descending.Dosage, Galvanic.In electro-therapeutics the amount of electric current or discharge, andduration of treatment given to patients.Double Carbon Arc Lamp.An arc lamp designed to burn all night, usually constructed with twoparallel sets of carbons, one set replacing the other automatically, thecurrent being switched from the burnt out pair to the other by theaction of the mechanism of the lamp.191 STANDARD ELECTRICAL DICTIONARY.Double Fluid Theory.A theory of electricity. Electricity is conveniently treated as a fluidor fluids. According to the double fluid hypothesis negative electricityis due to a preponderance of negative fluid and vice versa. Like fluidrepels like, and unlike attracts unlike; either fluid is attracted bymatter; the presence in a body of one or the other induceselectrification; united in equal proportions they neutralize each other,and friction, chemical decomposition and other causes effect theirseparation. The hypothesis, while convenient, is overshadowed by thecertainty that electricity is not really a fluid at all. (See SingleFluid Theory--Fluid, Electric.)Synonym--Symmer's Theory.[Transcriber's note: Current is the motion of negative electrons in aconductor or plasma. Unequal distribution of electrons is staticelectricity. The relatively immobile nuclei of atoms are positive whenone or more of its electrons is absent and accounts for part of thecurrent in electrolysis and plasmas.]Double Fluid Voltaic Cell.A cell in which two fluids are used, one generally as depolarizersurrounding the negative plate, the other as excitant surrounding thepositive plate. A porous diaphragm or difference in specific gravitiesis used to keep the solutions separate and yet permit the essentialelectrolytic diffusion. Grove's Cell, Bunsen's Cell, and Daniell's Cell,all of which may be referred to, are of this type, as are many others.Double Wedge.A plug for use with a spring-jack. It has connection strips at its endand another pair a little distance back therefrom, so that it can maketwo loop connections at once.Synonym--Double Plug.Doubler.A continuously acting electrophorous, q.v.; an early predecessor of themodern electric machines. It is now no longer used.D. P.Abbreviation for Potential Difference.Drag.The pull exercised by a magnetic field upon a conductor moving throughit or upon the motion of an armature in it.Dreh-strom. (German)Rotatory currents; a system of currents alternating in periodicsuccession of phases and producing a rotatory field. (See Field,Rotatory--Multiphase Currents.)Drill Electric.A drill for metals or rock worked by an electro-magnetic motor. Formetals a rotary motion, for rocks a reciprocating or percussion actionis imparted. It is used by shipbuilders for drilling holes in plateswhich are in place in ships, as its flexible conductors enable it to beplaced anywhere. For rock-drilling a solenoid type of construction isadopted, producing rapid percussion.192 STANDARD ELECTRICAL DICTIONARY.Drip Loop.A looping downward of wires entering a building, so that rain water, asit runs along the wire, will drip from the lowest part of the loopinstead of following the wire into or against the side of the building.Driving Horns.Projections on the periphery of an armature of a dynamo for holding thewinding in place and preventing its displacement. Various arrangementshave been adopted. They are sometimes wedges or pins and are sometimesdriven into spaces left in the drum core. The toothed disc armaturecores make up an armature in which the ridges formed by the teeth formpractically driving horns.Dronier's Salt.A substance for solution for use in bichromate batteries. It is amixture of one-third potassium bichromate and two-thirds potassiumbisulphate. It is dissolved in water to make the exciting fluid.Drop, Automatic.A switch or circuit breaker, operating to close a circuit by droppingunder the influence of gravity. It is held up by a latch, the circuitremaining open, until the latch is released by a current passing throughan electro-magnet. This attracting an armature lets the drop fall. As itfalls it closes a local or second circuit, and thus may keep a bellringing until it is replaced by hand. It is used in burglar alarms, itsfunction being to keep a bell ringing even though the windows or door bywhich entrance was made is reclosed.193 STANDARD ELECTRICAL DICTIONARY.Fig. 133. THE MAGIC DRUM.Drum, Electric.A drum with a mechanism within for striking the head with a hammer orsome equivalent method so as to be used as a piece of magical apparatus.In the one shown in the cut a sort of telephone action is used toproduce the sound, the electro-magnet D and armature being quitescreened from observation through the hole. (See Fig. 133) A ring, C,shown in Fig. 133, with two terminals, the latter shown by the unshadedportions a a, and a suspending hook E, also with two terminals, and twosuspending conductors A, B, carry the current to the magnet. A suddenopening or closing of the circuit produces a sound.Dub's Laws.1. The magnetism excited at any transverse section of a magnet isproportional to the square root of the distance between the givensection and the end.2. The free magnetism at any given transverse section of a magnet isproportional to the difference between the square root of half thelength of the magnet and the square root of the distance between thegiven section and the nearest end.Duct.The tube or compartment in an electric subway for the reception of acable. (See Conduit, Electric Subway.)Dyad.A chemical term; an element which in combination replaces two monovalentelements; one which has two bonds or is bivalent.Dyeing, Electric.The producing mordanting or other dyeing effects on goods in dyeing bythe passage of an electric current.Dynamic Electricity.Electricity of relatively low potential and large quantity; currentelectricity as distinguished from static electricity; electricity inmotion.194 STANDARD ELECTRICAL DICTIONARY.Dynamo, Alternating Current.A dynamo-electric machine for producing an alternating current; analternator. They are classified by S. P. Thompson into three classes--I.Those with stationary field-magnet and rotating armature. II. Those withrotating field magnet and stationary armature. III. Those with bothfield magnet part and armature part stationary, the amount of magneticinduction from the latter through the former being caused to vary oralternate in direction by the revolution of appropriate pieces of iron,called inductors. Another division rests on whether they give one simplealternating current, a two phase current, or whether they give multiphase currents. (See Current, Alternating--Currents, Multiphase.)A great many kinds of alternators have been constructed. Only an outlineof the general theory can be given here. They are generally multipolar,with north and south poles alternating around the field. The armaturecoils, equal in number in simple current machines, to the poles, arewound in opposite senses, so that the current shall be in one direction,though in opposite senses, in all of them at anyone time. As thearmature rotates the coils are all approaching their poles at one timeand a current in one sense is induced in every second coil, and one inthe other sense in the other coils. They are all in continuous circuitwith two open terminals, each connected to its own insulated connectingring on the shaft. As the coils pass the poles and begin to recede fromthem the direction changes, and the current goes in the other directionuntil the next poles are reached and passed. Thus there are as manychanges of direction of current per rotation as there are coils in thearmature or poles in the field.Fig. 134. ALTERNATING CURRENT DYNAMO WITHSEPARATE EXCITER MOUNTED ON MAIN SHAFT.195 STANDARD ELECTRICAL DICTIONARY.The field-magnets whose windings may be in series are often excited by aseparate direct current generation. Some are self-exciting, one or moreof the armature coils being separated from the rest, and connected to aspecial commutator, which rectifies its current.By properly spacing the coils with respect to the poles of the field,and connecting each set of coils by itself to separate connecting rings,several currents can be taken from the same machine, which currentsshall have a constant difference in phase. It would seem at first sightthat the same result could be attained by using as many separatealternators as there were currents to be produced. But it would bealmost impossible to preserve the exact relation of currents and currentphase where each was produced by its own machine. The currents wouldoverrun each other or would lag behind. In a single machine withseparate sets of coils the relation is fixed and invariable.Fig. I35. DIAGRAM OF ARRANGEMENT OF ARMATURE COILS ANDCOLLECTING RINGS IN AN ALTERNATING CURRENT DYNAMO.Dynamo, Alternating Current, Regulation of.Transformers, converters, or induction coils are used to regulatealternating current dynamos, somewhat as compound winding is applied inthe case of direct-current dynamos. The arrangement consists inconnecting the primary of an induction coil or transformer into theexternal circuit with its secondary connected to the field circuit. Thusthe transformer conveys current to the field picked up from the maincircuit, and represents to some extent the shunt of a direct-currentmachine.Dynamo, Commercial Efficiency of.The coefficient, q. v., obtained by dividing the mechanically useful oravailable work of a dynamo by the mechanical energy absorbed by it. Thisonly includes the energy available in the outer circuit, for doinguseful work.196 STANDARD ELECTRICAL DICTIONARY.Fig. 136. COMPOUND WOUND DYNAMO.Dynamo. Compound.A compound wound dynamo; one which has two coils on its field magnet;one winding is in series with the external circuit and armature; theother winding is in parallel with the armature winding, or else with thearmature winding and field winding, both in series. (See Winding, LongShunt--Winding, Short Shunt.)Such a dynamo is, to a certain extent, self-regulating, the two coilscounteracting each other, and bringing about a more regular action forvarying currents than that of the ordinary shunt or series dynamo.The extent of the regulation of such a machine depends on theproportions given its different parts. However good the self-regulatingmay be in a compound wound machine, it can only be perfect at oneparticular speed.To illustrate the principle on which the approximate regulation isobtained the characteristic curve diagram may be consulted.Fig. 137. CURVES OF SERIES AND SHUNT WINDINGS SUPERIMPOSED.One curve is the curve of a series winding, the other that of a shuntwinding, and shows the variation of voltage in each with resistance inthe external or working circuit. The variation is opposite in each case.It is evident that the two windings could be so proportioned on acompound machine that the resultant of the two curves would be astraight line. This regulation would then be perfect and automatic, butonly for the one speed.197 STANDARD ELECTRICAL DICTIONARY.Dynamo, Direct Current.A dynamo giving a current of unvarying direction, as distinguished froman alternator or alternating current dynamo.Dynamo, Disc.A dynamo with a disc armature, such as Pacinotti's disc, q. v. (See alsoDisc, Armature.) The field magnets are disposed so that the disc rotatesclose to their poles, and the poles face or are opposite to the side orsides of the disc. The active leads of wire are those situated on theface or faces of the disc.Fig. 138. POLECHKO'S DISC DYNAMO.Dynamo-electric Machine.A machine driven by power, generally steam power, and converting themechanical energy expended on driving it into electrical energy of thecurrent form. The parts of the ordinary dynamo may be summarized asfollows: First, A circuit as complete as possible of iron. Such circuitis composed partly of the cores of an electro-magnet or of severalelectro-magnets, and partly of the cylindrical or ring-shaped core of anarmature which fits as closely as practicable between the magnet ends orpoles which are shaped so as to partly embrace it. Second, of coils ofinsulated wire wound upon the field-magnet cores. When these coils areexcited the field-magnets develop polarity and the circuit just spokenof becomes a magnetic circuit, interrupted only by the air gaps betweenthe poles and armatures. Thirdly, of coils of insulated wire upon thearmature core. These coils when rotated in the magnetic field cutmagnetic lines of force and develop electro-motive force.198 STANDARD ELECTRICAL DICTIONARY.Fourthly, of collecting mechanism, the commutator in direct currentdynamos, attached to the armature shaft and rotating with it. Thisconsists of insulated rings, or segments of rings to which the wirecoils of the armature are connected, and on which two springs of copperor plates of carbon or some other conductor presses. The electro-motiveforce developed by the cutting of lines of force, by the wires of thearmature, shows itself as potential difference between the two springs.If the ends of a conductor are attached, one to each of these brushes,the potential difference will establish a current through the wire. Byusing properly divided and connected segments on the commutator thepotential difference and consequent direction of the current may be keptalways in the same sense or direction. It is now clear that the externalwire may be connected with the windings of the field-magnet. In suchcase the excitement of the field-magnets is derived from the armatureand the machine is self-excited and entirely self-contained.The above is a general description of a dynamo. Sometimes the coils ofthe field-magnets are not connected with the armature, but derive theircurrent from an outside source. Such are termed separately exciteddynamos.Some general features of dynamo generators may be seen in thedefinitions under this head and elsewhere. The general conception is tocut lines of force with a conductor and thus generate electromotiveforce, or in some way to change the number of lines of force within aloop or circuit with the same effect.Dynamo, Electroplating.A dynamo designed for low potential and high current intensity. They arewound for low resistance, frequently several wires being used inparallel, or ribbon, bar or rectangular conductors being employed. Theyare of the direct current type. They should be shunt wound or they areliable to reverse. They are sometimes provided with resistance in theshunt, which is changed as desired to alter the electro-motive force.Dynamo, Equalizing.A combination for three and five-wire systems. A number of armatures orof windings on the same shaft are connected across the leads. If thepotential drops at any pair of mains, the armature will begin to bedriven by the other mains, acting to an extent as an element of a motor,and will raise the potential in the first pair.Dynamo, Far Leading.A motor dynamo, used to compensate the drop of potential in long mains.Into the mains at a distant point a series motor is connected, driving adynamo placed in shunt across the mains. The dynamo thus driven raisesthe potential difference between the two mains.199 STANDARD ELECTRICAL DICTIONARY.Dynamograph.A printing telegraph in which the message is printed at bothtransmitting and receiving ends.Dynamo, Inductor.A generator in which the armature or current-generating windings are allcomprised upon the poles of the field magnets. Masses of iron, whichshould be laminated and are the inductors, are carried past the fieldmagnet poles concentrating in their passage the lines of force, thusinducing currents in the coils. In one construction shown in the cut thefield magnets a, a .. are U shaped and are arranged in a circle, theirpoles pointing inwards. A single exciting coil c, c ... is wound aroundthe circle in the bend of the V-shaped segments. The poles carry thearmature coils e, e ... The laminated inductors i, i ... are mounted ona shaft S, by spiders h, to be rotated inside the circle of magnets,thus generating an alternating current.Synonym--Inductor Generator.Fig. 139. INDUCTOR DYNAMO.Dynamo, Interior Pole.A dynamo with a ring armature, with field magnet pole pieces whichextend within the ring.200 STANDARD ELECTRICAL DICTIONARY.Dynamo, Iron Clad.A dynamo in which the iron of the field magnet is of such shape as toenclose the field magnet coils as well as the armature.Dynamometer.A device or apparatus for measuring force applied, or rate ofexpenditure of energy by, or work done in a given time by a machine. Acommon spring balance can be used as a force dynamometer, viz: todetermine how hard a man is pulling and the like. The steam engineindicator represents an energy-dynamometer of the graphic type, theinstrument marking an area whence, with the aid of the fixed factors ofthe engine, the work done may be determined. Prony's Brake, q. v., is atype of the friction dynamometer, also of the energy type. In the lattertype during the experiment the whole power must be turned on or beexpended on the dynamometer.Dynamo, Motor.A motor dynamo is a machine for (a) converting a continuous current atany voltage to a continuous current of different strength at a differentvoltage or for (b) transforming a continuous current into an alternatingone, and vice versa.For the first type see Transformer, Continuous Current; for the secondtype see Transformer, Alternating Current.Dynamo, Multipolar.A dynamo having a number of field magnet poles, not merely a singlenorth and a single south pole. The field magnet is sometimes of agenerally circular shape with the poles arranged radially within it, thearmature revolving between the ends.Dynamo, Non-polar.A name given by Prof. George Forbes to a dynamo invented by him. In it acylinder of iron rotates within a perfectly self-contained iron-cladfield magnet. The current is taken off by brushes bearing near theperiphery, at two extremities of a diameter. A machine with a disc 18inches in diameter was said to give 3,117 amperes, with 5.8 volts E. M.F. running at 1,500 revolutions per second. The E. M. F. of suchmachines varies with the square of the diameter of the disc or cylinder.Dynamo, Open Coil.A dynamo the windings of whose armatures may be grouped in coils, whichare not connected in series, but which have independent terminals. Theseterminals are separate divisions of the commutator and so spaced thatthe collecting brushes touch each pair belonging to the same coilsimultaneously. As the brushes come in contact with the sections formingthe terminals they take current from the coil in question. This coil isnext succeeded by another one, and so on according to the number ofcoils employed.Dynamo, Ring.A dynamo the base of whose field magnets is a ring in general shape, orperhaps an octagon, and with poles projecting inwardly therefrom.201 STANDARD ELECTRICAL DICTIONARY.Dynamo, Coupling of.Dynamos can be coupled exactly like batteries and with about the samegeneral results. An instance of series coupling would be given by thedynamos in the three wire system when no current is passing through theneutral wire, and when the lamps on each side of it are lighted in equalnumber.Dynamo, Self-exciting.A dynamo which excites its own field. The majority of dynamos are ofthis construction. Others, especially alternating current machines, areseparately excited, the field magnets being supplied with current from aseparate dynamo or current generator.Dynamo, Separate Circuit.A dynamo in which the field magnet coils are entirely disconnected fromthe main circuit, and in which current for the field is supplied byspecial coils carried for the purpose by the same armature, or by aspecial one, in either case a special commutator being provided tocollect the current.Dynamo, Separately Excited.A dynamo whose field magnets are excited by a separate currentgenerator, such as a dynamo or even a battery. Alternating currentdynamos are often of this construction. Direct current dynamos are notgenerally so. The term is the opposite of self-exciting.Fig. 140. SERIES DYNAMO.Dynamo, Series.A dynamo whose armature, field winding, and external circuit are all inseries.In such a dynamo short circuiting or lowering the resistance of theexternal circuit strengthens the field, increases the electro-motiveforce and current strength and may injure the winding by heating thewire, and melting the insulation.202 STANDARD ELECTRICAL DICTIONARY.Dynamo, Shunt.A dynamo whose field is wound in shunt with the external circuit. Twoleads are taken from the brushes; one goes around the field magnets toexcite them; the other is the external circuit.In such a dynamo the lowering of resistance on the outer circuit takescurrent from the field and lowers the electro-motive force of themachine. Short circuiting has no heating effect.Fig. 141. SHUNT DYNAMO.Dynamo, Single Coil.A dynamo whose field magnet is excited by a single coil. Several suchhave been constructed, with different shapes of field magnet cores, inorder to obtain a proper distribution of poles.Dynamo, Tuning Fork.A dynamo in which the inductive or armature coils were carried at theends of the prongs of a gigantic tuning fork, and were there maintainedin vibration opposite the field magnets. It was invented by T. A.Edison, but never was used.Dynamo, Uni-polar.A dynamo in which the rotation of a conductor effects a continuousincrease in the number of lines cut, by the device of arranging one partof the conductor to slide on or around the magnet. (S. P. Thomson.)Faraday's disc is the earliest machine of this type.203 STANDARD ELECTRICAL DICTIONARY.Dyne.The C. G. S. or fundamental unit of force. It is the force which canimpart an acceleration of one centimeter per second to a mass of onegram in one second. It is equal to about 1/981 the weight of a gram,this weight varying with the latitude.