The Standard Electrical Dictionary / A Popular Dictionary of Words and Terms Used in the Practice of Electrical Engineering

by T. O'Conor Sloane

Fig. 129. DIFFERENTIAL WINDING WORKING
OF ELECTRO-MAGNETIC APPARATUS.
Diffusion.
A term properly applied to the varying current density found in
conductors of unequal cross sectional area. In electro-therapeutics it
is applied to the distribution of current as it passes through the human
body. Its density per cross-sectional area varies with the area and
with the other factors.
Diffusion Creep.
When electrodes of an active circuit are immersed in a solution of an
electrolyte, a current passes electrolytically if there is a sufficient
potential difference. The current passes through all parts of the
solution, spreading out of the direct prism connecting or defined by the
electrodes. To this portion of the current the above term is applied. If
the electrodes are small enough in proportion to the distance between
them the current transmission or creep outside of the line becomes the
principal conveyor of the current so that the resistance remains the
same for all distances.
Dimensions and Theory of Dimensions.
The expression of the unitary value of a physical quantity in one or
more of the units of length (L), time (T) and mass (M) is termed the
dimensions of such quantity. Thus the dimension or dimensions of a
distance is simply L; of an angle, expressible by dividing the arc by
the radius is L/L; of a velocity, expressible by distance divided by
time--L/T; of acceleration, which is velocity acquired in a unit of
time, and is therefore expressible by velocity divided by time--L/T/T or
L/T2; of momentum, which is the product of mass into velocity--M*L/T; of
kinetic energy taken as the product of mass into the square of
velocity--M*(L2/T2); of potential energy taken as the product of mass
into acceleration into space-M*(L/T2)*L reducing to M*(L2/T2). The
theory is based on three fundamental units and embraces all electric
quantities. The simple units generally taken are the gram, centimeter
and second and the dimensions of the fundamental compound units are
expressed in terms of these three, forming the centimeter-gram-second or
C. G. S. system of units. Unless otherwise expressed or implied the
letters L, M and T, may be taken to indicate centimeter, gram and second
respectively. It is obvious that very complicated expressions of
dimensions may be built up, and that a mathematical expression of
unnamed quantities may be arrived at. Dimensions in their application by
these symbols are subject to the laws of algebra. They were invented by
Fourier and were brought into prominence by J. Clerk Maxwell. Another
excellent definition reads as follows: "By the dimensions of a physical
quantity we mean the quantities and powers of quantities, involved in
the measurement of it." (W. T. A. Emtage.)
185 STANDARD ELECTRICAL DICTIONARY.
Dimmer.
An adjustable choking coil used for regulating the intensity of electric
incandescent lights. Some operate by the introduction and withdrawal of
an iron core as described for the choking coil (see Coil, Choking),
others by a damper of copper, often a copper ring surrounding the coil
and which by moving on or off the coil changes the potential of the
secondary 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 in
cleaning or pickling solutions before plating in the electroplater's
bath.
(b) Plating by dipping applies to electroplating without a battery by
simple immersion. Copper is deposited on iron from a solution of copper
sulphate in this way.
Synonym--Simple Immersion.
Dipping Needle.
A magnet mounted in horizontal bearings at its centre of gravity. Placed
in the magnetic meridian it takes the direction of the magnetic lines of
force of the earth at that point. It is acted on by the vertical
component of the earth's magnetism, as it has no freedom of horizontal
movement. (See Magnetic Elements, and Compass, Inclination.)
Directing Magnet.
In a reflecting galvanometer the magnet used for controlling the
magnetic needle by establishing a field. It is mounted on the spindle of
the 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 a
positively charged electrode or terminal to a negatively charged one in
the outer circuit. (See Current.)
(b) The direction of magnetic and electro-magnetic lines of force is
assumed to be from north to south pole of a magnet in the outer circuit.
It is sometimes called the positive direction. Their general course is
shown in the cuts diagrammatically. The circles indicate a compass used
in tracing their course. The magnetic needle tends to place itself in
the direction of or tangential to the lines of force passing nearest it.
(c) The direction of electrostatic lines of force is assumed to be out
of 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 the
magnetic meridian, possessed by the magnetic needle.
Discharge, Brush.
The static discharge of electricity into or through the air may be of
the brush or spark form. The brush indicates the escape of electricity
in continuous flow; the spark indicates discontinuity. The conditions
necessary to the production of one or the other refer to the nature of
the conductor, and of other conductors in its vicinity and to the
electro-motive force or potential difference; small alterations may
transform one into the other. The brush resembles a luminous core whose
apex touches the conductor. It is accompanied by a slight hissing noise.
Its luminosity is very feeble. The negative conductor gives a smaller
brush than that of the positive conductor and discharges it more
readily. When electricity issues from a conductor, remote from an
oppositely excited one, it gives an absolutely silent discharge, showing
at the point of escape a pale blue luminosity called electric glow, or
if it escapes from points it shows a star-like centre of light. It can
be seen in the dark by placing a point on the excited conductor of a
static-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 through
air or rarefied gas; it is also called the quiet or silent discharge.
The luminous effect in air or gas at atmospheric pressures takes the
form of a little brush from a small positive electrode; the negative
shows a star. The phenomena of Gassiot's cascade, the philosopher's egg
and Geissler tubes, all of which may be referred to, are instances of
convective discharge.
Discharge, Dead Beat.
A discharge that is not oscillatory in character.
Discharge, Disruptive.
A discharge of a static charge through a dielectric. It involves
mechanical perforation of the dielectric, and hence the mere mechanical
strength of the latter has much to do with preventing it. A disruptive
discharge is often oscillatory in character; this is always the case
with the discharge of a Leyden jar.
188 STANDARD ELECTRICAL DICTIONARY.
Discharge, Duration of.
The problem of determining this factor has been attacked by various
observers. Wheatstone with his revolving mirror found it to be 1/24000
second. Fedderson, by interposing resistance, prolonged it to 14/10000
and again to 138/10000 second. Lucas & Cazin made it from 26 to 47
millionths of a second. All these experiments were performed with Leyden
jars.
Discharge, Impulsive.
A disruptive discharge produced between conductors by suddenly produced
potential differences. The self-induction of the conductor plays an
especially important part in discharges thus produced.
Discharge, Lateral.
(a) A lightning discharge, which sometimes takes place between a
lightning rod and the building on which it is.
(b) In the discharge of a Leyden jar or condenser the discharge which
takes the alternative path, q. v.
Discharge, Oscillatory.
The sudden or disruptive discharge of a static condenser, such as a
Leyden jar, or of many other charged conductors, is oscillatory in
character. The direction of the currents rapidly changes, so that the
discharge is really an alternating current of excessively short total
duration. The discharge sends electro-magnetic waves through the ether,
which are exactly analogous to those of light but of too long period to
affect the eye.
Synonym--Surging Discharge.
[Transcriber's note: Marconi's transmission across the English channel
occurs in 1897, five years after the publication of this book.]

Fig. 132. DISCHARGER.
Discharger.
An apparatus for discharging Leyden jars. It consists of a conductor
terminating in balls, and either jointed like a tongs or bent with a
spring-action, so that the balls can be set at distances adapted to
different sized jars. It has an insulating handle or a pair of such. In
use one ball is brought near to the coating and the other to the spindle
ball 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. It
consists of a base with three insulating posts. The central post carries
an ivory table to support the object. The two side posts carry
conducting rods, terminating in metal balls, and mounted with universal
joints. 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 and
sometimes to the condition of electric effluvium. (See Discharge,
Brush--Effluvium, Electric.)
Discharge, Spark.
The discontinuous discharge of high tension electricity through a
dielectric or into the air produces electric sparks. These are quite
strongly luminous, of branching sinuous shape, and in long sparks the
luminosity varies in different parts of the same spark. A sharp noise
accompanies each spark. High density of charge is requisite for the
formation of long sparks.
Disconnection.
The separation of two parts of, or opening a circuit, as by turning a
switch, unscrewing a binding screw, or the like. The term is sometimes
used to indicate a class of faults in telegraph circuits. Disconnections
may be total, partial or intermittent, and due to many causes, such as
open 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 all
on the surface. This fact being granted, the theory of displacement
holds that charging a body is the displacing of electricity, forcing it
from the interior on to the surface, or vice versa, producing a positive
or negative charge by displacement of electricity. While displacement is
taking place in a dielectric there is assumed to be a movement or
current of electricity called a displacement current.
Disruptive Tension.
When the surface of a body is electrified, it tends to expand, all
portions of the surface repelling each other. The film of air
surrounding such a body is electrified too, and is subjected to a
disruptive 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 a
sufficiently high degree of heat. All compounds are susceptible of
dissociation, so that it follows that combustion is impossible at high
temperatures.
190 STANDARD ELECTRICAL DICTIONARY.
Distance, Critical, of Alternative Path.
The length of air gap in an alternative path whose resistance joined to
the impedance of the rest of the conductors of the path just balances
the impedance of the other path.
Distance, Sparking.
The distance between electrodes, which a spark from a given Leyden jar
or other source will pass across.
Synonym--Explosive Distance.
Distillation.
The evaporation of a liquid by heat, and sometimes in a vacuum, followed
by condensation of the vapors, which distil or drop from the end of the
condenser. It is claimed that the process is accelerated by the liquid
being electrified.
Distributing Box.
In an electric conduit system, a small iron box provided for giving
access to the cable for the purpose of making house and minor
connections.
Synonym--Hand Hole.
Distributing Switches.
Switch systems for enabling different dynamos to supply different lines
of 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 in
or out as desired, without putting out the lights.
Distribution of Electric Energy, Systems of.
The systems of electric current distribution from central stations or
from private generating plants, mechanical or battery, the latter
primary or secondary. They include in general the alternating current
system and direct current systems. Again, these may be subdivided into
series and multiple arc, multiple-series and series-multiple
distribution, and the three, four, or five wire system may be applied to
multiple arc or multiple series systems. (See Alternating
Current--Current System--Multiple Arc--Multiple Series--Series
Multiple--Three Wire System.)
Door Opener, Electric.
An apparatus for opening a door by pushing back the latch. A spring then
draws the door open, and it is closed against the force of the spring by
the person entering. Electro-magnetic mechanism actuates the latch, and
is operated by a switch or press-button. Thus a person on the upper
floor can open the hall door without descending.
Dosage, Galvanic.
In electro-therapeutics the amount of electric current or discharge, and
duration of treatment given to patients.
Double Carbon Arc Lamp.
An arc lamp designed to burn all night, usually constructed with two
parallel sets of carbons, one set replacing the other automatically, the
current being switched from the burnt out pair to the other by the
action of the mechanism of the lamp.
191 STANDARD ELECTRICAL DICTIONARY.
Double Fluid Theory.
A theory of electricity. Electricity is conveniently treated as a fluid
or fluids. According to the double fluid hypothesis negative electricity
is due to a preponderance of negative fluid and vice versa. Like fluid
repels like, and unlike attracts unlike; either fluid is attracted by
matter; the presence in a body of one or the other induces
electrification; united in equal proportions they neutralize each other,
and friction, chemical decomposition and other causes effect their
separation. The hypothesis, while convenient, is overshadowed by the
certainty that electricity is not really a fluid at all. (See Single
Fluid Theory--Fluid, Electric.)
Synonym--Symmer's Theory.
[Transcriber's note: Current is the motion of negative electrons in a
conductor or plasma. Unequal distribution of electrons is static
electricity. The relatively immobile nuclei of atoms are positive when
one or more of its electrons is absent and accounts for part of the
current in electrolysis and plasmas.]
Double Fluid Voltaic Cell.
A cell in which two fluids are used, one generally as depolarizer
surrounding the negative plate, the other as excitant surrounding the
positive plate. A porous diaphragm or difference in specific gravities
is used to keep the solutions separate and yet permit the essential
electrolytic 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 end
and another pair a little distance back therefrom, so that it can make
two loop connections at once.
Synonym--Double Plug.
Doubler.
A continuously acting electrophorous, q.v.; an early predecessor of the
modern 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 through
it or upon the motion of an armature in it.
Dreh-strom. (German)
Rotatory currents; a system of currents alternating in periodic
succession 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. For
metals a rotary motion, for rocks a reciprocating or percussion action
is imparted. It is used by shipbuilders for drilling holes in plates
which are in place in ships, as its flexible conductors enable it to be
placed anywhere. For rock-drilling a solenoid type of construction is
adopted, producing rapid percussion.
192 STANDARD ELECTRICAL DICTIONARY.
Drip Loop.
A looping downward of wires entering a building, so that rain water, as
it runs along the wire, will drip from the lowest part of the loop
instead 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 the
winding in place and preventing its displacement. Various arrangements
have been adopted. They are sometimes wedges or pins and are sometimes
driven into spaces left in the drum core. The toothed disc armature
cores make up an armature in which the ridges formed by the teeth form
practically driving horns.
Dronier's Salt.
A substance for solution for use in bichromate batteries. It is a
mixture of one-third potassium bichromate and two-thirds potassium
bisulphate. It is dissolved in water to make the exciting fluid.
Drop, Automatic.
A switch or circuit breaker, operating to close a circuit by dropping
under the influence of gravity. It is held up by a latch, the circuit
remaining open, until the latch is released by a current passing through
an electro-magnet. This attracting an armature lets the drop fall. As it
falls it closes a local or second circuit, and thus may keep a bell
ringing until it is replaced by hand. It is used in burglar alarms, its
function being to keep a bell ringing even though the windows or door by
which 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 or
some 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 to
produce the sound, the electro-magnet D and armature being quite
screened from observation through the hole. (See Fig. 133) A ring, C,
shown in Fig. 133, with two terminals, the latter shown by the unshaded
portions a a, and a suspending hook E, also with two terminals, and two
suspending conductors A, B, carry the current to the magnet. A sudden
opening or closing of the circuit produces a sound.
Dub's Laws.
1. The magnetism excited at any transverse section of a magnet is
proportional to the square root of the distance between the given
section and the end.
2. The free magnetism at any given transverse section of a magnet is
proportional to the difference between the square root of half the
length of the magnet and the square root of the distance between the
given section and the nearest end.
Duct.
The tube or compartment in an electric subway for the reception of a
cable. (See Conduit, Electric Subway.)
Dyad.
A chemical term; an element which in combination replaces two monovalent
elements; one which has two bonds or is bivalent.
Dyeing, Electric.
The producing mordanting or other dyeing effects on goods in dyeing by
the passage of an electric current.
Dynamic Electricity.
Electricity of relatively low potential and large quantity; current
electricity as distinguished from static electricity; electricity in
motion.
194 STANDARD ELECTRICAL DICTIONARY.
Dynamo, Alternating Current.
A dynamo-electric machine for producing an alternating current; an
alternator. They are classified by S. P. Thompson into three classes--I.
Those with stationary field-magnet and rotating armature. II. Those with
rotating field magnet and stationary armature. III. Those with both
field magnet part and armature part stationary, the amount of magnetic
induction from the latter through the former being caused to vary or
alternate in direction by the revolution of appropriate pieces of iron,
called inductors. Another division rests on whether they give one simple
alternating current, a two phase current, or whether they give multi
phase currents. (See Current, Alternating--Currents, Multiphase.)
A great many kinds of alternators have been constructed. Only an outline
of the general theory can be given here. They are generally multipolar,
with north and south poles alternating around the field. The armature
coils, equal in number in simple current machines, to the poles, are
wound in opposite senses, so that the current shall be in one direction,
though in opposite senses, in all of them at anyone time. As the
armature rotates the coils are all approaching their poles at one time
and a current in one sense is induced in every second coil, and one in
the other sense in the other coils. They are all in continuous circuit
with two open terminals, each connected to its own insulated connecting
ring on the shaft. As the coils pass the poles and begin to recede from
them the direction changes, and the current goes in the other direction
until the next poles are reached and passed. Thus there are as many
changes of direction of current per rotation as there are coils in the
armature or poles in the field.

Fig. 134. ALTERNATING CURRENT DYNAMO WITH
SEPARATE EXCITER MOUNTED ON MAIN SHAFT.
195 STANDARD ELECTRICAL DICTIONARY.
The field-magnets whose windings may be in series are often excited by a
separate direct current generation. Some are self-exciting, one or more
of the armature coils being separated from the rest, and connected to a
special 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 currents
shall have a constant difference in phase. It would seem at first sight
that the same result could be attained by using as many separate
alternators as there were currents to be produced. But it would be
almost impossible to preserve the exact relation of currents and current
phase where each was produced by its own machine. The currents would
overrun each other or would lag behind. In a single machine with
separate sets of coils the relation is fixed and invariable.