DIELECTRIC.--An insulating material between two electrically charged plates in which there is set up an _electric strain_, or displacement.

DIELECTRIC STRAIN.--The electric displacement in a dielectric.

DIRECTIONAL AERIAL.--See _Aerial, Directional_.

DIRECTION FINDER.--See _Aerial, Loop_.

DISCHARGE.--(1) A faintly luminous discharge that takes place from the positive pointed terminal of an induction coil, or other high potential apparatus; is termed a _brush discharge_. (2) A continuous discharge between the terminals of a high potential apparatus is termed a _convective discharge_. (3) The sudden breaking-down of the air between the b.a.l.l.s forming the spark gap is termed a _disruptive discharge_; also called an _electric spark_, or just _spark_ for short. (4) When a tube has a poor vacuum, or too large a battery voltage, it glows with a blue light and this is called a _blue glow discharge_.

DISRUPTIVE DISCHARGE.--See _Discharge_.

DISTRESS CALL. [Morse code:] ...---... (SOS).

DISTRIBUTED CAPACITY.--See _Capacity, Distributed_.

DOUBLE HUMP RESONANCE CURVE.--A resonance curve that has two peaks or humps which show that the oscillating currents which are set up when the primary and secondary of a tuning coil are closely coupled have two frequencies.

DUO-LATERAL COILS.--See _Coils, Inductance_.

DUPLEX COMMUNICATION.--A wireless telephone system with which it is possible to talk between both stations in either direction without the use of switches. This is known as the _duplex system_.

EARTH CAPACITY.--An aerial counterpoise.

EARTH CONNECTION.--Metal plates or wires buried in the ground or immersed in water. Any kind of means by which the sending and receiving apparatus can be connected with the earth.

EDISON STORAGE BATTERY.--See _Storage Battery, Edison_.

ELECTRIC ENERGY.--The power of an electric current.

ELECTRIC OSCILLATIONS.--See _Oscillations, Electric_.

ELECTRIC SPARK.--See _Discharge, Spark_.

ELECTRICITY, NEGATIVE.--The opposite of _positive electricity_.

Negative electricity is formed of negative electrons which make up the outside particles of an atom.

ELECTRICITY, POSITIVE.--The opposite of _negative electricity_.

Positive electricity is formed of positive electrons which make up the inside particles of an atom.

ELECTRODES.--Usually the parts of an apparatus which dip into a liquid and carry a current. The electrodes of a dry battery are the zinc and carbon elements. The electrodes of an Edison storage battery are the iron and nickel elements, and the electrodes of a lead storage battery are the lead elements.

ELECTROLYTES.--The acid or alkaline solutions used in batteries.

ELECTROMAGNETIC WAVES.--See _Waves, Electric_.

ELECTROMOTIVE FORCE.--Abbreviated _emf_. The force that drives an electric current along a conductor. Also loosely called _voltage_.

ELECTROMOTIVE FORCE, COUNTER.--The emf. that is set up in a direction opposite to that in which the current is flowing in a conductor.

ELECTRON.--(1) A negative particle of electricity that is detached from an atom. (2) A negative particle of electricity thrown off from the incandescent filament of a vacuum tube.

ELECTRON FLOW.--The pa.s.sage of electrons between the incandescent filament and the cold positively charged plate of a vacuum tube.

ELECTRON RELAY.--See _Relay, Electron_.

ELECTRON TUBE.--A vacuum tube or a gas-content tube used for any purpose in wireless work. See _Vacuum Tube_.

ELECTROSE INSULATORS.--Insulators made of a composition material the trade name of which is _Electrose_.

ENERGY, ELECTRIC.--See _Electric Energy_.

ENERGY UNIT.--The _joule_, which see, Page 308 [Appendix: Definitions of Electric and Magnetic Units].

FADING.--The sudden variation in strength of signals received from a transmitting station when all the adjustments of both sending and receiving apparatus remain the same. Also called _swinging_.

FARAD.--The capacitance of a condenser in which a potential difference of 1 volt causes it to have a charge of 1 coulomb of electricity.

FEED-BACK ACTION.--Feeding back the oscillating currents in a vacuum tube to amplify its power. Also called _regenerative action_.

FERROMAGNETIC CONTROL.--See _Magnetic Amplifier_.

FILAMENT.--The wire in a vacuum tube that is heated to incandescence and which throws off electrons.

FILAMENT RHEOSTAT.--See _Rheostat, Filament_.

FILTER.--Inductance coils or condensers or both which (1) prevent troublesome voltages from acting on the different circuits, and (2) smooth out alternating currents after they have been rectified.

FILTER REACTOR.--See _Reactor, Filter_.

FIRE UNDERWRITERS.--See _Code, National Electric_.

FIXED GAP.--See _Gap_.

FLEMING VALVE.--A two-electrode vacuum tube.

FORCED OSCILLATIONS.--See _Oscillations, Forced_.

FREE OSCILLATIONS.--See _Oscillations, Free_.

FREQUENCY, AUDIO.--(1) An alternating current whose frequency is low enough to operate a telephone receiver and, hence, which can be heard by the ear. (2) Audio frequencies are usually around 500 or 1,000 cycles per second, but may be as low as 200 and as high as 10,000 cycles per second.

Carrier.--A radio frequency wave modulated by an audio frequency wave which results in setting of _three_ radio frequency waves. The princ.i.p.al radio frequency is called the carrier frequency, since it carries or transmits the audio frequency wave.

Commercial.--(1) Alternating current that is used for commercial purposes, namely, light, heat and power. (2) Commercial frequencies now in general use are from 25 to 50 cycles per second.

Natural.--The pendulum and vibrating spring have a _natural frequency_ which depends on the size, material of which it is made, and the friction which it has to overcome. Likewise an oscillation circuit has a natural frequency which depends upon its _inductance_, _capacitance_ and _resistance_.

Radio.--(1) An oscillating current whose frequency is too high to affect a telephone receiver and, hence, cannot be heard by the ear.

(2) Radio frequencies are usually between 20,000 and 2,000,000 cycles per second but may be as low as 10,000 and as high as 300,000,000 cycles per second.