Spark.--The number of sparks per second produced by the discharge of a condenser.

GAP, FIXED.--One with fixed electrodes.

GAP, NON-SYNCHRONOUS.--A rotary spark gap run by a separate motor which may be widely different from that of the speed of the alternator.

GAP, QUENCHED.--(1) A spark gap for the impulse production of oscillating currents. (2) This method can be likened to one where a spring is struck a single sharp blow and then continues to set up vibrations.

GAP, ROTARY.--One having fixed and rotating electrodes.

GAP, SYNCHRONOUS.--A rotary spark gap run at the same speed as the alternator which supplies the power transformer. Such a gap usually has as many teeth as there are poles on the generator. Hence one spark occurs per half cycle.

GAS-CONTENT TUBE.--See _Vacuum Tube._

GENERATOR TUBE.--A vacuum tube used to set up oscillations. As a matter of fact it does not _generate_ oscillations, but changes the initial low voltage current that flows through it into oscillations.

Also called an _oscillator tube_ and a _power tube._

GRID BATTERY.--See _Battery C._

GRID CHARACTERISTICS.--The various relations that could exist between the voltages and currents of the grid of a vacuum tube, and the values which do exist between them when the tube is in operation. These characteristics are generally shown by curves.

GRID CONDENSER.--See _Condenser, Grid._

GRID LEAK.--A high resistance unit connected in the grid lead of both sending and receiving sets. In a sending set it keeps the voltage of the grid at a constant value and so controls the output of the aerial.

In a receiving set it controls the current flowing between the plate and filament.

GRID MODULATION.--See _Modulation, Grid._

GRID POTENTIAL.--The negative or positive voltage of the grid of a vacuum tube.

GRID VOLTAGE.--See _Grid Potential._

GRINDERS.--The most common form of _Static,_ which see. They make a grinding noise in the headphones.

GROUND.--See _Earth Connection._

GROUND, AMATEUR.--A water-pipe ground.

GROUND, WATERPIPE.--A common method of grounding by amateurs is to use the waterpipe, gaspipe or radiator.

GUIDED WAVE TELEPHONY.--See _Wired Wireless._

HARD TUBE.--A vacuum tube in which the vacuum is _high,_ that is, exhausted to a high degree.

HELIX.--(1) Any coil of wire. (2) Specifically a transmitter tuning inductance coil.

HENRY.--The inductance in a circuit in which the electromotive force induced is 1 volt when the inducing current varies at the rate of 1 ampere per second.

HETERODYNE RECEPTION.--(1) Receiving by the _beat_ method. (2) Receiving by means of superposing oscillations generated at the receiving station on the oscillations set up in the aerial by the incoming waves.

HETERODYNE RECEPTOR.--See _Receptor, Heterodyne._

HIGH FREQUENCY CURRENTS.--See _Currents, High Frequency._

HIGH FREQUENCY RESISTANCE.--See _Resistance, High Frequency._

HIGH POTENTIAL CURRENTS.--See _Currents, High Potential._

HIGH VOLTAGE CURRENTS.--See _Currents, High Potential._

HONEYCOMB COILS.--See _Coils, Inductance._

HORSE-POWER.--Used in rating steam machinery. It is equal to 746 watts.

HOT WIRE AMMETER.--See _Ammeter, Hot Wire._

HOWLING.--Where more than three stages of radio amplification, or more than two stages of audio amplification, are used howling noises are apt to occur in the telephone receivers.

IMPEDANCE.--An oscillation circuit has _reactance_ and also _resistance,_ and when these are combined the total opposition to the current is called _impedance._

INDUCTANCE COILS.--See _Coils, Inductance._

INDUCTANCE COIL, LOADING.--See _Coil, Loading Inductance._

INDUCTIVE COUPLING.--See _Coupling, Inductive._

INDUCTIVE REACTANCE.--See _Reactance, Inductive._

INDUCTION COIL.--See _Coil, Induction._

INDUCTION, MUTUAL.--Induction produced between two circuits or coils close to each other by the mutual interaction of their magnetic fields.

INSULATION.--Materials used on and around wires and other conductors to keep the current from leaking away.

INSPECTOR, RADIO.--A U. S. inspector whose business it is to issue both station and operators' licenses in the district of which he is in charge.

INTERFERENCE.--The crossing or superposing of two sets of electric waves of the same or slightly different lengths which tend to oppose each other. It is the untoward interference between electric waves from different stations that makes selective signaling so difficult a problem.

INTERMEDIATE WAVES.--See _Waves._

IONIC TUBES.--See _Vacuum Tubes._

INTERNATIONAL CODE.--See Code, International.

JAMMING.--Waves that are of such length and strength that when they interfere with incoming waves they drown them out.

JOULE.--The energy spent in 1 second by a flow of 1 ampere in 1 ohm.

JOULE'S LAW.--The relation between the heat produced in seconds to the resistance of the circuit, to the current flowing in it.