hacking Colloquial term for the deliberate gaining of unauthorised access to private computer systems over the telecommunications network. For some hackers this is simply an intellectual challenge; others cause serious problems by tampering with confidential stored data or by making fraudulent use of the data accessed.

halation In a cathode ray tube, degradation of the image caused by an area of light surrounding the spot where the electron beam strikes the screen. This unwanted area is caused by light from the spot reaching the screen by reflection at the front and rear surface of the faceplate of the tube.

half-adder In logic circuitry a combination of logic elements with two inputs and two outputs so related that one output is the sum of the two inputs and the other is an exclusive-OR function of the two inputs. Two such circuits can be combined to form a full adder which can perform binary addition. See exclusive-OR gate.

half-duplex operation Of a telegraph system, a duplex system permitting operation in either direction, but not in both directions simultaneously.

half-section Of a filter. See ladder network.

half-wave rectification Rectification in which power for the load is taken from the AC supply only during alternate half-cycles of the supply. A typical circuit for a half-wave rectifier is given in Figure H.1.

Hall constant (or coefficient) Factor relating the voltage generated as a result of the Hall effect with the product of the current and the magnetic field.

Hall effect The production of a transverse EMF in a current-carrying conductor or semiconductor subjected to a magnetic field. When a conductor or a semiconductor carrying a current is subjected to a magnetic field perpendicular to the direction of current flow a voltage proportional to the current and to the magnetic field is generated across the faces of the conductor perpendicular to the direction of current flow and to the direction of the magnetic field.

Hamming code In digital transmission a code so designed that errors in signals can be detected and corrected. See parity check.

Hamming distance Same as signal distance.

    handset An instrument containing a microphone and an earphone designed to be hand held, as in the familiar telephone instrument.

hard copy In data processing a copy of a document on paper or similar material, in contrast with a copy on magnetic media such as a floppy disk or in some electronic format. Hard copy is so called because, once committed to paper, it cannot be easily changed; in contrast, a file in a data storage medium, such as a hard or floppy disk, can be readily edited.

hard disk (or hard drive) A data storage system consisting usually of several magnetic disks on a common spindle and sealed with their read/write heads in a container. The disks are rigid and double-sided, each surface having its own read/write head. To minimise wear, the heads do not contact the disk surface, but are suspended very close to it, the sealed container preventing the ingress of dust which would have a catastrophic effect on the drive’s reliability. Physical drive sizes vary widely. Storage capacities range from 20 Mbytes to 10 Gbytes and data transfer is significantly faster than that available from floppy disks or CD-ROMs. Hard disks are sometimes called fixed disks because, in contrast with floppy disks, they are usually fixed permanently in computers where they are used for the storage of commonly used software, such as operating systems and applications. This software is transferred on to the hard disk from floppy disks, CD-ROMs, tape streamers or over a network. There are, however, some demountable hard disks. In these the sealed container housing the disks and heads slides out of the drive assembly containing the motor and control electronics. These are used not only for software storage, but also for the distribution of large collections of software.

hard valve Same as vacuum tube.

hardware Term used particularly in computer technology to describe the electronic equipment, its circuits and components. See software.

hardware key A device used to protect a software package against unauthorised copying. Also known as a dongle, the device is plugged into a computer port, often the parallel printer port; the printer is connected to a printer port on the hardware key itself. At start-up the protected software checks for the presence of coded data in the hardware key; if it is absent, the application is terminated. Since the software cannot be used without a suitable hardware key, unauthorised copies of it are useless. The hardware key itself is far more difficult to copy than the software it protects.

harmonic analyser Equipment for measuring the amplitude and phase of the harmonic components of a complex waveform. It is usually mechanical in nature.

harmonic component Of a complex waveform, one of the components with a frequency equal to a multiple of the fundamental frequency. The analysis of a complex waveform into its harmonic components can be carried out mathematically by Fourier analysis or practically by use of a harmonic analyser.

harmonic distortion Distortion arising from the non-linearity of the input-output characteristic of a system, equipment or component and resulting in the production of new signals at harmonics of the frequency of a sinusoidal input signal.

harmonic generator Same as frequency multiplier.

Harries tetrode Same as critical-space tetrode.

Hartley oscillator A sinusoidal oscillator in which the frequency-determining element is a parallel-tuned LC circuit connected between the input and output terminals of a active device, positive feedback being obtained by connecting a tapping on the inductor to the common terminal (cathode, emitter, source) of the active device. Figure H.2 gives the circuit diagram of a transistor Hartley oscillator.

Hay bridge A bridge circuit generally used for the measurement of inductance in terms of capacitance, resistance and frequency. It differs from the Maxwell bridge in that the capacitor is in series with its associated resistor as shown in Figure H.3. The conditions for balance are:

head A device which records information on a storage medium, reproduces the information or erases it. The storage medium may be tape, film or disk and the information may be in digital form as in data-processing equipment or may be in analogue form as in audio and TV recording.

head amplifier An audio or video amplifier incorporated in a microphone, television camera or motion picture projector to raise the level of the output signal before it is sent along a cable. This technique is used as a means of improving the signal-to-noise ratio.

headphones An electro-acoustic transducer designed to feed sound directly into the ear and mounted on a headband. Some headbands carry only one transducer so as to leave one ear free. Others carry two transducers which are connected in series or in parallel for monophonic reproduction or can be connected to the two channels of stereo equipment for stereophonic reproduction.

headset A combination of headphone and microphone mounted on a headband and used for two-way communications by, for example, telephone operators and aircraft pilots.

hearing aid An equipment consisting of a microphone, an audio amplifier and an earphone and powered by an internal battery which allow persons suffering hearing deficiency to take part in normal conversations and to enjoy radio and television broadcasts. Modern hearing aids are extremely small and can be hidden behind or within the ear. Many also feature a pickup coil which may be selected instead of the microphone for use in a theatre, church or other auditorium fitted with an induction loop.

heater In general any resistor carrying current and used to supply heat. In particular the resistor which supplies the heat necessary for thermionic emission from an indirectly-heated cathode. The heater is usually a tungsten wire contained within the cathode cylinder but electrically insulated from it.

heat sink A metal structure arranged to be in intimate thermal contact with a heat-generating component such as a power transistor to aid heat dissipation and so limit temperature rise in the component. To be efficient a heat sink requires adequate thermal capacity (mass × specific heat). It should also be a good conductor of heat and have large surface area. Often heat sinks are finned to increase surface area.

Heising modulation A circuit for amplitude modulation in which the anodes of a modulation-frequency amplifier and a carrier-frequency amplifier are coupled and fed via a common inductor with a high reactance at modulation frequencies.

    If the carrier-frequency amplifier operates in class C its output is proportional to its HT supply voltage. If, therefore, the HT voltage is varied by the modulation-frequency amplifier, amplitude modulation can be achieved. When the anode current of one tube is instantaneously high, that of the other is low and in fact the sum of the two currents is constant. The circuit is sometimes called constant-current modulation.

    The basic circuit described here is, in practice, modified to enable very deep modulation to be achieved without excessive distortion, e.g. by the use of a transformer in place of a choke.

Helmholtz resonator A cavity open to the external environment via a small aperture in the cavity wall. Such an arrangement resonates at a frequency dependent on the dimensions of the cavity. Helmholtz resonators are used extensively as tuned elements for electromagnetic waves in microwave tubes such as klystrons. The vented enclosures used in loudspeaker designs and the tuned absorbers used in acoustic treatment of sound studio walls are also examples of Helmholtz resonators.

Henry The unit of self-inductance or mutual inductance, symbol H. An inductor is said to have an inductance of 1 H when a current in it, changing at the rate of 1 A per second, generates a voltage across it of 1 V. Practical inductors have inductances ranging from many H to fractions of a μH.

heptode An electron tube with five grids situated between cathode and anode. It was usually used as a frequency changer, the first two grids acting together as the grid and anode of the oscillator. Grids 4 and 5, together with the anode, constitute an RF tetrode acting as an RF mixer. Grid 3 is a screen to minimise capacitive interaction between oscillator and mixer sections. Mixing occurs by virtue of the oscillator-frequency component impressed on the electron stream by grids 1 and 2. However the signals on these grids are usually in antiphase and their effects on the electron stream are therefore mutually destructive. To minimise this effect grid 2 is not of conventional construction but may take the form, for example, of a number of rods which can carry sufficient current to sustain oscillation but have little modulating effect on the electron stream. Because grids 3 and 5 both function as electrostatic screens they are often connected together inside or outside the tube.

    The graphical symbol for a heptode is given in Figure H.4 which also indicates the function of each grid.

heterodyne The process of combining two signals of different frequencies so as to produce an output at the sum or difference frequency. The process is thus equivalent to non-linear additive mixing or multiplicative mixing. The process is extensively used in sound and TV receivers where the received signal is combined with the output of the local oscillator to produce a difference term at the intermediate frequency. Because the difference frequency is above audibility this type of receiver is known as a supersonic heterodyne, abbreviated to superheterodyne or superhet.

Hertz The unit of frequency and equal to 1 cycle per second, symbol Hz. The multiple units of kilohertz (kHz), megahertz (MHz) and gigahertz (GHz) are commonly used in electronics.

hexadecimal (hex) A counting scale containing sixteen digits, each digit in a number representing a power of sixteen. Conventionally the digits of the hexadecimal scale are represented by the decimal numbers from 0 to 9 followed by the letters A to F. Thus the decimal equivalent of the hexadecimal number 5A6 can be evaluated in the following manner:

    Hexadecimal numbers are often used as a shorthand method of writing binary numbers. The base 16 of the hexadecimal scale is 2⁴ and this facilitates conversion between the hexadecimal and the binary scales. Each four digits of a binary number can be uniquely represented by one digit of a hexadecimal number. For example an 8-bit byte such as 01101100 can be represented by the two-digit hexadecimal number 6C, both having a decimal equivalent of 108. Similarly a 16-bit binary word can be represented by a four-digit hexadecimal number.

hexode An electron tube with four grids between cathode and anode. The tube may be regarded as an RF tetrode (grid 4 being the screen grid) with two control grids (1 and 3) separated by a screen grid (2) to minimise capacitive coupling between the two inputs. Grids 2 and 4 may be connected together inside or outside the tube.

    The tube was designed for use as an RF mixer, the oscillator and signal-frequency inputs being applied to grids 1 and 3. The tube is often combined with a triode oscillator in a single envelope to form a triode-hexode frequency changer.

    The graphical symbol for a hexode is given in Figure H.5 which also indicates the function of each grid.

hi-fi, high-fidelity A vague term used to describe audio equipment in whose design some effort has been made to ensure that sound is reproduced with minimal distortion.

high-definition television (HDTV) A colour television system providing wide-screen flicker-free pictures with a resolution comparable to that of 35-mm cinematograph film. Considerable research has been invested in the development of such systems using a 16:9 aspect ratio and 1125 lines, but there are difficulties in establishing standards that are acceptable worldwide.

high frequency resistance Same as effective resistance (2).

high level modulation Same as high-power modulation.

high-noise-immunity logic (HNIL) A form of logic similar to diode-transistor logic but including a zener diode to increase voltage levels. It is largely obsolete.

high-pass filter A filter designed to pass signals at frequencies above a specified cut-off frequency. Thus the passband extends from the cut-off frequency to an infinitely-high frequency. In general such filters comprise series capacitors and shunt inductors. The block symbol for a high-pass filter is given in Figure H.6.

high-power modulation In an AM transmitter amplitude modulation of the carrier by introducing the modulating signal into the anode circuit of the final amplifying stage. The power of the modulating signal thus approximates to that at the transmitter output. See low-power modulation.

high tension (HT) The high-voltage supply required for the anodes and screen grids of electron tubes. In battery-operated equipment the HT supply was commonly between 45 V and 90 V but in mains-operated equipment supplies for early stages were often about 200 V and for output stages up to 500 V.

high threshold logic (HTL) Same as high-noise-immunity logic.

highway Same as bus.

H-network A five-element network in which both legs contain two elements in series, a shunt element bridging their junctions as shown in Figure H.7. It can be regarded as a balanced form of T-network.

hold control In early TV receivers the controls which determined the free-running frequency of the line and field time bases. The controls were adjusted to bring the time-base frequencies into the range at which the time bases lock at the frequencies of the line and field synchronising signals. Hence receivers had two hold controls: line or horizontal hold; field or vertical hold. Modern receivers do not have hold controls.

hole A deficiency of one electron in the atomic structure of an atom in an extrinsic semiconductor. As a result of this deficiency the atom has a net positive charge equal to that of an electron and neutralisation of such charges by electrons can give rise to a current in the form of a unidirectional movement of holes as described under p-type semiconductor.

hole conduction The process by which current flows through a p-type semiconductor.

hole injection The basic process underlying the action of pnp transistors. The base-emitter junction is forward-biased and the current which flows between these regions is largely carried by holes moving from emitter to base. These enter the very thin base region and are swept into the collector region by the collector-base voltage so giving rise to a considerable collector current. It is the holes injected into the base region from the emitter region which are responsible for the collector current.

hole storage The process which causes the collector current of a pnp transistor (which has been driven hard into conduction) to continue for a brief period after the emitter current has been cut off. When the transistor has been driven hard on, the emitter injects into the base region more holes than are required to give the collector current and the excess holes are stored ready to be swept into the collector region to prolong the collector current when the emitter current has been cut off. See carrier storage.

homodyne A system of heterodyne reception in which the received signal is mixed with another at the carrier frequency of the signal and derived from the received signal. Thus there is no local oscillator as in normal heterodyne reception.

horizontal amplifier In oscilloscopes the circuits which amplify the signals responsible for horizontal deflection of the beam. Also known as an X amplifier.

horizontal blanking Same as line blanking.

horizontal hold See hold control.

horizontal polarisation Property of an electromagnetic wave in which the plane of polarisation of the electric field is horizontal.

horizontal time base In a cathode ray tube the circuits generating the signals which give horizontal deflection of the beam. In TV this is usually termed the line time base.

host The computer controlling a network. More specifically, the mainframe or other large computer served by a network of terminals or the computer running a bulletin board or other communications system to which other computers are temporarily connected for data exchange purposes.

hot carrier diode Same as Schottky diode.

hot-electron diode Same as Schottky diode.

hot spot (1) In tubes with a mercury-pool cathode a small heated area formed on the mercury surface by the ignition electrode and which initiates the discharge. (2) In a high-power electron tube a small heated area on the anode.

H-parameters Same as hybrid parameters.

HSV colour model See colour model.

hue That quality of a colour which enables it to be classified as red, green, blue, etc. For colours appearing in the spectrum, hue can be quantitatively defined by quoting the wavelength of the colour.

hum An unwanted low-frequency noise originating from mains-driven equipment and comprising harmonics of the mains frequency. The sound may be radiated directly from components such as mains transformers or may be caused by ripple on the DC supply to an amplifier or receiver and heard in the output of loudspeakers or headphones.

hunting In a controlled system an undesired oscillation, usually at a low frequency, resulting from over-correction in which the controlled quantity fluctuates about the required value.

hybrid coil A transformer with three windings and four pairs of terminals, two of the windings being designed for use in a bridge circuit which isolates one pair of terminals from another pair provided the remaining two pairs are correctly terminated. The circuit diagram of one application of a hybrid coil is given in Figure H.8.

hybrid computer A computer in which analogue and digital techniques are used.

hybrid integrated circuit An integrated circuit using monolithic and thin-film techniques.

hybrid network A network with four pairs of terminals which, when two pairs are correctly terminated, transmits power from the third pair into the terminated pairs but not into the fourth pair.

hybrid parameters Of a transistor a method of expressing the electrical characteristics by representing it as a four-terminal equivalent network, for which the input voltage and output current are expressed in terms of input current and output voltage. The fundamental equations are:

from which h_i has the dimensions of an impedance, h_o of an admittance, whilst h_r and h_f are both pure numbers. The parameters are thus mixed or hybrid in nature. In fact h_f is the current gain of the transistor and h_i is the input impedance (both with the output terminals short-circuited), h_o is the output admittance and h_r the voltage feedback ratio (ratio of input voltage to output voltage) both for open-circuited input terminals. See Y parameters, Z parameters.

hypermedia Interactive multimedia, that is, a computer application in which the user can steer his way through presentations consisting of hypertext, still images, video and sound.

hypertext A form of interactive text display used in computer applications and often forming part of on-line help and hypermedia systems. Unlike a plain text file through which the user must navigate using scroll bars or other means to control the portion of the document displayed, hypertext uses an active cross-reference system. Certain words in the text are highlighted (often using a different colour). When the user moves the pointer over one of these and clicks the mouse button, the screen displays an entry giving further information on that topic. Facilities are usually provided to allow the user to retrace his steps and to jump to an interactive index to the whole document.

hysteresis In general a relationship between a force and its effect in which the effect lags on the force so that the magnitude of the effect depends not only on the present value of the force but also on its previous value.

    As a result the magnitude of the effect for a given value of force depends on whether the force is increasing or decreasing and any particular value of effect can be produced by two values of force. If the force is varied cyclically the relationship between force and effect, if shown graphically, is a closed loop, the most familiar example of which is the B–H curve for a magnetic material.

hysteresis loop Of a magnetic material, the closed figure obtained by plotting the magnetic flux density B against the magnetising force H as the magnetising force is varied throughout a complete cycle. An example of this closed loop is shown in Figure H.9.

hysteresis losses In a magnetic material the energy loss whenever varying flux is produced in the material. The loss is proportional to the area of the hysteresis loop.