为了保证数据库的完整性(正确性)，数据库系统必须维护事务的以下特性(51)。

(71)

(69)

(68)

(70)

(66)

Ink-jet　printer　belongs　to　nonimpact　printers　in　which　the　characters　are　formed　by　projecting　particles　or　droplets　of　ink　on　to　paper.(71) printer　is　also　a　nonimpact　printer　in　which　the　characters　are　produced　by　applying　hot　elements　to　heat-sensitive　paper　directly　or　by　melting　ink　from　a(72) on　to　plain　paper.　Electrostatic　printer　creates　an　electrostatic　latent　image,　which　is　then　made　visible　by　a　toner　and　transferred　and　fixed　on　paper.　An　electrostatic　printer　may　be　used　in　some　instances　as　a(73).　Laser　printer　creates,　by　means　of　a　laser　beam　directed　on　a(74) surface,　a　latent　image　which　is　then　made　visible　by　a　toner　and　transferred　and　fixed　on　paper.　Magnetographic　printer　creates,　by　means　of　magnetic　heads　operating　on　a　metallic　drum,　a　latent　image　which　is　then　made　visible　by　a　toner　and　transferred　and　fixed　on　paper.　Computer-output-microfilm (COM) printer　produces　on　a　photographic　film　a(75) of　each　page.

(64)

(65)

(63)

An　analog　signal's　characteristic　quantity　representing　data　may(66).　at　any　instant,　any　value　within　a　continuous(67).　For　example,　an　analog　signal　may　follow　continuously　the　values　of　another　physical　quantity　representing　data.A　discrete　signal　is　composed　of(68)　elements　in　time,　each　element　having　one　or　more　characteristic　quantities　which　can　represent　data.　Examples　of　characteristic　quantities　are　amplitude,　waveform,(69),　and　position　in　time.　A　digital　signal　is　a　special　case　of　discrete　signals,　in　which　data　are　represented　by　a(70) number　of　well-defined　discrete　values　that　one　of　its　characteristic　quantities　may　take　in　time.