reference will now be made to the drawings to describe in detail embodiments of the present ionization vacuum gauge . fig1 and 2 are schematic axial and radial cross - sectional views , respectively , showing the present ionization vacuum gauge 100 . the ionization vacuum gauge 100 includes a linear cathode 102 , an anode 104 , and an ion collector 106 . the linear cathode 102 , the anode 104 , and the ion collector 106 are coaxial , and the arrangement thereof is in the order from center to outer . the linear cathode 102 , the anode 104 , and the ion collector 106 are spaced from one another , in such a manner so as to not be in direct electrical contact with one another . a radial space between the linear cathode 102 and the anode 104 ( referred as r ) is , beneficially , from about 1 millimeter ( mm ) to about 8 mm and , rather usefully , about 1 ˜ 2 mm . a radial space between the linear cathode 102 and the ion collector 106 ( referred to as r ) is from about 10 mm to about 15 mm , advantageously , about 12 mm . the ionization vacuum gauge 100 further includes an enclosure 120 and three leads 122 . the enclosure 120 is part of a vacuum system in which the linear cathode 102 , the anode 104 and the ion collector 106 are disposed . the enclosure 120 is fluidly connected to a chamber ( not shown ) whose pressure is measured . the first ends of the leads 122 are electrically connected to the linear cathode 102 , the anode 104 , and the ion collector 106 , respectively , and the second ends of the leads 122 are extended out of the enclosure 120 . it is to be understood that the vacuum system incorporates one or more known evacuation mechanisms ( not shown ), as needed to achieve the desired level of vacuum . the anode 104 and the ion collector 106 are made of an oxidation - resistant , conducting metal , such as aluminum ( al ), copper ( cu ), tungsten ( w ), or alloy thereof . the anode 104 has an apertured structure , such as a metallic ring , a metal - enclosed aperture , or a metallic net . the ion collector 106 has an apertured and / or plane structure , such as a metallic ring , a metal - enclosed aperture , a metallic net , or a metallic sheet . the linear cathode 102 includes a linear base 108 and a field emission film 110 coated thereon . the linear base 108 is an electric conductive thread , such as an oxidation - resistant metal thread made , e . g ., of nickel ( ni ), tungsten , or copper . a diameter of the linear base 108 is about from 0 . 2 mm to 2 mm , advantageously , about 0 . 3 mm . the field emission film 110 is , usefully , composed , initially , of carbon nanotubes ( cnts ), low - melting - point glass powders , conductive particles , and a an organic carrier / binder . the mass percents of the foregoing ingredients are respectively : about 5 %˜ 15 % of cnts , about 10 %˜ 20 % of conductive particles , about 5 % of low - melting - point glass powders , and about 60 %˜ 80 % of an organic carrier / binder , this latter component being evaporated and / or burned off in a drying step , leaving the other three ingredients in the final film composition . cnts can be obtained by a conventional method such as chemical vapor deposition , arc discharging , or laser ablation . rather suitably , cnts are obtained by chemical vapor depositon . a length of cnts is , advantageously , about from 5 microns ( μm ) to 15 μm , because cnts less than 5 μm tend to be weak electron emitters , and cnts more than 15 μm are often easily broken . the an organic carrier / binder is composed of terpineol , acting as a solvent ; dibutyl phthalate , acting as a plasticizer ; and ethyl cellulose , acting as a stabilizer . the low - melting - point glass melts at an approximate temperature from 400 ° c . to 500 ° c . the function of the low - melting - point glass is to attach cnts firmly to the linear base 108 , in order to avoid cnts becoming dislodged / unbonded from the linear base 108 . the conductive particles can , usefully , be silver and / or indium tin oxide ( ito ). the conductive particles help ensure , to at least a certain degree , that the cnts are electrically connected to the linear base 108 . a process for forming such an the linear cathode 102 is illustrated as per the following steps : step 1 , providing and uniformly mixing carbon nanotubes ( cnts ), low - melting - point glass powders , conductive particles , and an organic carrier / binder in a certain ratio to form a composite slurry ; step 2 , coating the composite slurry on the outer surface of the linear base 108 ; and step 3 , drying and sintering the composite slurry to form the field emission film 110 on the linear base 108 . in step 2 , the composite slurry is beneficially provided onto the linear base 108 by a silk - screen printing process . in step 3 , drying the composite slurry is performed to remove ( e . g ., evaporate and / or burn off ) the an organic carrier / binder , and sintering the composite slurry is to melt the low - melting - point glass powders for firmly attaching the cnts to the linear base 108 . after step 3 , the field emission film 110 can , opportunely , further be scrubbed with rubber to expose end portions of cnts , thus enhancing the electron emission capability thereof . in another alternative , the field emission film 110 can be made essentially of cnts . in this alternative , cnts are deposited on the linear base 108 by a conventional method , i . e ., cnts are formed directly on the linear base 108 . in operation of the ionization vacuum gauge 100 , an electric voltage is applied between the linear cathode 102 and the anode 104 to cause electron emission . after emitting , electrons are drawn and accelerated toward the anode 104 by the electric potential , then tending to pass through the anode 104 because of the inertia of the electrons and because of the apertured structure thereof . the ion collector 106 is supplied with a negative electric potential and thus decelerates the electrons . therefore , before arriving at the ion collector 106 , electrons are drawn back to the anode 104 , and an electric current ( i electron ) is formed . in the travel , electrons collide with gas molecules , ionize some of gas molecules , and produce ions . typically , the ions are in the form of positive ions and are collected by the ion collector 106 , and , thus , an ion current ( i ion ) is formed . a ratio of i ion to i electron is proportional to a pressure in the ionization vacuum gauge 100 , within a certain pressure range , covering the primary range of interest for most vacuum devices . therefore , the pressure in the ionization vacuum gauge 100 and , by extension , the vacuum device ( not shown ), to which it is fluidly attached , can be measured according to the above relation . referring to fig3 , the ionization vacuum gauge 100 , according to the present invention , 25 volts of electric potential is supplied to the ion collector , 750 volts of electric potential is supplied to the anode , and ground potential is supplied to the linear cathode . the ratio of i ion to i electron is perfectly proportional to the pressure in the range from 10 − 7 torr to 10 − 3 torr , as can be seen in fig3 . the present ionization vacuum gauge , employing a cathode having cnts thereon , can be used instead of many kinds of the ionization vacuum gauges that use a hot - filament cathode , without dissipating heat and / or light . further , the ionization vacuum gauge can be widely used in many fields ( e . g ., cases where a high degree of vacuum is necessary ) in which use of the hot - filament cathode is not appropriate . finally , it is to be understood that the above - described embodiments are intended to illustrate rather than limit the invention . variations may be made to the embodiments without departing from the spirit of the invention as claimed . the above - described embodiments illustrate the scope of the invention but do not restrict the scope of the invention .