GENERATION, CONTROL, AND MEASUREMENT 



165 



efficient emitters and may be operated at relatively low temperatures. 

 The oxide-coated cathode is used chiefly in the low- to medium-pressure 

 discharges, and the more rugged thoriated-tungsten and pure-tungsten 

 cathodes are employed in the high-pressure arcs. 



The low-pressure discharge tubes, such as the fluorescent lamps, are 

 commercially available with both cold and hot cathodes; the latter are 

 manufactured as preheat and instant-start types. In the preheat cathode 

 a current is passed through a filament for a sufficient time to bring it to 

 an efficient emitting temperature before the discharge is initiated. This 



1- 



o 

 a 



/ 



/ 



OHMIC 



RESIST 



METAL 



-TUNGSTEN 



/^^RES,S>ORS, / ^LAMENT 



^ ..^^». / LAMPS 



I /medium-pressure I / 



00 W ZIRCONIUM 



/ 

 / 



/ 



/-- -r- 



/ I 100 W ZIRCONIUM 

 / 



/ iJungsten) 



r'ARC ^ 



I high-pressure _j 

 Hg AND xe lamps'^ 



CARBON ARC LAMPS 



-H 



CURRENT, amp 



Fig. 3-10. The volt-ampere characteristics ut" arcs and uhmic resistors. 



is usually accomplished l)y some type of automatic switch or starter which 

 shorts the high voltage of the ballast through the cathodes for a few 

 seconds. After the cathodes are heated, the switch opens, producing a 

 high-voltage impulse which initiates the discharge. The instant-start 

 lamp is powered by a ballast supplying a voltage sufficient to start the 

 discharge with cold cathodes, which then quickly attain the normal emit- 

 ting temperature. The instant-start lamp requires no complex starting 

 mechanism, and there is no flickering after the lamp is switched on. 



Electrical Characteristics. The electrical discharge sources have a nega- 

 tive electrical resistance characteristic. Ohmic resistors, such as those of 

 metal, have a positive resistance characteristic; i.e., the current is pro- 

 portional to the voltage, according to Ohm's law, and the plot of current 

 as a function of voltage has a positive slope (Fig. 3-10). For a negative 

 resistance the current decreases with increasing voltage, and the charac- 

 teristic curve has a negative slope. When an ionic conductor such as a 

 discharge lamp, with a negative resistance characteristic, is applied to a 



