Carbon Arc Lamps (A.C. carbon arc lamp)

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A.C. carbon arc lamp :

Working principle :
Two carbon rods kept face to face touching to each other connected to the source of electricity and if they are departed from each other to form a small air gap between them, an arc is produced with tremendous heat and a light is produced.

Construction and working is similar to D.C. type.
  • The difference is only instead of D.C. supply A.C. supply is connected and instead of a controlling resistance (R) a controlling reactor (L) is  connected as shown in the following figure. 
  • In its case burning of both positive and negative rod is same and hence their cross section area is same in A.C. operation.
  • Since the carbon is consumed during operation frequent adjustment for the gap between rods is required.
  • An automatic device is provided for feeding the carbon into the arc for big lamps.

Requirements of voltage :
         V. = (39 + 2.8 1) where l: length of arc in mm

  • A series resistance or reactor as shown in the figure is used for stabilizing the arc.
  • Overall is luminous efficiency very low 12 lumens/watt.
  • In A.C. arc lamp, instead of a series resistance, inductor is used for stability.
  • % production of light :

              85% of light by positive carbon
              10% of light by negative carbon
              5% of light by the air.

  • Temperature : 

              Of the carbon : 3500°C to 4000°C and that of negative carbon : about 2500°C.
Uses of Arc lamps :

  1. Cinema projectors
  2. Search lights

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