104 THE ELECTRIC ARC 



in an air-tight globe by other means. The length of the 

 column of luminous mercury vapor is only a few millimeters. 

 The color is bluish white, very similar to sunlight, and 

 it is said to require but 0.45 watt per candle. It operates 

 on a potential difference as low as 12 volts. The voltage 

 and candle power may be made greater by having an inert 

 gas in the tube. 



The lack of red in the mercury arc is occasionally reme- 

 died by combining the arc with a tungsten incandescent 

 lamp. It is usually necessary to have some resistance in 

 series with the arc. The tungsten lamp furnishes this 

 resistance and at the same time serves the useful purpose 

 of giving the lacking colors, thus making an efficient com- 

 bination. 



