SOUECES OF ILLUMINATION 



97 



light does not settle down and become constant. This crater 

 is really the source of light, and in microscopic work it 

 only is used, the negative carbon being so placed that the 

 light from it does not reach the microscope. 



The ends of both the carbons are generally tapered off, 

 and in course of burning the tips tend to keep this shape. The 

 crater on the positive carbon should be of such diameter that 

 it is well confined within the limits of the tapered point that 

 is, there should be no tendency for the white-hot portion of 

 the crater to go over the edge of the carbon. This will never 

 happen if the size of the carbons is correctly chosen, but 

 the larger the current which the . 

 lamp takes, the larger must be 

 the size of the carbons used. 

 When the current is too large 

 for the carbons, the arc begins 

 to hiss, and the luminous point 

 spreads and extends over the edge 

 of the crater. To decrease the 

 chance of the luminous point alter- 

 ing its proper position on the end of 

 the positive carbon, the latter is 

 always ' cored ' that is, it has a 

 hole through its centre which is 

 filled with softer carbon than the 

 outer portion. The result is that 

 this softer carbon vapourises rather 



more rapidly than the surrounding harder carbon, and the 

 crater therefore tends to remain in its proper position. 



The position of the carbons with a continuous-current arc 

 should be as shown (Fig. 29) that is, the positive carbon should 

 be set somewhat behind the negative one so that the crater 

 tends to form rather towards the front and facing the direc- 

 tion in which the light is required. The carbons should be 

 placed so that their front surfaces are approximately in a 

 straight line. They should also be inclined at an angle of about 

 27 to the vertical ; at this angle two essential conditions are 

 secured viz. the greatest amount of light is thrown forward from 

 the crater of the positive carbon, and practically the whole of 

 the light from the negative is thrown to the back, and 



FIG. 29. Correct Position of 

 Electric-arc Carbons. 



