98 



PEACTICAL PHOTO-MICROGKAPHY 



consequently eliminated as a second and harmful source of 



With each current that is used that is, with any given 

 number of amperes there is a critical length of arc at which 

 the maximum light is obtained, and the greatest efficiency and 

 constancy secured. With a current of, say, ten amperes, 

 the best length of arc will be found to be from three to 

 four millimetres ; but with any hand-feed method of adjust- 

 ment, this can easily be determined by experiment. The 

 arc should not be so short that hissing occurs, and 

 should not be so long that there is any variation in the 



a b c 



FIG. 30. Appearance of Electric-arc Carbons under Different Conditions. 



position of the luminous crater on the extremity of the positive 

 carbon. 



The sizes of carbons that should be used may be taken to 

 be approximately as follows : 



Current in -f (Cored) 



amperes. mm. 



57 

 10 12 



(Solid) 

 mm. 



15 14 9 



Observation of the form of the negative carbon is very 

 useful in determining whether the lamp is burning properly 

 and with a normal current supply. Under proper conditions 

 it should have a regular cone-shaped head pointing towards 

 the crater on the positive carbon (Fig. 30 a). If the current is 



