96 PEACTICAL PHOTO-MICROGRAPHY 



The production of an electric arc is dependent upon the fact 

 that if an electric current of suitable voltage is allowed to pass 

 between two carbon points which are slightly separated, one 

 or both of these points becomes a highly incandescent source 

 of light. 



There are two methods of producing and maintaining an 

 arc light now in general use ; one, in which some automatic 

 method of regulating the distance between the carbon points 

 is used, and the other which depends on a simple arrangement 

 whereby they are adjusted by hand. 



Let us for a moment consider the essential conditions for 

 producing an arc light with continuous current. Most sources 

 of supply of electrical energy are now such that the current is 

 supplied at from 100 to 250 volts. It is, perhaps, unfortunate 

 that an electric arc requires only from 40 to 50 volts ; so that 

 some method of absorbing the difference between the voltage 

 of the supply and the potential difference required between 

 the carbons must be adopted. This is usually accomplished 

 by putting in series with the lamp, of whatever form it may 

 be, a resistance that is to say, an arrangement in which some 

 of the electrical energy is expended in producing heat. The 

 term * in series ' means that the lamp and resistance-coil are 

 arranged so that the current passes successively through each. 



When it is required to start an arc it is essential that the 

 two carbons should momentarily touch. Be the distance 

 between the carbons ever so small, a current of electricity of 

 low voltage will not pass between them unless this preliminary 

 contact takes place. On making contact, some small portion 

 of the extremity of the carbons is volatilised, so that on parting 

 them again, the volatilised carbon acts as a conductor, and 

 the electricity passes across the gap ; in so doing, it raises both 

 points of the carbons to a condition of incandescence. 



It is almost universally the practice in arc lamps to place 

 the positive carbon that is, the one that is connected up to the 

 positive wire coming from the electrical supply in the upper- 

 most position, the negative carbon being placed in the lower 

 holder of the lamp. When an arc is started, it does not at 

 first burn steadily, nor is the source of light a constant or 

 regular one. This is due to the fact that, until what is known 

 as a * crater ' is formed on the tip of the positive carbon, the 



