340 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



arc lamp in which the carbon rods are impregnated with metallic 

 salts, the vapors of which give an intensely luminous arc. An- 

 other form of arc lamp in which the arc itself is intensely luminous 

 is the magnetite arc lamp in which the arc is formed between a 

 rod of compressed titanium carbide and iron oxide (the cathode) 

 and a rod of copper (the anode). The result is the vaporization 

 of the iron oxide and the production of an intensely luminous 

 arc. 



40. The electrolytic interrupter (Wehnelt). The primary cir- 

 cuit of an induction coil is usually interrupted by a vibrating reed 

 or spring which makes and breaks contact between two platinum 

 points. Wehnelt discovered that the sudden generation of 

 oxygen on a small platinum anode in dilute sulphuric acid causes 

 an abrupt stoppage of the electric current. This effect is utilized 

 in the electrolytic interrupter as follows : A glass jar CC, Fig. 

 42, is filled with dilute sulphuric acid and provided with two 



ELEOTRIO MAIN 



Fig. 42. 



electrodes p and /. The anode p is a tip of platinum wire 

 projecting from a glass tube, and the cathode / is a large plate 

 of lead. The electromotive force between the mains, which must 

 be 30 volts or more, causes a sudden rush of current through the 

 cell CC and through the primary of an induction coil. This 

 rush of current generates a layer of oxygen over the platinum 



