CH. XV] 



EMISSION SPECTRA 



639 



and it is desirable to have an auto- 

 matic machine to relight the arc 

 again instantly. A very conven- 

 ient device for this purpose is an 

 enclosed arc lamp mechanism for 

 shunt circuits. The wiring of the 

 arc will need to be slightly modi- 

 fied to adapt it to the heavy 

 currents (15 amps.) required. 

 This is done by connecting the 

 wires to the rheostat of the lamp 

 at A, B, C and D (fig. 367) and by 

 putting a germ an silver wire be- 

 tween E and F of the "series 

 magnet" so that this magnet will 

 not overheat and at the same time 

 will not lift the upper carbon 

 too suddenly. 



905. Current to use. For 

 the projection of arc spectra the 

 current to use will depend upon 

 the substance in the arc. When 

 treated carbon electrodes are used, 

 either alternating or direct cur- 



FIG. 367. AUTOMATIC ARC FOR THE 

 PROJECTION OF ARC SPECTRA. 



The mechanism is that of an enclosed 

 shunt, direct current arc. In order to 

 get sufficient current the wire is connec- 

 ted to the resistor in the two points A 

 and B. The wire to the lifting magnet 

 is connected at the points D and E. 

 This gives three times the current for 

 which the arc was designed, i. e., about 

 1 5 to 1 8 amperes. The lifting solenoid 

 E F, must be shunted by a suitable 

 resistance easily found by experiment. 

 The clutch automatically lifts the upper 

 carbon whenever current is flowing. 

 The lower carbon is stuffed with salts 

 and connected to the positive wire. 



