120 THE ELECTRIC ARC 



alternating-current arc in hydrogen. 1 For example, Upson 

 found that he could not maintain such an arc between 

 carbon terminals with a length of more than 0.03 in., the 

 E.M.F. being 100 volts and the frequency 80 per second. 

 Under similar conditions in coal gas an arc longer than 

 0.02 in. could not be maintained. 



Oscillographs showing the current and voltage with 

 alternating-current arcs in coal gas have been obtained by 

 Morris. 2 These show that the voltage rose to 370 volts 

 before the current commenced. 



Mercury Arc Rectifier. As has been stated it is diffi- 

 cult to maintain an alternating-current arc in a vacuum, 

 even when the electrodes are carbon. With any other 

 electrodes it appears to be quite impossible to do so. The 

 difficulty appears to be due to the fact that the terminal 

 which is the anode during one half period does not become 

 hot enough during that time to become the cathode when 

 the E.M.F. is reversed, for a terminal must be very hot 

 in order to become the cathode of an arc. 



On the other hand a terminal can easily become the 

 anode, even when it is cold. Use has been made of these 

 two facts in devising an apparatus for rectifying alternating 

 currents. The first device of this sort was made by Cooper 

 Hewitt and called by him the static converter. This form 

 can be used only with a three-phase current. It is shown 

 diagrammatically in Fig. 37. The rectifier consists of a 

 tube, T, with one mercury electrode at the bottom and 

 three iron electrodes above. The iron electrodes are con- 

 nected to the three circuits of a star-connected three-phase 



1 Child, Phys. Rev., 20, 374; 1905. 



Upson, Lond. Elec., 60, 58; 1907. Phil. Mag., (5), 14, 141; 1907. 



2 Lond. Elec., 59, 707; 1907. 



