784 On the Alternating- Current Carbon Arc. 



The wastage is increased by the fact that during the second 

 half-period, when the same pole becomes the cathode, it is 

 hotter than it would have been in a direct- current arc. 



Successive contours o£ a very short alternating-current arc 

 were drawn by projecting an image upon transparent paper and 

 tracing the outlines at short intervals of time, the arc length 

 being kept constant throughout. There is a very marked 

 difference between the results and those obtained with the 

 direct-current arc already referred to ; here there is no cycle 

 of changes, but a contour of practically unvarying form. No 

 constriction develops in the rear of the pole face when the 

 arc is short, though a tendency to form one has been observed 

 in the case of longer alternating-current arcs. No deposition 

 of material takes place upon either pole as it does in the 

 direct-current arc; or, if deposition occurs in one half-period, 

 a still larger quantity is consumed in the second half-period. 



Three further points may be briefly mentioned :— We 

 noticed that the upper and lower carbons were not equally 

 consumed ; for short arcs the lower, and for long arcs the 

 upper pole lost more rapidly, the value of the current not 

 affecting the ratio seriously: the ratio of upper todower pole 

 loss varied in almost linear manner from 0"8 for the shortest 

 to 1*9 for the longest arc used, 15 millimetres. This is, no 

 doubt, due to convection currents, which have greater 

 freedom of access to the hot under-surface of the upper 

 carbon in a long arc. 



Notes are made when the arc was hissing, and such obser- 

 vations have been distinguished on the first diagram, from 

 which it appears possible to divide it into two regions, one cha- 

 racterized by hissing and the other by silence. Mrs. Ayrton 

 found that hissing seemed to depend upon the access of air 

 to the crater of the direct-current arc ; if more than a certain 

 amount reached it, hissing ensued on account of an automatic 

 protective mechanism whieh seemed designed to oppose the 

 access of too much oxygen. Diagram 1 seemed at first sight 

 to offer evidence in opposition to this, since silent arcs are 

 associated with high rates of carbon consumption per coulomb; 

 but, as we have stated elsewhere, the loss per coulomb is 

 governed more by the time taken for the coulomb to pass 

 between the hot poles than by the strength of the current 

 (since it obviously takes longer with small than with large 

 currents) ; hence the objection is not serious. If we draw 

 out curves representing the total loss in one second against 

 current strength for arcs of different lengths, we find that 

 hissing arcs are, in accordance with Mrs. Ayrton's view, 

 associated with high carbon consumption. 



