Cady and Arnold — Electric Arc. 387 



appears. At the same time the mantle and shell surrounding 

 the arc increase greatly in brightness. 



This sudden change is represented in fig. 5 by the broken 

 line AB. Between the points A and B no readings can be 

 taken. As the current is still further increased, the portion 

 BJV on the second stage is obtained. This is, of course, the 

 stage on which the iron arc is usually operated for spectro- 

 scopic or other purposes, and it constitutes the final state of 

 the arc. 



The energy expended at the beginning of the second stage 

 is always less than that at the end of the first stage, owing to 

 the great decrease in voltage, together with the fact that the 

 increase of current is limited by the external resistance. 



The photographs, fig. 1, give a fair idea of the appearance 

 of the arc, though it was impossible to reproduce the sur- 

 rounding mantle. When the arc is viewed directly through 

 dark glasses, the change in appearance between the two stages 

 is much more striking than in the photograph. The bright 

 spot seen on the front surface of the positive globule in each 

 photograph is the reflection of the light from the arc. 



If one begins taking observations on the second, stage and 

 decreases the current, it is usually found that the change back 

 to the first stage does not take place along the line BA. but 

 that a smaller value of current can be reached. Thus in the 

 observations shown in fig. 5, as the external resistance was 

 gradually increased, the break took place along the line CD. 

 The value of the current for the critical points A and C is 

 subject to variation, but in general it may be said that the 

 longer the arc, the wider is the parallelogram. In the obser- 

 vations shown in fig. 5, no difference was observed between 

 the lines corresponding to AB and CD for lengths of l ram 

 and 2 mm , though even here on other occasions wide dif- 

 ferences have been found. Thus at least for long arcs there 

 may exist two values of voltage for the same current. As 

 would be expected, the lines BA and CD cut the axis of ordi- 

 nates at a point corresponding to the supply voltage. These 

 characteristic curves are discussed more fully in §30. 



§5. The first step was to see how far these two stages are 

 analogous to the quiet and hissing states of the carbon arc. 

 The experiments described by Mrs. Ayrton* were repeated, 

 with the following results : When the arc is on the second 

 stage, the first stage can be precipitated by (1) decreasing the 

 current, (2) drawing the electrodes farther apart, or (3) length- 

 ening the arc by an air-blast or by a magnet. Also, as with 

 the carbon arc, the largest current that will maintain the 

 first stage is greater the longer the arc. 



*The Electric Arc, p. 279. 



