452 Messrs. Pollock, Wellisch, and Ranclaud on 



were arrauged to open and close the circuits of two electro- 

 magnetic scribers which marked a smoked plate fixed to the 

 pendulum ; the records for various distances between the 

 switch-levers were then compared with that on the same 

 plates of a style attached to the prong of a standardized 

 tuning-fork. A third key, also worked by the pendulum, 

 enabled the battery connexions to be reversed in the interval 

 between the break and the make of the circuit if desired. 



A scheme of the connexions is shown in fig. 2 (PL IX.), 

 where A is an ammeter, V a voltmeter, R a variable resistance, 

 B the arc, and X and Y the two switches. 



An observation consisted in finding, for a given potential- 

 difference between the carbons at the instant of the reclosing 

 of the circuit, the greatest distance between the switch-levers 

 for which the arc would relight. This maximum distance 

 could be determined to within two millimetres,, which cor- 

 responds to a time interval of about *002 second. The time 

 interval corresponding to the maximum distance may be 

 called the critical time for relighting under the given con- 

 ditions. This time, under otherwise fixed circumstances, 

 varies considerably with the carbons used, and the results 

 are only directly comparable when they refer to the one pair 

 of carbons. 



The observations were made in all cases with " normal " 

 arcs*. The lengths of the arc were measured, on images of 

 the carbons, vertically from the point of the negative to a 

 horizontal line passing through the edge of the crater, the 

 values obtained being reduced according to the magnification 

 of the image. 



3. Relighting with Carbons in Normal Position 

 when Potentials Reversed. 



When the connexions from the battery are reversed during 

 the interval between the opening and closing of the circuit, 

 so that at the remake of the circuit the still existing crater 

 becomes negative to the previous cathode, the crater being 

 on the upper carbon, the phenomena are simpler than in 

 other cases, and will therefore be the first described. 



In fig. 3 (PI. IX.) is shown the relation between the minimum 

 potential-difference for relighting and the interval between 

 the break and the make of the circuit, under the condition 

 of the reversal of the potentials of the carbons, for a previous 

 current of 10 amperes. Three curves are drawn, from ob- 

 servations with the same pair of carbons, for arc-lengths of 



* Mrs. Ayrton, 'The Electric Arc,' p. 104. 



