THE RELIGHTING OF THE CARBON ARC. 219 
were then compared with that on the same plates of a style 
attached to the prong of a standardised tuning fork. A 
third key, also worked by the pendulum, enabled the battery 
connections to be reversed in the interval between the 
break and the make of the circuit, if desired. 
Fig. 2. 
Scheme of connections. 
A scheme of the connections is shown in figure 2, where 
A isan ammeter, V a voltmeter, Ra 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 corresponds 
to a time interval of about ‘002 seconds. The time interval 
corresponding to the maximum distance may be called the 
critical time for relighting under the given conditions. 
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 
1 Mrs. Ayrtoa, ‘‘ The Electric Arc,” p. 104. 
