SCIENTIEIC SUMMAEY. 
139 
■witii an air chamber, a, after the manner of Tilley’s blow-pipe ; from the 
upper part of this air chamber passes a small pipe terminated by a flex- 
ible tube of vulcanized rubber, that is carried under a wedged shaped 
piece attached to the end of a rod that carries the lower carbon ; a portion of 
this rod is constructed of soft iron, which part passes through the centre of 
a hollow coil of stout insulated copper wire. The action is as follows : when 
the wires from a battery are connected with the respective poles of the regu- 
lator, and the carbons are brought into contact, the current passes through 
the arrangement and the coil becoming magnetic, pulls down the iron core, 
separates the carbons so as to produce the proper arc of light, and at the 
same time forces the wedge down upon the flexible tube and closes it as 
efiectually as a stopcock, so that no air can escape from the air chamber. As 
soon as the distance between the poles becomes too great for the current to 
pass freely, the coil ceases to be magnetic and the lower rod is raised slightly 
by the action of a lever and counterpoise spring ; immediately air is forced 
from the chamber; a, by the action of the column of water in w, the float, f, 
sinks and consequently brings the upper carbon down in contact with the 
lower one ; instantly the circuit is again completed, the coil becoming magnetic 
pulls down the iron cone, presses the stopcock wedge upon the rubber tube, no 
more air escapes and the upper pole is arrested in its descent, and remains 
