TRAIN- SIGNALLING IN THEORY AND PRACTICE. 
277 
ready to receive electricity. One mode of allowing these ten- 
dencies to operate — of causing the plates to discharge the 
electricity — is to touch one shp with the other^ or to solder a 
wire to each slip, and to twist the two ends of the wire 
together. Electricity will pass, and continue to pass, from one 
plate to the other along the wires, as long as the wires are in 
contact, — the copper plate giving off and the zinc plate re- 
ceiving electricity by the intervention of the conducting wire ; 
and at the same time, within the mass of acid water, the zinc 
plate is giving off and the copper plate receiving electricity 
hj the intervention of the conducting liquid. This condition 
of things, which is called the 
Voltaic circuity is shown in fig. 
1, where z and G are the zinc 
and the graphite plates, and 
the arrows the direction of the 
current ; and this cu’culation of 
electricity will go on, but with 
decreasing energy, until the 
ptctive property of the liquid is 
exhausted. The zinc will then 
be found to be more or less 
corroded, and the contents of 
the jar will be solution of sul- 
lohate of zinc, instead of dilute 
sulphuric acid, as in the outset. 
The graphite will beunchanged ; 
but dm’ing the operation small 
bubbles, which are hydrogen 
gas, will have appeared abundantly at its surface, and 
have risen into the air. No electricity is produced or made 
by this process ; it is merely disturbed; none of the mate- 
rials employed are lost; they merely enter into new com- 
binations. The water employed is a compound of oxygen 
and hydrogen gas : the latter, as we have said, rises and 
escapes into the air ; the former combines with zinc, and pro- 
duces oxide of zinc, which combines with the acid and forms the 
sulphate of zinc that remains in solution; and the electricity 
during these operations moves in a definite direction, in order 
again to attain to a state of equilibrium. The mercury also 
is unchanged ; it discharges the important office of protecting 
the zinc from being attacked by the acid (which would other- 
wise be the case), when the apparatus is not in use for develop- 
ing electricity. But an absolute circuit is not indispensable to 
a current of electricity ; it is not essential that the discharge 
of the zinc plate should take place by the wire into the com- 
panion copper plate : in fact, it does not follow that one end 
u 2 
Fig. 1. 
