200 
THE TELEGRAPH. 
varnished on the under surface, except at the corners where it 
is in electrical contact with the lead, rests on the bottom of 
each trough. An earthenware block rests on each corner of 
the copper plate to support the zinc grid. The latter is wrapt 
in parchment paper, carefully fastened up with sealing-wax. 
When this paper has been moistened it acts as a porous 
membrane, and retains 
uniformly the whole of 
the zinc grid which in 
time is worn away. A 
wooden framework is 
provided to hold the 
troughs, and this rests 
on four porcelain insu¬ 
lators. The whole is 
carefully levelled, and is 
so constructed that it 
can easily be turned 
round. 
At the be ttom of the 
first trough the copper 
plate is placed, having 
metallic contact with 
the lead lining at the 
corners and in the 
centre ; above the cor¬ 
ners are the four por¬ 
celain blocks of a 
cubical form, and upon 
these re^ts the zinc 
Fig. 137. grid. A solution of 
sulphate of zinc (specific 
gravity = IT) is then poured into the trough, first wetting the 
zinc and its parchment covering. The four upper corners of 
the zinc and the four lower corners of the lead of the next 
trough having then been ascertained to be clean and dry, 
trough No. 2 is placed upon zinc No. 1, and the same series of 
operations is begun for No. 2 trough as for No. 1, and so on 
in succession until the battery is complete. 
The crystals of sulphate of copper used in this battery must 
