2o6 
THE TELEGRAPH 
slate into a number of separate cells, of which in a 2 feet 
trough there are usually ten. The whole of the interior, 
including the slate partitions, is coated with a composition of 
India-rubber and shellac, so that there may be no leakage or 
any liquid communication between the cells. Each cell is then 
subdivided by a plate of porous porcelain into two parts. A 
slip of copper hanging oyer the top of each slate partition 
connects a sheet of copper 3 or 4 inches square, with a thicker 
piece or slab of zinc ; the last copper at one end of the trough is 
connected with the binding-screw, and the last zinc at the other 
end w T ith a similar binding-screw. The subdivision of each 
cell which contains the copper plate is filled with solution of 
sulphate of copper, and crystals of that substance also 
occupy the lower part of the division. The zinc cells are filled 
with very dilute acid, or with a saline solution, and the electro¬ 
chemical action takes place only when the poles are con¬ 
nected through the line wire, in the manner presently to be 
pointed out. The advantages claimed for this form of 
apparatus are simplicity of construction and immunity from 
the risk of accidental injury in use or in transport. 
We shall not dwell further on this subject. The batteries 
we have just described are those particularly used in tele- 
grapliy. 
We have purposely omitted the Marie-Davy, which has gone 
completely out of use, because the mercurial salts by which it 
works are dangerous to handle. Thermo-electric piles are 
little used in telegraphy. 
We have just seen that a voltaic battery may be made in 
many different ways, but in all cases a combination of three 
things is required, of which one at least must be a liquid ; the 
others are generally metals. When the combination is well 
chosen, the force to which it gives rise, or what is usually 
called the electro-motive, remains latent or manifests itself, 
according as the connection of the three parts of the combi¬ 
nation is interrupted or complete. This will more easily be 
understood if we suppose the three elements of the combi¬ 
nation to form a triangle, the base of which represents the 
acidulated water, and the sides the two metals. So long 
as the triangle is entire without a break, the electro-motive 
force shows itself in circulation, in a determined direction 
