SOURCES OF ELECTRICITY. 
203 
The form of Callaud cell used in America differs materially 
from the French type. The vessels are 9 inches high and 7 
inches in diameter. The zinc is cast into the shape of a four¬ 
armed wheel without teeth, and is supported by a rod in the 
centre (fig. 141). 
We have now to examine the other hydro-electric batteries 
for telegraphic purposes that differ from those of the Daniell 
type. Of Grove’s battery, used more particularly in England 
and in Germany, it will be sufficient to state that the form 
Fig. 139. Fig. 140 
known in telegraphy is that invented by Poggendorff. The 
porous vessel is cylindrical, the platinum that forms the con¬ 
ducting electrode is bent into the s shape, in order to afford a 
larger surface. This platinum passes through a porcelain 
stopper that closes the porous vessel. But in Germany 
Bunsen’s battery is preferred to Grove’s. 
The idea of substituting retort coke or carbon for platinum 
appears to be due to Grove, who made some trials of carbon in 
London. He abandoned this plan, which was again taken up 
by Bunsen in 1848. The typical form of this battery is shown 
in fig. 142. v is the external vessel of brown glazed stone¬ 
ware. The zinc z is made of a sheet } inch thick. 
