SOURCES OF ELECTRICITY. 
201 
be broken into pieces of the size of peas, and of this about SO 
grammes is put into each trough and distributed equally 
between the earthenware blocks. A short circuit is then 
immediately formed for each current, and in 10 or 15 minutes 
the battery is ready to act with all its power. 
It is necessary from time to time to add copper sulphate in 
equal quantities to each trough, as when the battery is first 
charged ; but the fresh sulphate of copper should not be added 
until the former supply has been completely used up. From a 
point below the extreme level of the sulphate of zinc, enough 
liquid must from time to time be drawn off by the siphon to 
lower the level about £ inch, and 
this liquid is then replaced by add¬ 
ing water to the height of the zinc 
grid. 
The internal resistance of these 
cells is very small, whilst their 
electro-motive force is considerable. 
It will be understood that the large 
consumption of copper sulphate 
must be kept constant, if a per¬ 
manent effect is to be attained. 
M. Callaud, in his treatise on 
batteries, thus describes the one 
that is known by his name :— 
“ The inventor's object was to 
obviate the necessity of using a 
porous vessel in Daniell’s battery. This was accomplished by 
the agents used by Daniell himself; the difference of the 
densities of these liquids is greater than with any others used 
in batteries. The invention is one in which its author found 
more than he sought for; an electrical opposition is established 
between the two liquids, which keeps them separate in their 
super-position, and this circumstance ensures a long and good 
working in cells so charged. It is, in fact, a new departure in 
science." 
The typical form of this battery is represented in fig. 138. 
The outer vessel has the usual dimensions of those used in 
batteries for telegraph purposes. It is perforated with two 
holes, through which pass supports; the upper one ha 7 the 
Fig. 138. 
