£ 3 * 0 . 
SOURCES OF ELECTRICITY. 
*95 
uniform degree of saturation might be maintained in the 
liquids. After he had tried diaphragms of bladder, of leather, 
and of sail-cloth, Daniell fixed upon the porous vessels so well 
known to telegraphists. In Darnell’s form of the cell, the zinc 
was inside of the porous vessel, and the copper outside. Breguet 
reversed this arrangement, and definitely settled the best form 
of telegraph battery. This is formed of cells with two liquids, 
as shown in fig. 131, which represents the form that has been 
adopted by the French authorities. But this battery had some 
inconveniences, the removal of which has been attempted in 
certain modified forms where the 
porous diaphragm has been dis¬ 
pensed with altogether. 
In all these forms of DanielFs 
battery, the sulphate of copper at 
last attacks the ^inc, and thus the 
battery is impaired, and Daniell was 
perfectly justified in desiring to 
keep the form of battery he origin¬ 
ally devised, because it was free 
from this defect. Sir W. Thomson 
has contrived a Daniell’s battery 
designed to postpone this untoward 
result as long as possible. It is 
constructed as follows :— 
In each cell or element of the 
battery the plate of copper is 
placed horizontally at the bottom of a glass vessel, and a 
saturated solution of sulphate of zinc is poured over it. The 
zinc is shaped like a grating, and is placed horizontally near 
the surface of the solution. A glass tube (fig. 132) is placed 
vertically in the solution, with its lower end immediately above 
the surface of the copper. Crystals of sulphate of copper are 
put down this tube, and as they dissolve in the liquid they 
make a solution of a greater density than that of the sulphate 
of zinc alone, so that the copper solution can reach the zinc 
only by diffusion. To retard this effect, a siphon made of 
glass tube filled with a cotton wick, is placed with one end 
about half way between the zinc and the copper, while the 
other end, by the action of pressure and capillarity, allows the 
