SOURCES OF ELECTRICITY. 
199 
use at sea a battery not liable to polarization effects will be 
easily understood. 
Another form of Minotto’s battery consists of vessels made 
of sheet copper (fig. 135), at the bottom of which is placed 
about 6 lb. of sulphate of copper. This is covered with damp 
fir sawdust, above which rests the zinc plate pierced with a 
few holes. This battery is so simple that the most un¬ 
practised operator can put it together. It continues in 
working order for from 3 to 12 months, without being inter¬ 
fered with, otherwise than replacing from time to time the 
Fig. 135. Fig. 136. 
water lost by evaporation. Its internal resistance is com¬ 
paratively small, and it is very useful for short circuits where 
rather strong and continuous currents are needed. 
Sir W. Thomson, in the cells he uses for working the mill 
and charging the bobbins of his siphon recorder , has yet 
further diminished the internal resistance of the Daniell 
battery and increased its power. These batteries are made of 
troughs about 16 inches square at the base and widened at the 
top ; they are lined inside with lead, and contain zinc grids 
(figs. 136 and 137) resting on blocks of glazed earthenware. 
A plate of copper is soldered on the outer edge of each trough, 
to serve when necessary as an electrode. To facilitate^ the 
removal of the deposits of copper, a plate of that metal, 
