SOURCE OF ELECTRICITY. 
209 
to the manner in which the cells are combined. The batteries, 
like conducting wires, have their own resistances, which may 
be increased and diminished by various arrangements. Let ns 
suppose, for instance, that we have six pairs of metals, and let 
fig. 145 represent various combinations of these, the thick lines 
being the zinc and the thin lines the copper. 
In A (fig. 145) the elements are arranged “ for tension,” and if 
E stand for the amount of the electro-motive force of each 
element, and r for the resistance, it is clear that the battery of 
six cells will have an electro-motive force of f> e, and a resist¬ 
ance of 6 R. But the cells may be combined for quantity,” 
as shown at b. The electro-motive force of this arrangement 
will still be 
e, but its resistance will only be 
The cells may also be joined in two’s for tension, and in 
three’s for quantity as at C; in this case the electro-motive 
force will be 2 e and the resistance f R. Again, the cells may 
be connected as at D, in three’s for tension and in two’s for 
quantity; the electro-motive force will be 3 e and the resist- 
3 R 
It will be observed, therefore, that with a sufficient number 
of cells, a battery can be arranged which shall have as great 
an electro-motive force and as small a resistance as may be 
desired. Practically, in electric telegraphy we require only to 
increase the electro-motive force without paying any attention 
to the resistance. This, however, is easily provided for by 
increasing the size of the battery cells. 
We have stated that the current of a battery passing through 
a wire deflects a magnetic needle. If the needle be suspended 
vertically on its centre with its north pole upwards, and a wire 
in front of it be also vertical with a descending current, the 
upper part of the needle will be deflected to the right. If the 
current be made to rise behind the needle after it has passed 
down in front, the action will be concurrent with the former, 
and in fact the power of the current in the needle will thus 
have been doubled; and by repeatedly passing the wire before 
and behind the needle the apparatus can be made sensitive to 
very feeble currents. This invention of the multiplier or 
galvanometer is due to Schweigger. By using two parallel 
