45 
be balanced. In the first instance half the current was 
employed; in the second the force of the whole current 
was reduced to half, by doubling all the resistances; but 
the resistance of the galvanometer was doubled by removing 
the diverting coil; therefore, the added resistance, minus the 
equivalent of the necessary connecting wires, is equal to 
the battery resistance. 
The dynamic effect observed with the 10 feet coil will 
be equal to the effect of the battery reduced by the addi- 
tional resistance of the coil, so that the apparent force of 
the current from a pair having a considerable resistance, 
will be too large in relation to the actual force of a pair 
having a smaller resistance. The observations should, there- 
fore, be reduced by the proportion between the resistance 
of the battery, plus 10 feet, and the battery resistance alone. 
The theory of the voltaic battery involves many points 
for consideration, as well of practical as of speculative 
interest ; of the utmost practical interest in relation to the 
economical applications of the battery, as a source of motive 
power, and for the production of light, which have recently 
been so prominently brought before the public. Some points 
arising on this subject are also interesting, as showing that 
we may have a mathematical theory satisfactorily accounting 
for a great number of phenomena, and even enabling us to 
predict other phenomena, but which has, nevertheless, been 
derived from premises which other observers may believe to 
be false, and which they may think equally derivable from 
other premises. I allude to the theory of Ohm. I cannot 
briefly abstract the original paper, so as to point out dis- 
tinctly the process of reasoning by which he deduced his 
theory, as his views, from adopting the contact theory of 
voltaic electricity, are so entirely at variance with my own, 
as adopting the chemical theory. According to the contact 
theory, the electro-motive force has its source in the contact 
