the lamp the current has done something. It has overcome the resistance
of the carbons, heated them to a dazzling white heat, and so performed
work. In doing this the current of electricity has lost something. Led
from the first lamp to a second, it is found powerless--if the first
lamp be of sufficient size. What is it that the electricity has lost?
It has parted with what electricians would term "potential," or the
capacity for performing work. What this is precisely, or in what way the
presence or absence of potential modifies the nature of the electric
current, no one knows; but it is known that this potential can only be
conferred on electricity by doing work on the electricity in the first
instance. The analogy between electricity and a liquid like water will
now be recognized. So long as the water is at rest, it is inert. If we
pump it up to a height, we confer on it the equivalent of potential.
We can let the water fall into the buckets of an overshot wheel. Its
velocity leaving the tail race may be identical with that at which it
left the supply trough to descend on the wheel. Its quantity will be
the same. It will be in all respects unchanged, just as the current
of electricity passing through a lamp is unchanged; but it has,
nevertheless, lost something. It has parted with its potential--capacity
for doing work--and it becomes once more inert. But the duty which it
discharged in turning the mill wheel was somewhat less than the precise
equivalent of the work done in pumping it up to a level with the top of
the wheel. In the same way the electric current never can do work equal
in amount to the work done on it in endowing it with potential.

It will thus be seen that electricity can only be used as a means of
transmitting power from one place to another, or for storing power up
at one time to be used at a subsequent period; but it cannot be used to
originate power in the way coal can be used. It possesses no inherent
potential. It is incapable of performing work unless something is done