pass from one electrode to the other, while, if the electrodes be left
entirely free in the bath, the gases, rising in a spreading form, will
mix at a certain height. It is necessary to separate them by a
partition (Fig. 1, C). If this is isolating and impermeable, there
will be no interest in raising the electrodes sensibly above its lower
edge. Now, the nearer together the electrodes are, the more it is
necessary to lower the partition. The extension of the electrodes and
the bringing of them together is the knotty part of the question. This
will be shown by a very simple calculation.

[Illustration: FIG. 1.--A, B, COMMONEST FORMS OF LABORATORY
VOLTAMETERS. C, DIAGRAM SHOWING ASCENT OF BUBBLES IN A VOLTAMETER.]

The visible electrolysis of water begins at an E.M.F. of about 1.7 V.
Below this there is no disengagement of bubbles. If the E.M.F. be
increased at the terminals of the voltameter, the current (and
consequently the production of gas) will become proportional to the
excess of the value over 1.7 V; but, at the same time, the current
will heat the circuit--that is to say, will produce a superfluous
work, and there will be waste. At 1.7 V the rendering is at its
maximum, but the useful effect is _nil_. In order to make an
advantageous use of the instruments, it is necessary to admit a
certain loss of energy, so much the less, moreover, in proportion as
the voltameters cost less; and as the saving is to be effected in the
current, rather than in the apparatus, we may admit the use of three
volts as a good proportion--that is to say, a loss of about half the
disposable energy. Under such conditions, a voltameter having an
internal resistance of 1 ohm produces 0.65 liter of hydrogen per hour,
while it will disengage 6.500 liters if its resistance be but 0.0001
of an ohm. It is true that, in this case, the current would be in the