LIGHT METHOD.--Lastly, by means of an electric light, which
shows, by its brightness, a greater or less current.

THE PREFERRED METHODS.--It has been found that the first and second
methods are the only ones which will accurately register current
strength, and these methods have this advantage--that the chemical
effect produced is not dependent upon the size or shape of the apparatus
or the plates used.

HOW TO MAKE A SULPHURIC ACID VOLTAMETER.--In Fig. 41 is shown a simple
form of sulphuric acid voltameter, to illustrate the first method. A is
a jar, tightly closed by a cover (B). Within is a pair of platinum
plates (C, C), each having a wire (D) through the cover. The cover has a
vertical glass tube (E) through it, which extends down to the bottom of
the jar, the electrolyte therein being a weak solution of sulphuric
acid. When a current passes through the wires (D), the solution is
partially decomposed--that is, converted into gas, which passes up into
the vacant space (F) above the liquid, and, as it cannot escape, it
presses the liquid downwardly, and causes the latter to flow upwardly
into the tube (E). It is then an easy matter, after the current is on
for a certain time, to determine its strength by the height of the
liquid in the tube.

HOW TO MAKE A COPPER VOLTAMETER.--The second, or copper voltameter, is
shown in Fig. 42. The glass jar (A) contains a solution of copper
sulphate, known in commerce as blue vitriol. A pair of copper plates
(B, B') are placed in this solution, each being provided with a
connecting wire (C). When a current passes through the wires (C), one
copper plate (B) is eaten away and deposited on the other plate (B'). It
is then an easy matter to take out the plates and find out how much in
weight B' has gained, or how much B has lost.

In this way, in comparing the strength of, say, two separate currents,
one should have each current pass through the voltameter the same length
of time as the other, so as to obtain comparative results.

It is not necessary, in the first and second methods, to consider the
shapes, the sizes of the plates or the distances between them. In the
first method the gas produced, within a given time, will be the same,
and in the second method the amount deposited or eaten away will be the
same under all conditions.

DISADVANTAGES OF THE GALVANOSCOPE.--With the third method (using the
galvanoscope) it is necessary, in order to get a positively correct
reading