osit was formed with so high a current-density
as 280 amperes per sq. ft., but if the jet was checked, the deposit
(now in a still liquid) was instantaneously ruined. When two or more
jets were used side by side the deposit was good opposite the centre
of each, but bad at the point where two currents met, because the rate
of flow was reduced. By introducing perforated shields of ebonite
between the electrodes, so that the full current-density was only
attained at the centres of the jets, these ill effects could be
prevented. One of the chief troubles met with was the formation of
arborescent growths around the edges of the cathode, due to the
greater current-density in this region; this, however, was also
obviated by the use of screens. By means of a very brisk rotation of
cathode, combined with a rapid current of electrolyte, J. W. Swan has
succeeded in depositing excellent copper at current-densities
exceeding 1000 amperes per sq. ft. The methods by which such results
are to be obtained cannot, however, as yet be practised economically
on a working scale; one great difficulty in applying them to the
refining of metals is that the jets of liquid would be liable to carry
with them articles of anode mud, and Swan has shown that the presence
of solid particles in the electrolyte is one of the most fruitful
causes of the well-known nodular growths on electro-deposited copper.
Experiments on a working scale with one of the jet processes in
America have, it is reported, been given up after a full trial.

In copper-refining practice, the current-density commonly ranges from
7.5 to 12 or 15, and occasionally to 18, amperes per sq. ft. The
electrical pressure required to force a current of this intensity
through the solution, and to overcome a certain opposing electromotive
force arising from the more electro-negative impurities of the anode,
depends upon the composition of the bath and of the anodes, the
distance between the electrodes, and the temperature, but under the
usual working conditions averages 0.3 volt for every pair of
electrodes in series. In nearly all the processes now used, the
solution contains about 1-1/2 to 2 lb. of copper sulphate and from 5
to 10 oz. of sulphuric acid per gallon of water, and the space between
the electrodes is from 1-1/2 to 2 in., whilst the total area of
cathode surface in each tank may be 200 sq. ft., more or less. The