% silver, 33% platinum), and also the alloy called
manganin, seem to possess the qualities of permanence essential for a
wire-resistance standard. A comparison made in 1892 and 1894 of all
the manganin wire copies of the ohm made at the Reichsanstalt in
Berlin, showed that these standards had remained constant for two
years to within one or two parts in 100,000. It appears, however, that
in order that manganin may remain constant in resistivity when used in
the manufacture of a resistance coil, it is necessary that the alloy
should be _aged_ by heating it to a temperature of 140 deg. C. for ten
hours; and to prevent subsequent changes in resistivity, solders
containing zinc must be avoided, and a silver solder containing 75% of
silver employed in soldering the manganin wire to its connexions.

The authorities of the Berlin Reichsanstalt have devoted considerable
attention to the question of the best form for a wire standard of
electric resistance. In that now adopted the resistance wire is
carefully insulated and wound on a brass cylinder, being doubled on
itself to annul inductance as much as possible. In the coil two wires
are wound on in parallel, one being much finer than the other, and the
final adjustment of the coil to an exact value is made by shortening the
finer of the two. A standard of resistance for use in a laboratory now
generally consists of a wire of manganin or platinum-silver carefully
insulated and enclosed in a brass case. Thick copper rods are connected
to the terminals of the wire in the interior of the case, and brought to
the outside, being carefully insulated at the same time from one another
and from the case. The coil so constructed can be placed under water or
paraffin oil, the temperature of which can be exactly observed during
the process of taking a resistance measurement. Equalization of the
temperature of the surrounding medium is effected by the employment of a
stirrer, worked by hand or by a small electric motor. The construction
of a standard of electrical resistance consisting of mercury in a glass
tube is an operation requiring considerable precautions, and only to be
undertaken by those experienced in the matter. Opinions are divided on
the question whether greater permanence in resistance can be secured by
mercury-in-glass standards of resistance or by wire standards, but the
latter are at least more portable and less fragile.

A full description of the co