arison with an
electrodynamometer, the reading of which varies as the square of the
current. But in either case it is neccessary, in order to obtain the
readings in amperes, to standardize the instrument for some particular
value of the current by comparison with a voltameter, or in some
equivalent manner. Whenever possible, ammeters and voltmeters are
calibrated by comparison of their readings with those of a
potentiometer, the calibration of which can be reduced to the comparison
and adjustment of resistances, which is the most accurate of electrical
measurements. The commoner kinds of mercury thermometers are generally
calibrated and graduated by comparison with a standard. In many cases
this is the most convenient or even the only possible method. A mercury
thermometer of limited scale reading between 250 deg. and 400 deg. C.,
with gas under high pressure to prevent the separation of the mercury
column, cannot be calibrated on itself, or by comparison with a mercury
standard possessing a fundamental interval, on account of difficulties
of stem exposure and scale. The only practical method is to compare its
readings every few degrees with those of a platinum thermometer under
the conditions for which it is to be used. This method has the
advantage of combining all the corrections for fundamental interval,
&c., with the calibration correction in a single curve, except the
correction for variation of zero which must be tested occasionally at
some point of the scale.

AUTHORITIES.--Mercurial Thermometers: Guillaume, _Thermometrie de
Precision_ (Paris, 1889), gives several examples and references to
original memoirs. The best examples of comparison and testing of
standards are generally to be found in publications of Standards
Offices, such as those of the Bureau International des Poids et
Mesures at Paris. Dial Resistance-Box: Griffiths, _Phil. Trans._ A,
1893; Platinum Thermometry-Box: J.A. Harker and P. Chappuis, _Phil.
Trans._ A, 1900; Thomson-Varley Potentiometer and Binary Scale Box:
Callendar and Barnes, _Phil. Trans._ A, 1901. (H. L. C.)

CALICO, a general name given to plain cotton cloth. The word was spelt
in various forms, including "calicut," which shows its derivation from
the Indian city of Calicut or Kolikod, a seaport in the presidency of
Madras, and one of the chief ports of intercourse with Europe in the
16th century, where cotton cloths were made. The name seems to have bee