by auxiliary resistances by
which the scale may be indefinitely extended. In accurate electrical
thermometry, for example, the slide-wire itself would correspond to
only 1 deg., or less, of the whole scale, which is less than a single
step in the calibration of a mercury thermometer, so that an accuracy
of a thousandth of a degree can generally be obtained without any
calibration of the slide-wire. In the rare cases in which it is
necessary to employ a long slide-wire, such as the cylinder
potentiometer of Latimer Clark, the calibration is best effected by
comparison with a standard, such as a Thomson-Varley slide-box.

_Graphic Representation of Results._--The results of a calibration are
often best represented by means of a correction curve, such as that
illustrated in the diagram, which is plotted to represent the
corrections found in Table III. The abscissa of such a curve is the
reading of the instrument to be corrected. The ordinate is the
correction to be added to the observed reading to reduce to a uniform
scale. The corrections are plotted in the figure in terms of the whole
section, taking the correction to be zero at the beginning and end. As a
matter of fact the corrections at these points in terms of the
fundamental interval were found to be -29 and -9 thousandths
respectively. The correction curve is transformed to give corrections in
terms of the fundamental interval by ruling a straight line joining the
points +29 and +9 respectively, and reckoning the ordinates from this
line instead of from the base-line. Or the curve may be replotted with
the new ordinates thus obtained. In drawing the curve from the
corrections obtained at the points of calibration, the exact form of the
curve is to some extent a matter of taste, but the curve should
generally be drawn as smoothly as possible on the assumption that the
changes are gradual and continuous.

The ruling of the straight line across the curve to express the
corrections in terms of the fundamental interval, corresponds to the
first part of the process of calibration mentioned above under the term
"Standardization." It effects the reduction of the readings to a common
standard, and may be neglected if relative values only are required. A
precisely analogous correction occurs in the case of electrical
instruments. A potentiometer, for instance, if correctly graduated or
calibrated in parts of equal resistance, will give correct relative
v