alues of any differences of potential within its range if connected to
a constant cell to supply the steady current through the slide-wire. But
to determine at any time the actual value of its readings in volts, it
is necessary to standardize it, or determine its scale-value or
reduction-factor, by comparison with a standard cell.

[Illustration: CALIBRATION CURVE.]

A very neat use of the calibration curve has been made by Professor
W.A. Rogers in the automatic correction of screws of dividing machines
or lathes. It is possible by the process of grinding, as applied by
Rowland, to make a screw which is practically perfect in point of
uniformity, but even in this case errors may be introduced by the
method of mounting. In the production of divided scales, and more
particularly in the case of optical gratings, it is most important
that the errors should be as small as possible, and should be
automatically corrected during the process of ruling. With this object
a scale is ruled on the machine, and the errors of the uncorrected
screw are determined by calibrating the scale. A metal template may
then be cut out in the form of the calibration-correction curve on a
suitable scale. A lever projecting from the nut which feeds the
carriage or the slide-rest is made to follow the contour of the
template, and to apply the appropriate correction at each point of the
travel, by turning the nut through a small angle on the screw. A small
periodic error of the screw, recurring regularly at each revolution,
may be similarly corrected by means of a suitable cam or eccentric
revolving with the screw and actuating the template. This kind of
error is important in optical gratings, but is difficult to determine
and correct.

_Calibration by Comparison with a Standard._--The commonest and most
generally useful process of calibration is the direct comparison of the
instrument with a standard over the whole range of its scale. It is
necessary that the standard itself should have been already calibrated,
or else that the law of its indications should be known. A continuous
current ammeter, for instance, can be calibrated, so far as the relative
values of its readings are concerned, by comparison with a tangent
galvanometer, since it is known that the current in this instrument is
proportional to the tangent of the angle of deflection. Similarly an
alternating current ammeter can be calibrated by comp