the mercury reaches some
easily recognized division. At this point the broken thread is
rejoined to the mercury column from the bulb, and a microscopic bubble
of gas is condensed which generally suffices to determine the
subsequent breaking of the mercury column at the same point of the
tube. The bulb is then allowed to cool till the length of the thread
above the point of separation is equal to the desired length, when a
slight tap suffices to separate the thread. This method is difficult
to work with short threads owing to deficient inertia, especially if
the tube is very perfectly evacuated. A thread can always be separated
by local heating with a small flame, but this is dangerous to the
thermometer, it is difficult to adjust the thread exactly to the
required length, and the mercury does not run easily past a point of
the tube which has been locally heated in this manner.

Having separated a thread of the required length, the thermometer is
mounted in a horizontal position on a suitable support, preferably
with a screw adjustment in the direction of its length. By tilting or
tapping the instrument the thread is brought into position
corresponding to the steps of the calibration successively, and its
length in each position is carefully observed with a pair of reading
microscopes fixed at a suitable distance apart. Assuming that the
temperature remains constant, the variations of length of the thread
are inversely as the variations of cross-section of the tube. If the
length of the thread is very nearly equal to one step, and if the tube
is nearly uniform, the average of the observed lengths of the thread,
taking all the steps throughout the interval, is equal to the length
which the thread should have occupied in each position had the bore
been uniform throughout and all the divisions equal. The error of each
step is therefore found by subtracting the average length from the
observed length in each position. Assuming that the ends of the
interval itself are correct, the correction to be applied at any point
of calibration to reduce the readings to a uniform tube and scale, is
found by taking the sum of the errors of the steps up to the point
considered with the sign reversed.

Table I.--_Calibration by Method of Gay Lussac_.

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