it:

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CHAPTER V
THE VISCOMETER
A very full discussion has been given of the theory of the trans-
piration method. Much matter of great historical interest in
regard to the various other methods has been passed over. This
has been done in order to present the matter which will be of
greatest use to the worker. At present the transpiration method
is by all odds the most important, this superiority being based
upon the following advantages: (1) It is susceptible of simple
mathematical treatment. (2) It is rapid. (3) Only a small
amount of fluid is required. (4) It can be used under the widest
variety of conditions as regards temperature, pressure et cetera.
(5) The preliminary measurements and adjustments are not
difficult to make. (6) Finally, it has been tested out most
thoroughly and found to be capable of the highest degree of
precision.
For certain purposes other methods must apparently continue
to be used. Thus the pendulum method seems best suited for
investigating superficial viscosity and the viscosity of solids like
steel. The fall method is of great use also for certain purposes.
It is quite impracticable to discuss here the almost innumerable
forms of instruments which have been suggested for use, so we
propose to consider briefly some of the instruments which have
shown the greatest advance toward meeting the conditions of
an ideal disposition of apparatus.
Naturally all transpiration instruments are based upon that
of PoiseuiUe; but for general purposes, his instrument was defi-
cient, since the capillary terminated directly into the bath and
hence the apparatus had to be refilled after each measurement.
This difficulty was overcome hi two forms of apparatus designed
by Pribrarn and Handl (1880), which consisted of acapillary placed
between two tubes of larger bore. In the better form shown in
Fig. 20, the two tubes are vertical, the capillary being bent.
The advantage of this arrangement is immediately apparent
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