the
pointer, a delicate manometer is attached to the can by means of a
pet-cock and the oxygen is admitted by operating the switch B until the
desired tension is reached.

[Illustration: FIG. 32.--Part of the oxygen cylinder and connections to
tension-equalizer. At the left is shown the upper half of the oxygen
cylinder with a detail of the electro-magnet and reducing-valve. D is
the cylinder; K, the band supporting the oxygen cylinder and
electro-magnet arrangement; F, the electro-magnet; E, the tension
spring; and L, the rubber tubing at a point where it is closed by the
clamp. The tension-equalizer and the method of closing the circuit
operating it are shown at the right. C and C' are two mercury cups into
which the wire loop dips, thus closing the circuit. B is a lever used
for short-circuiting for filling the diaphragm J. G is a sulphuric-acid
container; H, the quick-throw valve for shutting off the tension
equalizer J; M, part of the ingoing air-pipe; N, a plug connecting the
electric circuit with the electro-magnet; and O, a storage battery.]

In order to provide for the maximum sensitiveness for weighing D and its
appurtenances, the electric connection is broken at the cylinder by
means of the plug N and the rubber tube is connected by a glass
connector which can be disconnected during the process of weighing.
Obviously, provision is also made that there be no leakage of air out of
the system during the weighing. The current at F is obtained by means of
a storage battery O. The apparatus has been in use for some time in the
laboratory and has proved successful in the highest degree.

TENSION-EQUALIZER.

The rigid walls of the calorimeter and piping necessitate some provision
for minor fluctuations in the absolute volume of air in the confined
system. The apparatus was not constructed to withstand great
fluctuations in pressure, and thin walls were used, but it is deemed
inadvisable to submit it even to minor pressures, as thus there would be
danger of leakage of air through any possible small opening.
Furthermore, as the carbon dioxide and water-vapor are absorbed out of
the air-current, there is a constant decrease in volume, which is
ordinarily compensated by the admission of oxygen. It would be very
difficult to adjust the admission of oxygen so as to exactly compensate
for the contraction in volume caused by the absorption of water-vapor
and carbon dioxide. Consequently it is necessary to adjust some