hey are all fed from the same mercury reservoir and run
down into the same mercury receiver. It is much easier to make three
pumps, each with separate pinch cocks to regulate the mercury supply,
than it is to make three jets, each delivering exactly the proper
stream of mercury to three fall tubes.

Sprengel pumps only work at their highest efficiency when the mercury
supply is carefully regulated to suit the peculiarities of each fall
tube, and this is quite easily done in the model figured. Since on
starting the pump the rubber connections have to stand a considerable
pressure, the ends of the tubes must be somewhat corrugated to enable
the rubber to be firmly wired on to them. The best binding wire is the
purest Swedish iron wire, previously annealed in a Bunsen gas flame.

The wire must never be twisted down on the bare rubber, but must
always be separated from it by a tape binding. By taking this
precaution the wire maybe twisted very much more tightly than is
otherwise possible without cutting the rubber.

The only difficulty in making such a pump as is described lies in the
bending of the heads of the fall tubes. This bending must be done
with perfect regularity and neatness, otherwise the drops of mercury
will not break regularly, or will break just inside the top of the
fall tube, and so obstruct its entrance that at high vacua no air can
get into the tube at all.

The connections at the head of the fall tubes must also be well put on
and the joints blown out so that the mercury in dropping over the head
is not interfered with by the upper surface of the tube. However, a
glance at the enlarged diagram will show what is to be aimed at better
than any amount of description. In preparing the fall tubes it is
generally necessary to join at least two "canes" together. The joint
must be arranged to occur either in the tube leading the mercury to
the head of the fall, or in that part of the fall tube which remains
full of mercury when the highest vacuum is attained. On no account
must the joint be made at the fall itself (at least not by an
amateur), nor in that part of the fall tube where the mercury falls
freely, particularly at its lower end, where the drops fall on the
head of the column of mercury.

When a high vacuum is attained the efficacy of the pump depends
chiefly on the way in which the drops fall on the head of the column.
If the fall is too long the drops are apt to break up and allow the
s