All organettes are fitted with a reservoir which is pneumatically
connected to the crank driven exhausters which jointly are connected
to the playing reeds.  In the case of the Melodia, these are directly
played by the paper roll which sits over the tracker bar, but the
principle is the same for valve operated organettes such as the popular
Celestina or the keyed Phoenix range.  Most organettes have anything
between two and four exhausters, although one of mine has only a single
one.

As the crank is turned you will get this many separate charges of
air supplied to the reeds so this will give an uneven supply.  The
reservoir is there to even out the supply of air and ensure that
there is always air available at the correct pressure (or suction).
The correct pressure to allow the reeds to voice correctly is
determined by the spring force being applied to the reservoir.

The reservoir is also used to supply sudden large demands of air
which would otherwise be more than is being pumped at that moment.
If more air is being exhausted than is required by the reeds for a
particular part of the tune then the reservoir will collapse fully
(for a suction organette ) and cause the spill valve to open, allowing
atmospheric air to enter the reservoir and feed the exhausters.
Remember that exhausters are a fixed displacement pump and if not
allowed to pump air they will break.

Almost without exception, all bellows operated instruments use
a reservoir to even out the supply, set the correct pressure and
incorporate the spill.

As a matter of interest, it is quite possible to use a centrifugal
blower without a reservoir, blowing directly into the valve or pipe
chest.

Best wishes from Great Britain,  Nicholas Simons.