The function of the feed is to transport the ink from the reservoir to the nib and to control its flow. Simple? Most of us expect this, without being aware of what this “it” entails.
However, there are many parameters, which impact on and make the life of a feed very difficult.
1.) Supply ink and adjust the flow effected by writing speed (stop and start) and style and the hygroscopy (blotting strength) of the paper.
2.) Compensate the various characteristics of the ink (different colour and manufacturer, see my writing on ink)
3.) Compensate the effects of varying temperature, air pressure, gravity
4.) Initial supply of ink when the feed is dry
5.) Transporting ink through a dried and/or blocked feed
Are you slightly more impressed? These independent parameters may compensate or amplify each other in their effect on the constancy of ink flow. All this is done by a static control system, meaning, there is no mini computer inside, which registers these variations and adjusts the flow by altering the orifice of a valve.
The static control system is developed and set by the manufacturer, that’s it. For the rest of its natural life, the feed has to cope with changing conditions.
It is self‑regulating, meaning, there are no knobs and buttons for the user to control it (could be a good idea) or inbuilt control mechanisms, which vary the flow rate of ink in response to a change of conditions.
The flow rate of ink is not controlled directly. It is the inflow of air into the ink reservoir, which allows ink to escape from it, or not. The air inlet either passes air or not. It is a yes/no regulation, air bubble in or not.
The mechanical dimensions of the feed keep the air bubbles at a certain size; small enough and at a frequency so that the writer does not experience any variation in ink flow.
What determines the timing? There are forces (capillary, surface action, hydrodynamic, air-pressure) acting on the ink as it runs down the feed to the nib. Details you find under Surface Tension and Capillaries
Only briefly here: Initially the flow starts because the weight of the column of fluid (level in the reservoir down to the nib) pulls it down. The pulling of the capillary in the feed and nib supports this flow.
As the ink flows out, the air volume above the fluid in the (firm) reservoir develops a vacuum, and the ink flow reduces. When the pulling and sucking forces are equal, the flow stops entirely. (Bladders or sacks avoid this problem to some degree.)
The only way to start the flow again is to allow air to enter into the tank, the vacuum reduces, and ink can run out.
Detailed information you find in the chapter Application to the Feed.
Insiders could just gobble on. I will endeavour to introduce you gradually to the individual systems and modes of control carried out by this small black component of plastic. It holds the secrets of the function of a fountain pen. After you have read this article, your relationship with your fountain pen will have changed, forever.
Early fountain pen feeds only had a capillary (or two or three), transporting the ink from the ink reservoir to the nib but not air vents or overflow slits. The only means for controlling the flow was the cross-section of the capillary and the opening of the tines. I wonder, what would have been messier, using an inkwell and a dip-nib with a holder or such a prehistoric fountain pen. I guess it would explain the reason for the expression fountain pen.
How does it do all that?
Please read on Bubbles and Bottles
11 March 2016