Henk Kooiker Barker levers: technical aspects
Het ORGEL 100 (2004), nr. 5, 5-9 [summary]

The Barker lever helps the organist to open the valves in the wind chest. The need for such a lever arose in the 19th century, as organs grew bigger and had relatively larger pipes that thus required more wind.
The power needed to open a valve can be found by means of an easy formula: it equals (in grams) half of the product of the area of the opening and the wind pressure. The Barker lever needs to be able to realise quick note-repetitions, so one needs to calculate the extent to which the lever affects the speech of the pipes as well.
Calculations with respect to the reconstruction of the levers of the Cavaillé-Coll organ of the Concertgebouw at Haarlem (1875), which is now in restoration, indicate that a special built-in valve that regulates the rise and fall of the Barker motors makes the lever relatively quiet; and that a pressure between 100 and 150 mm is sufficient.