a ball cock includes but is not limited to the type of valve described by Talbot
see the images here: http://images.google.de/images?q=ball%20cock...
Funnily enough, if one googles for ballcock then the images are much more limited, and only refer to the type of valve Talbot mentioned.
if one googles for ball valve, then the images shown are identical to those shown on googling for Kugelhahn
AHD doesn't recognise 'ballcock' at all, but does have 'ball cock'( http://www.bartleby.com/61/36/B0043600.html
) in the meaning mentioned by Talbot.
Askoxford, on the other hand, does recognise 'ballcock':http://www.askoxford.com/results/?view=dict&f...
a valve which automatically tops up a cistern when liquid is drawn from it.
indicating that perhaps 'ballcock' is a BE special, and 'ball cock' an AE one? AE natives would have to decide this.
all in all:
ball cock: correct terminus technicus for Kugelhahn and other forms of valve
a Kugelhahn, but a form of 'ball cock'
ball valve: correct terminus technicus for Kugelhahn