A growler can also be used for crack detection, before I got the real powder I used brake lathe shavings from .001,.002 cuts
http://home.comcast.net/~k.fehr/pwpimages/IMG_0313(2).JPG --the pic is too big for me to post
Here is a link to operation--
http://books.google.com/books?id=yF...=onepage&q=armature growler operation&f=false