I found this fragment: The Black Penny was believed to be useful in curing mental disorders in cattle [mad cow disease!?] and was often borrowed by local English farmers for its supposed healing powers. The Black Penny was a coin belonging to the Turnbull family. To function properly, the coin had to be immersed into water that ran in a southerly direction; that water was then given to the sick animal to drink. The coin was smaller than an ordinary penny, but thicker. It was made of copper and zinc and had a raised rim. It was lost about 1827.

if this is connected, I'd think that you'd only spend this as a last resort.