12 
BULLETIN 347, U. S. DEPARTMENT OF AGRICULTURE. 
has a small cone set in it, which fits snugly into a concentric electro- 
magnet, and this is attached to the lower side of a crosshead (if) 
Fig. 6.— Details of grinding lap. 
which slides freely on the guide rod (J) . This crosshead is provided 
with a slot in its rear side through which a sprocket chain (S) passes, 
and this sprocket chain is supported on the sprocket wheels, which 
