1886.] Electromagnets and the Magnetisation of Iron. 491 



A point marked on the curve indicates the magnetic force under 

 which in some experiments with a rod of the same iron retraction 

 began. The point corresponding with the maximum magnetic force 

 reached in Rowland's experiments is also marked. 



The ring was made of very soft charcoal iron rod 6*4 mm. in 

 thickness, the joint being carefully welded. It was turned in a lathe 

 to a uniform circular section, and when finished its external diameter 

 was 8 cm., and the diameter of its transverse section 4*82 mm. The 

 ring was sawn into two equal portions, and the cut faces ground as 

 flat as possible, first with fine emery, and afterwards with rottenstone. 

 But I was unable to get rid of a very slight convexity which, though 

 it perhaps affected the permeability of the ring, was of some advan- 

 tage in preventing the possibility of complications arising from non- 

 magnetic adhesion. Upon the ends of one-half of the ring pieces of 

 thin brass tube 5 mm. in length were fitted : these projected 1 mm. 

 beyond the faces, forming shallow sockets into which the ends of the 

 other portion of the ring could be inserted, thus insuring exact 

 coincidence of the cut surfaces. Each half of the ring was then 

 covered with ten layers of insulated wire 0*7 mm. in thickness, the 

 radial gaps in every layer being filled with paraffin wax before a new 

 layer was wound on. The number of turns of wire upon the portion 

 having the brass tabes was 980, and upon the other portion, the ends 

 of which protruded about 1*2 mm. beyond the wire, 949. The total 



