44 



HEXET A. KOWLAND 



homogeneous bar; in the case of a ring I believe it is better to have it 

 welded than forged solid; it should then be well annealed, and after- 

 wards have the outside taken off all round to about -J of an inch deep in 

 a lathe. This is necessary, because the iron is " burnt " to a consider- 

 able depth by heating even for a moment to a red heat, and a sort of 

 tail appears on the curve showing the permeability, as seen on plotting 

 Table III. To get the normal curve of permeability, the ring must only 

 be used once; and then no more current must be allowed to pass through 

 the helix than that with which we are experimenting at the time. If 

 by accident a stronger current passes, permanent magnetism is given to 

 the ring, which entirely changes the first part of the curve, as seen on 

 comparing Table I with Table II. The areas of the bars and rings were 

 always obtained by measuring their length or diameter across, and then 

 calculating the area from the loss of weight in water. The following 

 is a list of a few of the rings and bars used, the dimensions being given 

 in metres and grammes. In the fourth column " annealed " means 

 heated to a red heat and cooled in open air, " C annealed " means placed 

 in a large crucible covered with sand, and placed in a furnace, where, 

 after being heated to redness, the fire was allowed to die out ; " natural " 

 means that its temper was not altered from that it had when bought. 



The first three Tables are from the same ring. 



Besides these I have used very many other bars and rings ; but most 

 of them were made before I had discovered the effect of burning upon 



14 Almost chemically pure before melting. 



