and the Maximum of Magnetism of Iron, Steel, and Nickel. 149 



the helix is of fine wire closely surrounding the bar or ring, all 

 the lines of force which affect F must pass through the bar, and 

 so no correction should be made. However, the correction is 

 so small that it will hardly affect the result. If it were to be 



Q' 



made, — (equation 5) should be diminished by 47rM ; but, for 



the above reasons, it has not been subtracted. As a test of the 

 whole arrangement, I have obtained the number of lines of force 

 in a very long solenoid : the mean of two solenoids gave me 



Q'=12'67M(ttR 2 ); 



while from theory we obtain, by equation (7) (fi=l), 



Q'=12-57M(7tR 2 ), 



which is within the limits of error in measuring the diameter of 

 the tubes &c. 



All the rings and bars with which I have experimented have 

 had a circular section. In selecting the iron, care must be used 

 to obtain a 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 afterwards 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 considerable 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, per- 

 manent 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 ob- 

 tained 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. 



