1887.] On the Magnetisation of Iron in Strong Fields. 



203 



induction occurred, a small quantity of wire was wound over the first 

 induction coil, to form a distance-piece, and on the top of that a 

 second induction coil was wound, the second coil, like the first, con- 

 sisting of a single layer of very fine wire. The space between the 

 two coils was accurately determined. "When the test-piece was 

 reversed or drawn out of the field the operation was in each case 

 performed several times, and two groups of observations were 

 recorded, one giving the induction in the inner coil, and the other the 

 induction in the outer coil ; the difference of course served to 

 determine the field in the space between the coils. When this field 

 was known it was easy to correct for the induction in the non-ferrous 

 space enclosed by the inner coil. 



Three kinds of wrought iron were tested ; soft hammered scrap, 

 Swedish iron, and Lowmoor iron. The hammered scrap proved less 

 susceptible than the other two, and was not used in the final experi- 

 ments, which were made with test- pieces of the form of Sample B. 

 Pieces of cast iron were also tested, informs resembling both A and B. 



To determine in absolute measure the value of the ballistic effects, 

 a large earth-coil was kept in circuit with the induction coil and 

 galvanometer, and was turned over in either the vertical or horizontal 

 earth-field at the beginning, and again at the end of each group of 

 observations. To avoid the possibility of error in this important 

 particular, two separate earth-coils of entirely different dimensions 

 were employed, and the galvanometer constant was determined inde- 

 pendently by means of both, with results which were in excellent 

 agreement. The values of the induction stated below are worked out 

 on the basis that the horizontal force in the grounds of University 

 College, Dundee, at a place sufficiently removed from local magnetic 

 influence, is 0*160 in C.G.S. units. 



The following experiments are representative of a considerably 

 larger number -: — 



Lowmoor iron, annealed before turning the bobbin from a forged 

 bar. Sample B, of shape and dimensions shown in fig. 2. Diameter 

 uf iron neck = 0'65 cm. ; length = 0*44 cm. Diameter to middle of 

 inner induction coil, 0*6765 cm. Diameter to middle of outer induction 

 coil, 0*9364 cm. 



Area of section of iron (S x ) = 0*3318 sq. cm. 



Area of space to be corrected for under inner induction coil (S 2 ) = 

 0*0276 sq. cm. 



Area of space between inner and outer coil (S 3 ) = 0*3293 sq. cm. 

 Number of turns on inner induction coil = 16 ; number on outer 

 coil = 12. 



In the following table D x is the throw of the ballistic galvanometer 

 given by the inner coil when the test-piece was turned round, and Do 

 is the throw given by the outer coil. X x and X 2 are the corre- 



