71 
1912-13.] Magnetic Induction in Ferric Oxide. 
of the bar. The magnetometer readings are taken by noting the position 
of the spot of light when a given magnetic field is applied to the specimen 
and when the same field is reversed. The difference of the two readings, 
called the double deflection, is taken as a measure of the magnetic state of 
the bar at the particular time or temperature. This is considered sufficient 
in the case of such weakly magnetic substances. 
Material. 
The ferric oxide employed was in the form of : 
(1) The ordinary dark-red artificial ferric oxide. 
(2) A dark- brown ferric oxide obtained from Merck. 
(3) A yellow-brown ferric oxide, also from Merck. 
(4) Blocks of haematite, obtained from cargoes of Spanish ore dis- 
charged at Glasgow, containing about 60 per cent, of iron. 
The ferric oxide mentioned under (2) and (3) was of the purest obtain- 
able commercial form. 
The ores tested were known as Lucinena, red Seriphos, brown Seriphos, 
Agulas, and Bacares. Of these, the Lucinena was a soft ore, and brown 
Seriphos a very hard one. 
Treatment of the Material. 
The amorphous powder specified under (1), (2), and (3) was tested in the 
form of : 
(a) Loose, dry powder held in a glass or copper tube open at both 
ends. 
( b ) Bars formed by the use of (i.) alcohol as a cement ; (ii.) water 
as a cement ; (iii.) compression in a Bramah press. 
Bars were also cut from pieces of the haematite ore. 
None of the material underwent any previous heating or drying 
treatment. 
Heat Treatment. 
Three heat processes were employed, which were as follows : — 
1. Heat Cycle . — In general the bars were first placed in the furnace- 
magnetometer and passed through a temperature cycle from the air 
temperature up to 1000° C. and back again, readings being taken usually 
at intervals of 50°, sometimes at smaller intervals. Observation of the 
bar during this treatment showed whether any magnetic change took 
