Barrett — On the Magnetic Moment of Steel, etc. 463 



The magnetometer consisted of a short fragment of magnetised 

 watch spring, with attached mirror, suspended by a silk fibre in a 

 glass shade — the deflection being read on a millimeter scale 

 placed 100 centims from the mirror. The specimen to be tested 

 was placed on a level with the needle, and its centre at a given 

 distance (varying from 20 to 60 centims) due east behind the 

 magnetometer. After reading the deflection the specimen was 

 reversed end for end, and the deflection in the opposite direction 

 read. Half the diiference of the scale readings gives the deflec- 

 tion produced by a single pole. As the length of the magnetometer 

 needle may be neglected, the magnetic moment Mis : — 



(V 2 - FY 

 M = { n ' tan 0. H. 

 2 r 



where r is the distance of the magnetometer needle from the centre 

 of the specimen under trial, /, half the distance between the poles 

 of the specimen (practically its semi-length) 0, the angle of deflec- 

 tion produced by the specimen, and H, the horizontal force of the 

 earth's magnetism in c. g. s. units, this was determined for the 



d 

 place of observation and found to be 0-16. 9 is equal to =-= (the 



angular displacement of the needle being doubled by reflection) 

 where d is the scale reading, and JD the distance of the scale from 

 the mirror, in the same units. 



When the specimen is placed at some distance from the magne- 

 tometer its length may be neglected, and the moment is then given 

 by the approximate formula 



M = § r 3 tan . H 



r being as before the distance of the specimen from the magneto- 

 meter. 



The magnetizing force, F, of the field to which the specimens 

 were subjected was calculated for each helix employed from the 

 formula 



„ 4nrn O 



where n is the number of turns of the current of strength C (in 

 c. g. s. units), and / the total length of the helix in centims. 



In order to ascertain the effect of the duration and strength of 

 the current on the saturation of the specimen under trial the 



