ON THE MEASUREMENT OF MAGNETIC HYSTERESIS. 
99 
magnetic meridian, and a magnetometer was so adjusted that its magnet was verti¬ 
cally above the centre of the solenoid, and as near to the solenoid as possible. On 
sending a current through the solenoid a small deflexion of the magnet ensued, diie to 
the finite length of the solenoid ; by adjusting the wires connecting the solenoid to the 
rest of the apparatus this effect was easily annulled. On now placing the rod inside 
the solenoid there was a deflexion of the magnet, due entirely to the distribution of 
magnetism on the bar. When the diameter of the rod and the distance of the 
O 
magnetometer needle from the rod are both small compared with the length of the 
rod, the magnetic force at the magnetometer, due to the rod, differs very little from 
the magnetic force at the centre of the rod due to the rod itself Under these 
conditions the deflexion of the magnetometer may be taken as a nearly exact measure 
of the demagnetising force due to the rod. 
When the specimen was a single thin wire, a mirror magnetometer was used ; for 
bundles of wire, a mirror magnetometer had too small a range, and then a magneto¬ 
meter, with a pointer moving over a circular scale, was employed. 
If 6 be the deflexion of the magnetometer, and M be the earth’s horizontal 
magnetic force, then the demagnetising force h is given by h = M tan 6. 
§ 75. When iron is tested for permeability, the specimen, j^reviously demagnetised 
“ by reversals,” is subjected to a magnetic force which is reversed many times between 
the limits flz Hg. The maximum induction Bg is then determined from the galvano¬ 
meter “ throw ” due to a single reversal of the magnetic force between the same 
limits. Starting from a small value, Hg is increased by suitable steps, and the Audue 
of Bg corresponding to each value of Hg is determined, the process yielding a single 
point on the Bg—Hg diagram for any given value of Hg. 
We made a test of this kind upon an annealed soft iron wire, determining also, by 
the method of § 74, the demagnetising force Ag for every value of Hg employed. The 
area of section of the wire was '00412 sq. centim., and its length was 48'5 centims. 
As the solenoid was 60 centims. in length and of small diameter, the magnetic force 
due to the current Avas very nearly uniform over the whole length of the specimen. 
A mirror magnetometer was used to determine h^, the needle l)eing about 3 centims. 
from the wire. 
The results of this experiment are shoAvn graphically in fig. 18. The abscissas of 
the curves marked Hg, p, and h represent the values of Bg, the mean maximum 
induction at the centre of the wire, Avhile the ordinates represent the corresponding 
values of Hg, p [= Bg/Hg] and Iiq respectively. The nearly straight line passing 
through the origin indicates the value Ag would have if the flow of induction from the 
wire occurred entirely at its ends, in which case the “ poles” would be concentrated 
at the ends of the wire. This line represents Ag = 2lgA/P, where Ig [= (Bg—Hg)/47r] 
is the mean maximum of intensity of magnetisation at the centre of the wire, and 2/ 
is the length of the wire. 
Initially Ag is nearly proportional to Bg. Since, however, much of the induction 
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