ON THE MEASUREMENT OF MAGNETIC HYSTERESIS. 
lOl 
the rod, increases until Bq reaches B'q approximately. Thus as Bq increases we may 
regard the “poles” as moving inwards towards the centre. There is thus a double 
reason why Iiq should increase with Bg during this stage. When Bg increases beyond 
B'g, he found that the “poles” move out again towards the ends of the rod, and we 
must suppose that this motion more than compensates for the increasing strength of 
the poles with increasing Bg.* 
In our experiments, for the largest values of Bg, Iiq was less than 2lgA//b This 
effect can only arise from the development of a subsidiary pole between the centre and 
either end of the wire, wdth sign opposite to that of the pole at the end. This 
result is easily seen to be impossible with a bar of uniform material as long as the 
induction is a definite single-valued function of the magnetic force, which increases as 
the magnetic force increases. It must therefore be due to the effects of hysteresis, 
which becomes an increasingly important factor as the point of maximum perme¬ 
ability is reached and passed. A different result might perhaps have been obtained 
if the magnetic force had been gradually increased from zero. But in our experiments, 
as in those of Lamb, the magnetic force was put through several cycles between the 
limits iHg before the observations wmre made. 
§ 77. When iron is tested by the ballistic method for hysteresis, the specimen is 
subjected to an applied magnetic force which is reversed between the limits fizHg until 
the iron has reached a cyclic state. This attained, the points on the cyclic B—H 
diagram are found from the throws of the galvanometer which occur when the 
magnetic force is suddenly changed, by means of a special key, from Hg to a series 
of values between Hg and — Hg. 
We made hysteresis tests by this method ip^on two specimens, determining also 
the demagnetising force, h, at tlie centre of the wire, for each value of H which was 
employed in constructing the B— H curve. Thus, the magnetic force was changed from 
Hg to H, and the throw of the galvanometer, giving Bg — B, was observed, and then, 
without altering the magnetic force from its value H, the deflexion of the magneto¬ 
meter was noted. We reckon positive when its direction is opposite to that of the 
induction at the centre of the specimen. 
§ 78. The first specimen w'as a bundle of ten iron wires with a total area of section 
of •0412 scp centim. The length of the wire was equal to the length of the solenoid, 
* Dr. L. Holborx, in a paper “ On the Distribution of Induced Magnetism in Cylinders ” (‘ Sitzungs- 
berichte der Akademie der Wissenschaften zu Berlin,’ 17th February, 1898), has obtained a result similar 
to that found by Mr. Lamb. He used two secondary coils, one a uniformly wound solenoid closely fitting 
the rod, the other a coil wound about the centre of the rod. The rod and the secondary coils w^ere placed 
inside a long magnetising solenoid. By comparing the changes of induction through these two 
secondaries due to a reversal of the primary current, he found the distance A. between the “ centres of 
gravity ” of the free magnetism on the two halves of the rod. He found that the “ centres of gravitj^ ” 
move towards the centre as Bq increases, until Bq reaches B'q. A further increase of Bq caused the 
“ centres of gravity ” to move out again from the centre. Dr. Holborn made similar experiments on 
ellipsoids, and found that for them X was remarkably constant. 
