ON THE MEASUREMENT OF MAGNETIC HYSTERESIS. 
49 
Application of the Method to Rods of Finite Length. 
§ 16. In many experiments it is convenient, and in most of our expeiiments it was 
necessary, that the specimen should be a straight rod of finite length instead of a 
ring. We must therefore consider what modification of the theory is necessary in 
order to make it fit this case. 
The magnetising solenoid will not be infinite in length, hut tlie correction due to 
the finite length is very small when the diameter is small compared with the length. 
For if the mean radius of the windings is r, and the whole length of the solenoid 
is 2/, then at a ])oint within the solenoid whose distances from the central plane and 
from the axis are .x and y, the components of the magnetic force are 
I 
INO + 
. . . . (25), 
. . . . ( 26 )- 
The magnetic force is tl)us very nearly constant in the central parts of the solenoid, 
and it is practically sufficient to substitute for N in the expression IttNC the quantity 
N' = N/ {I- -f F)-K 
In our experiments we had / = 24, r = 2 approximately, and thus the magnetic 
force at any point within the central 20 centims. of the solenoid did not differ from 
that at the centre by more than i j)er cent. 
§ 17. Wlien the specimen is a finite straight rod instead of a ring, “poles” are 
developed upon it, and these give rise to a demagnetising force, //, thus causing the 
magnetic force at the centre of the rod to differ from that calcidated from the 
currents in tlie primary and secondary circuits. As we are now dealing with a 
correction it will suffice to find the effect of the demagnetising force on the assumption 
that the elementary theory is applicable so that the magnetic force due to the 
secoiidary current is negligil)le. We further suppose that the secondary coil, which is 
placed r(jund the centre of tlie rod, is short compared with both solenoid and rod ; we 
can then treat the demagnetising force, h, as well as the magnetic force, 47rN'C, due 
to the solenoid, as constant within the secondary coil. The rod niay he either longer 
or shorter than the solenoid. 
Let be the area of the section of the iron and G the mean area of one turn of 
the secondary coll. We only restrict A to be constant in the neighbourhood of the 
secondary coil; the section may, if convenient, increase or decrease considerably at a 
distance from the secondary coil. If H he the magnetic foi'ce at the centre of the 
specimen, we liave 
H = IttN'C - h 
VOL. CXOVIII.—A. 
H 
(27), 
