ON THE MAGNETISATION OF COBALT. 
345 
The new coil had a length of 18‘15 cm., an internal diameter of 1’4 cm., and an 
external diameter of 7 cm. It consisted of thirteen layers of copper wire of ’24 cm. 
diameter when covered. In the solid end of the brass tube holding the cobalt a 
diametral hole was bored at right angles to the length of the tube, and a corre¬ 
sponding hole was made in each of two cheeks projecting from one of the faces of the 
coil. By slipping a pin through these holes the tube was firtnly secured in a fixed 
position relative to the coil, and when the pin was withdrawn the tube and its 
contained cobalt could be removed and replaced without the least risk of disturbing 
the remainder of the apparatus. 
A current of 1 ampere in the new coil produces a field varying from 65'7 C.G.S. 
units at the centre to 43’9 units at the ends of the cobalt rod. Throughout the 
greater portion of the rod, howmver, the field is very nearly the same as at the centre. 
These figures refer entirely to the action of the current, no attempt having been made 
to allow for the action of the rod itself 
In all the following calculations the values assigned to the field <§ are calculated 
from the action of the current alone at the centre of the rod, and so are somewhat 
higher than the mean values of the actual fields. The diflGculty of determining the 
actual fields at the different elements of the rod would be very great even treating 
the permeability as constant, a most erroneous supposition; and, considering the 
differences to be expected betwmen different specimens of the same metal, an attempt 
at great accuracy in tins departnnent seems entirely supererogatory. 
§ 20. Throughout the whole investigation the ordinary magnetometric method was 
employed. The axis of the coil was perpendicular to the magnetic meridian, and the 
magnetometer was situated in the direction of this axis produced. The magnetometer 
needle carried a mirror which formed on a millimetre scale, set parallel to the magnetic 
meridian, an image of a vertical ware placed across a slit in the centre of the scale 
behind which a lamp stood. The direct action of the coil current wms neutralised by 
a compensating coil traversed by the same current. The strength of the current wms 
recorded by a Thomson graded ammeter in circuit wfith the coil. The current was 
derived from the storage cells of the laboratory, and its strength was varied by 
changing the number of cells and the resistance in a wire bridge in circuit with the 
coil. 
§ 21. In order to render apparent the relative magnitudes of the several phenomena 
some common system of magnetic measurements was essential. Thus all the scale 
readings have been reduced, and in the following tables and diagrams only C.G.S. 
units appear. From the previous remarks as to the method of calculating it will 
be clearly understood that the values given for the strengths of the fields, and con¬ 
sequently for the coefficients of magnetic induction k, cannot rigidly be held to apply 
to the actual condition of the rod, in which ^ and k varied from point to point, but 
must be regarded as giving an approximation to the mean state of the rod, and more 
MDCCCXC,—A. 2 Y 
