RESEARCHES IN MAGNETISM. 
557 
These experiments were in most cases made by the direct magnetometric method. 
A straight piece of wire, long enough to approximate as nearly as was practicable to 
the condition of endlessness, was hung vertically behind the magnetometer in the 
manner described in § 18. The magnetising force was applied by means of a solenoid 
wound close to the wire, while the vertical component of the earth’s field was 
neutralised by maintaining a constant current of the necessary strength in another 
solenoid wound on a tube which enclosed the first. The correct value of the current 
in the second solenoid was calculated with reference to the known value of the earth’s 
force, and then verified experimentally by subjecting to the process of demagnetising 
by reversals (§19) a long piece of soft iron in the interior. This process gives 
complete demagnetisation when the piece subjected to it lies in a perfectly neutral 
field, but leaves a large residue when the field is not fairly balanced. It thus affords 
an accurate test of the exact neutralisation of the earth’s vertical component. 
In some instances the ballistic method was used, and then the rods experimented on 
were laid at right angles to the terrestrial magnetic field. A few experiments were 
also made by the ballistic method on rings. The results obtained from long rods by 
the two methods, and those from rings, were in substantial agreement. 
Plate 60, fig. 17, shows an actual example of the direct magnetometric method of 
investigating the ratio of residual to temporary magnetisation. Starting from a state 
of no magnetism, a force was applied and removed, then again applied and carried 
to a somewhat higher value, again removed, again reapplied and carried to a somewhat 
higher value, again removed, and so on. The application and removal of the force was 
conducted very gradually, and in this particular example intermediate readings of the 
magnetism during the application and removal were taken, from which the curves in 
fig. 17 have been plotted. Generally, however, only the induced and the residual 
magnetism for each stage in the increase of <§ were determined. The following table 
gives the successive values of the applied force <§, the intensity of induced magnetism, 
and the residual magnetism which survived the removal of the force, and finally the 
ratio of the residual to the induced or temporary magnetism. This ratio increases 
with increased strengths of magnetisation, and in this case does not pass a maximum 
even at the highest value of >§. The piece tested here was the soft annealed iron 
wire of fig. 5 (§ 21), 0'077 centim. in diameter, and 30‘5 centims. or 400 diameters in 
length. 
4 c 
MDCCCLXXXV. 
