189 
1908-9.] An Improved Form of Magnetometer. 
result. The current having been interrupted, a small permanent magnet is 
next placed east and west on the stand E, and the stand moved along the 
cross channel in the magnetometer bed-plate until the spot rests near one 
extremity of the scale. The current is again made and reversed, and if any 
appreciable deflection of the spot on the scale is observed coil C 3 is in- 
cluded in the circuit, the current being so directed through it that the de- 
viations of the needle from its equilibrium position are diminished. The 
coil is gradually moved closer to the magnetometer until the Erhard effect 
is completely wiped out, and is then clamped in position. The compensation 
now holds for all parts of the scale, and the apparatus is ready for carrying 
out magnetic tests. 
The several sections in which the three compensating coils are built 
allow the adjustment to be completely made with the coils in several 
different positions. This is a great advantage, as it always affords a means 
of escape from any arrangements of the coils which might prove awkward 
when specimens are in the solenoid. 
With reference to the adjustment of the coils described above, it should be 
noted (1) that the method is systematic, and that there is no possibility of 
failure to secure a balance — all the adjustments are carried out in a perfectly 
definite manner ; (2) that the method is delicate ; for, owing to the great dis- 
tance of coil C 2 from the magnetometer needle, it may generally be moved 
several millimetres without causing any appreciable error in the compensa- 
tion ; (3) that the method is capable of furnishing a high degree of 
accuracy ; with the near end of the solenoid at a distance of only 12 cm. 
from the magnetometer needle it is quite possible to arrange that the 
change in the scale reading brought about by reversing a current of 15 
amperes in the circuit is only a fraction of 1 mm. with the scale at a 
distance of 175 cm. from the needle ; (4) that the operations can be carried 
out with great rapidity ; unless the solenoid is very close up to the magnet- 
ometer, the changing over of the apparatus from one degree of sensibility 
to another can be carried out within the space of two minutes. 
The magnitude of the directive force at the needle is easily determined 
by passing a measured current through one of the balancing coils and noting 
the deflection of the magnetometer needle produced. The value of the 
directive force is then easily calculated. 
Fig. 4 is a photograph of the apparatus when adjusted for the examina- 
tion of a strongly magnetic specimen ; fig. 5 shows the arrangement when 
a feebly magnetic specimen is being dealt with. When the solenoid has to 
be placed very close to the magnetometer needle to allow of a very feebly 
magnetic specimen being examined, the coil C 1 is placed on the opposite side 
