182 
Proceedings of the Royal Society of Edinburgh. [Sess. 
XII. — On an Improved Form of Magnetometer and Accessories for 
the Testing of Magnetic Materials at Different Temperatures. 
By James G. Gray, B.Sc., Lecturer on Physics in the University of 
Glasgow, and Alexander D. Ross, M.A., B.Sc., Assistant to the 
Professor of Natural Philosophy in the University of Glasgow. 
Communicated by Professor A. Gray, F.R.S. 
(MS. received January 29, 1909. Read February 1, 1909.) 
In the usual form of magnetometer the magnetising solenoid is placed 
with its axis in the magnetic east and west line passing through the 
magnetometer needle. The effect of the current is balanced at the needle 
by means of a compensating coil connected up in the circuit. This latter 
coil has its axis coincident, or nearly so, with that of the solenoid. When 
a feebly magnetic specimen is under examination the solenoid, and con- 
sequently the compensating coil, must of necessity be brought up close to 
the needle. If large magnetising currents are employed, any small shift 
of the coils from their correct positions may be sufficient to seriously 
impair the balance. In consequence of this the operation of adjusting the 
position of the compensating coil (the solenoid is usually clamped once for 
all in a convenient position) is a difficult one, especially as the slight 
inevitable movement of the coil which results from clamping it in position 
generally results in the balance being interfered with. 
Even if this adjustment be accomplished with the requisite accuracy for 
the undisturbed position of the magnetometer needle, it does not necessarily 
follow that the compensation is complete for the needle in its deflected 
position. In practice the axes of the solenoid and compensating coils are 
in general slightly inclined to one another and to the east and west line 
passing through the needle. The effect of this is to increase the directive 
force on the needle for one direction of the current and to diminish it for 
the other. That this is the case will be seen from fig. 1, in which the want 
of alignment of the coil and solenoid has been greatly exaggerated. The 
magnetometer needle is situated at the point P, and it has been assumed 
that the solenoid and coil are so placed that they produce fields at P in the 
directions P S and P C respectively. If the intensity of the field due to the 
solenoid be denoted by F s , and that due to the coil by F c , then since the 
coils balance for the undisturbed position of the needle it follows that 
F s cos (h = F c cos 6 2 - There are left, however, the components of the intensities 
