184 Proceedings of the Koyal Society of Edinburgh. [Sess. 
was therefore made to design a form of magnetometer which would over- 
come these disadvantages which are common to instruments of the usual 
type. In planning the apparatus the following requirements were kept 
constantly in view : (1) the magnetometer must be capable of accurate and 
rapid adjustment, (2) there must be no resultant Erhard effect, (3) the 
instrument must be suited for testing specimens at all temperatures from 
that of liquid hydrogen to the critical temperature, (4) it must be alike 
efficient for testing strongly magnetic and feebly magnetic specimens, 
(5) the magnetising solenoid must be capable of furnishing fields up to at 
least 400 C.G.S. units, (6) the instrument must be rigid, all parts being fitted 
on one bed-plate, and the coils must be capable of being clamped without 
danger of destroying the compensation in so doing. 
The general principle of the instrument which has been evolved will be 
seen from fig. 2. n s represents the magnetometer needle provided with a 
L 
* 
n 
I 
s 
C, 
H 
s 
C 3 
Fig. 2. 
concave mirror, by means of which and a source of light L, its movements 
are observed on the scale S S. H is the magnetising solenoid placed due 
east or west of the magnetometer needle and clamped in a convenient 
position. C 1 and C 2 are compensating coils placed with their axes approxi- 
mately in coincidence with that of the solenoid. In adjusting the apparatus 
the effect of the current in H on the needle ns is first approximately 
annulled by means of Cj , which is then clamped in position. The final 
adjustment of the compensation, so far as the undisturbed position of the 
needle is concerned, is carried out by means of C 2 , which on account of its 
great distance from the needle contributes only a small fraction of the 
balancing field, and thus provides an adjustment of great refinement. The 
position of C 2 necessary for balance having been obtained, it is clamped in 
position ; obviously, since the distance of C 2 from the needle is great, any 
slight movement caused by doing so produces no perceptible effect on the 
compensation. 
