185 
1908-9.] An Improved Form of Magnetometer. 
If the axes of C x , H, and C 2 were coincident and passed through the 
magnetometer needle, the adjustment would now be complete. If, however, 
the needle ns is deflected by means of a permanent magnet, and a large 
current is reversed in the circuit, in general an alteration in the scale 
reading on S S will be observed. A coil C 3 , placed with its axis in the 
magnetic north and south line passing through the needle, is now included 
in the circuit. By properly adjusting the direction of the current in C 3 , 
and altering the distance of C 3 from the needle, the compensation can be 
made perfect for all positions of n s* In a magnetometer where n s, C l5 H, 
and C 2 are all carried on stands moving in one channel in the bed-plate 
there should be little departure of the axes of the coils from coincidence. 
Accordingly the resultant magnetic field, due to the coils and solenoid, in 
the north and south direction will be small. The coil C 3 is therefore made 
of little power, and a small change in its position brings about only a very 
slight alteration in its effect upon the needle. It can therefore be clamped 
without any risk of upsetting the balance. The manner of making the 
adjustments will be fully explained later. 
The instrument with its compensating coils and other fittings is shown in 
plan in fig. 3. The bed-plate is in the form of a cross, and is built of well- 
seasoned mahogany planks 22 cm. broad and 2*5 cm. thick. The length 
over all is 350 cm., and the breadth from end to end of the arms 135 cm. 
The cross-piece is at a distance of 100 cm. from one end of the main length. 
Like the main portion of the bed-plate, it is formed from one piece of wood, 
the two lengths being set accurately at right angles and half checked into 
one another. The junction is made rigid by means of glue and brass 
screws. A channel 11 5 cm. broad is formed over the entire length of the 
cross-piece by means of two mahogany strips which are square in section 
and fixed parallel to the edges of the arms. A similar channel runs down 
the main length of the bed-plate, being discontinued where it is crossed by 
the channel already mentioned. The wooden strips forming these channels 
are permanently fixed by glueing and by brass screws driven in from the 
under side of the base-board. After they have been constructed they are 
made of perfectly uniform width by sand-papering, the width being tested 
from time to time during the process by means of a wooden gauge. 
A is a mahogany box consisting of bottom, sides, and top, the ends which 
face east and west being left open. In the bottom is a slot running parallel 
to the cross-piece of the bed-plate. A brass screw projecting upwards from 
the base-board of the magnetometer passes through this slot and is provided 
* A side coil lias been used by Dr G. E. Allan in liis magnetometric work for giving 
compensation throughout the scale, but it does not permit of the adjustment here described. 
