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Proceedings of the Royal Society of Edinburgh. [Sess. 
mounted in the “ B ” position of Gauss. The construction of the stand is 
similar to that of the stand which carries the magnetometer proper. On 
the top of it is fixed a rectangular block of wood provided with a groove 
for receiving the magnet. 
The bed-plate of the magnetometer is mounted on six pairs of mahogany 
feet, which are fastened to a rigid table by means of brass screws. 
The process of setting up the apparatus is as follows. The centre of the 
magnetometer needle has first to be placed on the axis of the solenoid. To 
accomplish this, coil C 1 (fig. 3) is removed, and the solenoid H is moved 
along the bed-plate towards A until its inner end is almost in contact with 
the back of the magnetometer casing. The stand A is then moved in its 
channel until the needle is brought exactly on to the axis of the helix, and is 
then permanently fastened in this position by means of the clamping screw 
already mentioned. The table carrying the magnetometer is now placed so 
that the long channel of the bed-plate lies due east and west, the adjust- 
ments being carried out and tested by means of the well-known method 
described in Gray’s Absolute Measurements in Electricity and Magnetism. 
The method is as follows : — A wire is stretched out vertically beneath the 
needle, and accurately parallel to the short channel of the bench. On passing 
a current through this wire a deflection of the needle is produced. If the 
current is reversed in direction the deflection will have the same numerical 
value as before, provided that the wire lies exactly magnetic north and south. 
The table is so placed that this condition is fulfilled, and its feet are then 
clamped to the floor by means of L -shaped brass brackets. The scale is 
erected on a separate table in order that the movements of the observer may 
not set up oscillations of the needle. The coils C 15 H, and C 2 are now con- 
nected up in series with the storage battery, ammeter, and variable resist- 
ances, etc., care being taken that the direction of the current in C x and C 2 is 
opposite to that in H. The permanent adjustments of the instrument are 
now complete. 
When a specimen has to be tested the solenoid H is moved to a convenient 
distance from the magnetometer needle and firmly clamped. The coil C 2 is 
placed at the far end of the magnetometer table, and a current two or three 
times greater than the maximum to be used in the subsequent test is sent 
through the complete circuit. Coil C x is then moved until it just falls short 
of balancing the effect of the solenoid on the needle. It is then securely 
clamped. Coil C 2 is next slowly moved up towards the magnetometer needle 
until the deflection of the latter is brought exactly to zero ; C 2 is now clamped, 
and the accuracy of the compensation verified by suddenly reversing the 
current in the coils. No measurable change in the scale reading should 
