Thomson spheres in the binnacle, causes, tests and procedure for abnormal deviations
In some vessels I have found some binnacles without the spheres and normally the captains do not know why this abnormal incidence has happened since they do not find any reason for that and in many occasions they were not on board by the time this happened
In theory the spheres serve to correct the deviations which appear in the quadrantal magnetic headings(NE-SE-SW-NW) due to the horizontal induced magnetism .
If the spheres have been taken out ,there are two reasons for that:
1/The spheres are magnetized ,which means that the whole adjustment will be affected by this circumstance and it is better to leave out the spheres.
In order to be sure that the spheres are magnetized ,there is a procedure as follows:
Move the spheres as close to the compass as possible and slowly rotate each sphere separately.
Any appreciable deflection (2º or more) of the compass needles resulting from this rotation indicates residual magnetism in the spheres.This test may be made with the ship on any steady heading or alongside.
Spheres which shows residual magnetism must be annealed, i.e heated to a dull red and allowed to cool slowly or changed for other new spheres.
2/Sometime they do not correct all the deviations which appear and it happens due to these reasons:
a/If you move the spheres towards the compass and the deviations are not corrected in its totality , it means that you have to use larger spheres.
b/ On the contrary if you move the sphere outward and also the deviations are not corrected totally , it means that you have to use smaller spheres or lengthen the brackets of the binnacles , which almost very difficult or impossible…
Since all the compasses have the proper spheres with its diameter provided by the manufacturer , the only way is to state the deviations that you have finally corrected, either 2º or 3º or whatever.
Always is better to have the spheres installed since they give the compass more stability.
The other possibility is to remove the spheres taking into account that they produce high deviations and taking them out ,the deviations decrease….
Other impracticable solution is to slew the spheres clockwise till through required angle.
As you can imagine the quadrantal spheres brackets are fixed and cannot be moved…
To sum up the situation , always is better to have the spheres installed ,even it they do not correct all the deviations which have appeared, you just state them in the certificate.
Only, if the deviations are bigger with the spheres than without , in this particular case, we will proceed to take them out and write the proper remark in the certificate.
Also it would be convenient to ask the manufacturer if bigger or smaller spheres can be provide (I reckon it will be very difficult)
CAPT. A.ROS COMPASS ADJUSTER Nº 86 BY DGMM (SPANISH ADMINISTRATION)