Improvements in Appliances for rendering Astatic Galvanometer Needles Independent of Disturbances by Earth Magnetism.
Year: 1896

page_text: Date of Application, 12th Sept., 1896
Complete Specification Left, 2nd June, 1897-Accepted, 3rd July, 1897
Improvements in Appliances for rendering Astatic Galvanometer
Needles Independent of Disturbances by Earth Magnetism.
We, SIEMENS BROTHERS & Co. LIMITED, of 12 Queen Anne's Gate, Westminster,
Electrical Engineers, do hereby declare the nature of this invention to be as
follows:-
In order to render an astatic mirror galvanometer needle independent of external
magnetic influences which affect its zero position and its sensitiveness, it suffices to
render it so completely astatic that the influence still exerted by the earth's
magnetism in opposition to the directing influence exerted by the torsion of the
suspending fibre or by a permanent magnet, vanishes.
It is however very difficult so exactly to adjust the two magnets of the system
that their magnetic moments are quite equal. And it is scarcely possible so to
adjust their relative positions, that no deflecting resultant of their forces is left.
According to the present invention, in order to produce an astatic condition as
exact as may be desired, two auxiliary magnets are arranged so that they can be
readily turned so as to vary the resultants of their magnetic moments, both as to
extent and direction.
These auxiliary magnets are much lighter than the main magnets, and they are so
magnetised that the sum of their magnetic moments is only slightly greater than the
resultant of the two main magnets, while the difference thereof is smaller.
In this case, a considerable shifting round of the auxiliary magnets will only
slightly alter the total resultants of the system, and it is therefore easy to render the
latter infinitely small, that is to say, to render the system perfectly astatic.
Instead of employing two magnets adjustable round the vertical axis, only one may
be used, which can be turned round two axes at right angles to each other, as, in
this case, its horizontal moment can be varied at will as regards extent and
direction.
A galvanometer rendered astatic in the above described manner, is entirely
independent of any disturbances of the field so long as these act equally upon all
magnets of the system.
But if the cause of disturbance is not at a great distance from the instrument,
there will still be deviations in the latter, as the different magnets will be affected to
a different degree.
This influence can be neutralised up to a certain extent, by arranging three main
magnets at equal distances apart in the system, of which the upper and the lower
ones are each half as powerful as the middle one, and are arranged in the reverse
direction to the latter.
By means of suitably arranged auxiliary magnets the above condition can be
fulfilled with any required degree of accuracy. The instrument is then independent

page_text: of all disturbances in which the strength of the field increases or decreases from the
position of the upper to that of the middle magnet to the same extent as from the
position of the middle magnet to that of the lower one.
This will always be the case when the source of disturbance does not approach too
near to the instrument; any suitable appliances can be provided on the instrument
for facilitating the accurate adjustment of the auxiliary magnets.
The above described astatic system can also be employed for making
magnetometrical measurements.
Dated this 19th day of September 1896.
ABEL & IMRAY,
Agents for the Applicants.
Improvements in Appliances for rendering Astatic Galvanometer
Needles Independent of Disturbances by Earth Magnetism.
We, SIEMENS BROTHERS & Co. LIMITED, of 12 Queen Anne's Gate, Westminster,
Electrical Engineers do hereby declare the nature of this invention and in what
manner the same is to be performed, to be particularly described and ascertained in
and by the following statement:-
In order to render an astatic mirror galvanometer needle independent of external
magnetic influences which affect its zero position and its sensitiveness, it suffices to
render it so completely astatic that the influence still exerted by the earth's
magnetism in opposition to the directing influence exerted by the torsion of the
suspending fibre or by a permanent magnet, vanishes.
It is however very difficult so exactly to adjust the two magnets of the system that
their magnetic moments are quite equal. And it is scarcely possible so to adjust their
relative positions, that no deflecting resultant of their forces is left.
According to the present invention, in order to produce an astatic condition as
exact as may be desired, two auxiliary magnets are arranged, as shewn in Fig. 1 of
the accompanying drawings, so that they can be readily turned so as to vary the
resultants of their magnetic moments, both as to extent and direction.
These auxiliary magnets are much lighter than the main magnets, and they are so
magnetised that the sum of their magnetic moments is only slightly greater than
the resultant of the two main magnets, while the difference thereof is smaller.
In this case a considerable shifting round of the auxiliary magnets will only
slightly alter the total resultants of the system, and it is therefore easy to render
the latter infinitely small, that is to say, to render the system perfectly astatic.
Instead of employing two magnets adjustable round the vertical axis, only one may
be used which can be turned round two axes at right angles to each other, as, in
this case, its horizontal moment can be varied at will as regards extent and direction.
A galvanometer rendered astatic in the above described manner, is entirely
independent of any disturbances of the field so long as these act equally upon all
magnets of the system.
But if the cause of disturbance is not at a great distance from the instrument,
there will still be deviations in the latter, as the different magnets will be affected to
a different degree.
This influence can be neutralised up to a certain extent, by arranging as shewn in
Fig. 2, three main magnets at equal distances apart in the system, of which the
upper and the lower ones are each half as powerful as the middle one, and are
arranged in the reverse direction to the latter.
By means of suitably arranged auxiliary magnets the above condition can be
fulfilled with any required degree of accuracy. The instrument is then independent

page_text: of all disturbances in which the strength of the field increases or decreases from the
position of the upper to that of the middle magnet to the same extent as from
the position of the middle magnet to that of the lower one.
This will always be the case when the source of disturbance does not approach too
near to the instrument. Any suitable appliances can be provided on the instrument
for facilitating the accurate adjustment of the auxiliary magnets. The above
described astatic system can also be employed for making magnetometrical
measurements.
Having now particularly described and ascertained the nature of this invention
and in what manner the same is to be performed, we declare that what we claim is :--
1. In astatic galvanometers in combination with the main magnet one or more
auxiliary magnets adjustable as to extent and direction of their magnetic moment,
substantially as and for the purpose set forth.
2. The combination of one or more auxiliary magnets, such as are above referred
to, with three equi-distant main magnets, each of the extreme being half as
powerful as the middle and having their poles opposite to those of the middle,
substantially as described.
ABEL & IMRAY,
Agents for the Applicants.