Improvements in Instruments for Measuring Electrical Currents.
Year: 1892

page_text: Date of Application, 17th Sept., 1892
Complete Specification Left, 6th June, 1893-Accepted, 22nd July, 1893
PROVISIONAL SPECIFICATION.
[Communicated from abroad by Messrs. SIEMENS & HALSKE, of Berlin, in the
German Empire.]
Improvements in Instruments for Measuring Electrical Currents.
We, SIEMENS BROTHERS & Co. LIMITED, of 12 Queen Anne's Gate, West-
minster, in the County of Middlesex, Electrical Engineers, do hereby declare the
nature of this invention to be as follows:-
Instruments for measuring electrical currents, in which magnetic needles are
employed, have the disadvantage that the magnetism of these needles can easily be
reversed or destroyed by excessive currents. It has been attempted to overcome
this difficulty by substituting for the magnetic needle a soft iron needle magnetised®
by an external permanent or electro-magnet.
This arrangement, however, is not free from the influence of strong currents.
The permanent magnets become transversely magnetised by the current under
measurement and thus the axis of the field which polarises the iron needle is
diverted, altering the zero of the instrument.
The sensitiveness is also affected, and consequently there is no certainty in its
action.
In what follows there is described a special arrangement of the permanent
magnet used for polarising the iron needle, by which the uncontrollable influence of
strong currents is avoided. The magnets are so arranged with respect to the
current leads that they cannot have their magnetism affected by them.
For this purpose, according to one arrangement, the horse shoe magnet which
polarises the iron needle, is fixed in a slot in the middle of the thickness of a copper
slab through which slab the current passes in a plane transverse to the axis of the
needle which is mounted so as to be in the middle of the horse shoe. According
to the strength and direction of the current, an index on the axis of the needle is
more or less deflected to right or left of the zero point on a graduated scale. As
the current passes equally on each side of the magnet, it is not influenced either by
its strength or direction, the current on its one side neutralising that on the other
side. In order to shield the instrument from the influence of external currents, it
may be duplicated, the slabs which hold in their slots the two magnets being
connected by a conductor the lower end of the one with the upper end of the other
so that the current passes through both in the same direction. The axis in this
case carries two iron needles, the one on the right of the one magnet, the other on
the left of the other magnet. Instead of fixing the permanent magnet in a slot of
a conducting slab, it may have a conducting wire or bar passed down close along
its one side, then bent and turned upwards at some distance from it, and again
turned down close along its other side, the current thus being made to pass equally
along both sides of it.
Dated this 17th day of Santamber 1800
ABEL & IMRAY,
Agents for the Applicants.
COMPLETE SPECIFICATION.
Improvements in Instruments for Measuring Electrical Currents.
We, SIEMENS BROTHERS & Co. LIMITED of 12 Queen Anne's Gate, West-
minster in the County of Middlesex, 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 :-
Instruments for measuring electrical currents in which magnetic needles are
Heine Rd 1

page_text: employed have the disadvantage that the magnetism of these needles can easily be
reversed or destroyed by excessive currents. It has been attempted to overcome
this difficulty by substituting for the magnetic needle a soft iron needle magnetised
by an external permanent or electromagnet. This arrangement, however, is not
free from the influence of strong currents. The permanent magnets become
transversely magnetised by the current under measurement and thus the axis of
the field which polarises the iron needle is diverted, altering the zero of the
instrument.
The sensitiveness is also affected and consequently there is no certainty in the
action of the instrument.
In what follows there are described with reference to the accompanying drawings
special arrangements of the permanent magnets used for polarising the iron needles
by which arrangements the uncontrollable influence of strong currents is avoided.
The magnets are so arranged with respect to the current leads that they cannot
have their magnetism affected by them.
For this purpose, according to one arrangement, shewn in front view in Fig. 1
and in vertical section in Fig. 2 the horse shoe magnet f which polarises the iron
needle c and brings it to its zero position is fixed in a slot in the middle of the
thickness of a copper slab a through which slab the current passes in a plane
transverse to the axis h of the needle which is mounted so as to be in the middle
between the limbs of the horseshoe.
According to the strength and direction of the current, an index d on the axis of
the needle is more or less deflected to the right or left of the zero point on a
graduated scale e. As the current passes equally on each side of the magnet f
there is no influence exerted on the magnet by the strength or direction of the
current, its action on the one side of the magnet neutralising that on the other side.
In order to shield the instrument from the influence of external currents, it may be
duplicated as shewn in Fig. 3, the slabs a a which hold in their slots the two
magnets ff being connected by a conductor k the lower end of the one with the
upper end of the other so that the current passes through both in the same
direction. The axis h in this case carries two iron needles cc the one on the right
of the one magnet, the other on the left of the other magnet. Instead of fixing the
permanent magnet in a slot of a conducting slab, it may be arranged as shewn in
front and side views in Figs. 4 and 5 respectively. A conducting wire or bar a is
passed down close along the one side of the magnet f. It is then bent and turned
upwards as at a¹ some distance from the magnet and again turned down as at a³
close along its other side, the current thus being made to pass equally along both
sides of the magnet.
Having now particularly described and ascertained the nature of our said inven-
tion and in what manner the same is to be performed we declare that what we
claim is :-
1. In an instrument for measuring electrical currents so arranging the conductor
that the current passes equally on both sides of the magnet employed to polarise
the needle, substantially as and for the purpose set forth.
2. Arranging the magnet or each of two magnets within the slot of a conducting
slab, substantially as described with reference to Figs. 1, 2 and 3.
3. Arranging the magnet between the limbs of a bent wire or bar, substantially
as described with reference to Figs. 4 and 5.
Dated this 7th dan of Tune 1802
ABEL & IMRAY,
Agents for the Applicants.