Improvements in Ammeters.
Year: 1899

page_text: COMPLETE SPECIFICATION,
Improvements in Ammeters.
We, WALTER CLAUDE JOHNSON and THOMAS JOHN MURDAY, both of Victoria
Works, Charlton, in the County of Kent, Electrical Engineers, do hereby declare
the nature of this invention and in 1 + manner the same is to be performed
to be particularly described and ascertained in and by the following statement:-
Our invention relates to amraters of the class wherein a solenoid is made
use of and has for its object to provide an ammeter wherein equal variations
of the current flowing through the same will produce equal movements of the
finger or pointer of the said ammeter at all points of the scale, the said
ammeter being especially applicable for use in connection with the integrating
electricity meter forming the subject matter of Letters Patent No. 12390
of 1896.
In the accompanying drawing.:
Figure 1 is an elevation of an ammeter constructed in accordance with our
invention; and
Figure 2 is a sectional side elevation of the same:
Figures 3 and 4 are sections on the lines 3-3 and 44 respectively of
Figure 2 drawn to an enlarged scale.
a is the frame of the apparatus, 6 is the coil of the solenoid which is firmly
clamped to the frame a in a vertical position and c is the soft iron core of the
solenoid which is attached to a beam or arm d on the spindle e to which the
pointer of the ammeter is also fixed. 9 is a counterweight on the said
pointer and h. i are two springs the operation of which is hereinafter described.
In order to determine the conditions under which the apparatus works we
have made a large number of experiments and as a result have arrived at the
following conclusions:
1. The solenoid coil must be very long in proportion to the internal diameter
and in practice from 15 to 20 times the latter, for example, we have found that
a coil having an internal diameter of a quarter of an inch, should have a length
of four inches.
2. The solenoid must be wound so as to give about 1,500 ampere turns with
the maximum current. This can be accomplished without exceeding the normal
maximum current density of 1000 amperes per square inch of conductor. In
practice we wind the solenoid up to an outside diameter of about 7 inch with
copper strip of a section capable of carrying the maximum current required
by the particular meter. For large currents we wind the inside and outside
layers in parallel.
3. The angular movement of the beam carrying the iron wire core must not
exceed 30°. In order to have an open scale of sufficient length we make use
of a long pointer, (say six inches radius). When a greater angular movement
is required, the iron wire must be hung on a silk fibre and the beam replaced by
a pulley for the fibre to run on.
4. The iron wire core must be very carefully annealed and must be of uniform

page_text: thickness throughout its length and suspended freely in the centre of the
solenoid.
5. When the pointer is at the middle of the scale the iron wire core must
then have exactly half of its length entered into the solenoid, that is to say,
when the beam is quite horizontal the distance between the upper end of the
solenoid and the beam must, if the iron wire is two inches in length, be exactly
one inch.
6. The length of beam or distance between the beam spindle and the hole in
which the iron wire is suspended may be varied between the limits of ¼ inch
and inch. The maximum amount of movement allowable for the iron
wire is 3 inch with a coil as described.
7. The spindle with pointer, beam and iron wire must be carefully balanced
by the counterweight 9 so that the pointer will remain stationary in any
position within 15º on either side of its vertical position, i.e. over the total
length of scale.
"
"
8. The power against which the pull of the current acts is preferably in the
form of the two spiral springs h, i which are of phosphor bronze or other non-
magnetic material. These springs are attached to collets h¹, respectively
fixed on the spindle and are so arranged that their coils are in reverse directions.
The spring which " closes or is coiled up as the pointer moves towards the
maximum of the scale is clamped by its outer convolution to an adjustable
arm j the movement of which determines the zero adjustment of the pointer
on the scale. The other spring i opens or is uncoiled as the pointer moves
towards the maximum, its outer convolution being bent out at an angle as at k
and passed through a loop or hole formed in the end of an arm m on an
adjusting plate n. The bent up portion of the spring is so arranged with
reference to the loop as to be quite free while the pointer stands at zero.
When, however, the pointer moves up the scale the bent up part k of the spring
comes against one side of the loop and thus it is prevented from turning with
the spindle. Until the iron becomes magnetically saturated the pull on the
iron wire is not so great in proportion and at this point we only oppose one
spring to the pulling force. When the iron is saturated the pull is then pro-
portional and the other spring comes into action. This point we usually find
to be about 40 part of the total scale from zero, thus if the ammeter is for a
maximum of 40 amperes the second spring should come on at about 1 ampere.
In practice we find slight alterations necessary to this position so we do not per-
manently fix it but find the exact position for each individual instrument.
9. The proportion of spring strength which we find acts best is approximately
six convolutions of the spring h to eight of the spring i. We leave the unfixed
spring i always its full length of eight convolutions and reduce the length of
the fixed" spring h by drawing it through its arm ; until the current value
is correctly indicated on the dial. This length we generally find comes out at
about six convolutions.
Without the loose spring adjustment, that is to say, with both springs fixed,
the first quarter of the scale is contracted, the error amounting to about two or
three per cent.
10. The iron wire we make use of and find most satisfactory is .014 inch
diameter in lengths of two inches weighing .65 of a grain.
Having now particularly described and ascertained the nature of our said
invention and in what manner the same is to be performed, we declare that
we claim is:-
.
1. An ammeter of the kind described wherein a solenoid and core are arranged
with relation to each other and to the pointer of the ammeter, substantially
as herein before described.
2. In an ammeter of the kind herein before described the combination with the
spindle carrying the finger of the ammeter, of two springs coiled in opposite

page_text: directions and each fixed at one end to the said spindle, the other end of one
of the said springs being fixed to an arm made adjustable for calibrating the
hand of the ammeter at the zero position whilst the outer end of the second
spring is movable relatively with an adjusting plate which serves for arresting
the rotation of the spring with the spindle at the point of the saturation of the
iron core, substantially as described.
3. The manufacture and use of the improved ammeter substantially as herein-
before described and illustrated in the accompanying drawing.
Dated the 2nd day of September 1899
G. F. REDFERN & Co.,
4, South Street, Finsbury, London, Agents for the Applicants.