Improvements in Electric Measuring Apparatus.
Year: 1892

page_text: Date of Application, 4th Feb., 1892
Complete Specification Left, 3rd Nov., 1892-Accepted, 3rd Dec., 1892
PROVISIONAL SPECIFICATION.
Improvements in Electric Measuring Apparatus.
I, ROOKES EVELYN BELL CROMPTON, of Mansion House Buildings, in the
City of London, Engineer do hereby declare the nature of this invention to be as
follows:-
One portion of my invention consists in a new form of or improvements in what
is known as the slide wire potentiometer. My improved form of the apparatus
consists in a framework of any desired length which may be made of carefully
seasoned wood so built up or connected together that it maintains itself without
distortion of form through very considerable changes of temperature or of moisture
in the atmosphere. In some cases I can make my framework of cast iron or any
rigid material suitable for the purpose, on which is secured free to slide in a
longitudinal direction an insulating portion on which the measuring wire is mounted.
It is of considerable importance that the temperature co-efficient of lineal expansion
of the wood or other material forming this insulating part on which the slide wire
is mounted should be the same as that of the slide wire itself. For instance in
cases where I use an alloy of platino-iridium containing 17% of iridium I find that
the temperature co-efficient of lineal expansion of this alloy is so nearly equal to
that of teak, mahogany and boxwood that these materials are very suitable for the
purpose of the insulating part frame or support. Instead of stretching or mounting
the whole of my slide wire in the manner usual in this kind of apparatus either over
or on one side of my framework carrying the corresponding scale, I do not find it
necessary to mount more than one fractional part of it in this manner, that is to say
that if I choose to employ a slide wire 10 metres in length and having a resistance
of 100 ohms, 1 metre of it need only be applied to the measuring scale which for
convenience may be divided into 1000 parts, and the remaining 9 metres may be
stretched alongside the first portion which is over its scale by being wound
alternately round certain insulating pulleys placed at each end of and adjacent to
the scale. In this manner I can obtain the advantages of a scale and wire 10 metres
long divided into 10,000 parts. I prefer however to so arrange and place these
portions of the wire that are not above the scale that they are in identical conditions
as regards heat radiation and connection with that portion applied to the scale, that
is to say that they are exposed as far as possible to the same influence of cooling
air currents.
In order to provide fixed contacts on this wire at the desired points viz. at one
point at the commencement of the scale and at 9 other points on the remainder of
the wire I provide 10 knife edge contacts over which the wire passes and against
which it presses. The arrangement that I prefer to maintain the wire in contact
with these knife edges is that of placing the centre line of the grooves of the pulleys
round which the wire is wound slightly out of line of the level of the knife edges
so that the wire on being drawn tightly round the pulleys has a tendency to press
against the knife edges.
In some cases I find it convenient to make the knife edges point downwards and
so press the wire up against them. I find it convenient to make these knife edges
adjustable, that is to say each of them mounted on a slide and actuated by a
micrometer screwed so that it can be traversed through a range amounting to
2 or 3% of the distance between the respective knife edges, the object of this
being that the respective resistances of each of these portions of the wire may be
adjusted and made to agree with the corresponding length which is applied to or
stretched immediately over the scale itself. In place of the contact usually
adapted for slide wire measuring bridges I prefer to have sliding boxes to slide

page_text: longitudinally on a suitable part of the framework, such boxes contain a part which
forms a continuous rubbing contact against a conductor running from end to end
of the scale and parallel to it. In addition to this the sliding boxes themselves are
provided with a key contact consisting of a knife edge to make contact with the
slide wire which can be made to press itself with a given definite amount of
pressure on the surface of the slide wire. I accomplish this in the following
manner. The knife edge contact is mounted on a flat or a spiral spring which
when in action is of sufficient strength to press this knife edge into contact with the
wire to the required degree This knife edge contact and spring is held out of
action by a catch actuated by a vertical piston, this vertical piston is maintained
in its normal out of contact position by a second spring and when the pressure of
the finger overcomes that of this second spring and causes the piston to descend it
thus releases the knife edge which is pressed by its own spring into contact at the
desired pressure against the slide wire. I prefer not to mount the knife edge
contact with its spring piston et cetera on the slide box itself but on a separate
slide which can have a longitudinal movement within the slide box. This slide can
be made to traverse longitudinally with reference to the slide box by means of a
micrometer screw. In this way a fine adjustment of the position of the knife edge
on the wire can be made without moving the slide box which can be brought near
the position required and the final adjustment of the knife edge contact can be
made by the micrometer screw and slide. In order to conveniently change the
electrical connections from the first to any one of the other 9 fixed knife edges I
connect them to the various contacts of a multiple switch, and so arranged that one
terminal of the circuit leading from the E.M.F. or difference of potential to be
measured can be attached successively to any one of these fixed knife edge contacts
the other conductor being brought in the usual way through the galvanometer to
the moving knife edge contact.
Dated the 4th day of February 1892.
WM. BROOKES & SON,
55 & 56, Chancery Lane, London, Agents for the Applicant.
COMPLETE SPECIFICATION.
Improvements in Electric Measuring Apparatus.
I ROOKES EVELYN BELL CROMPTON of Mansion House Buildings in the City
of London, Engineer 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 :-
One of my improvements consists in a new form of what is known as the slide
wire potentiometer. My form of the apparatus consists in a frame work of any
desired length which may be made of carefully seasoned wood so built up or con-
nected together that it maintains itself without distortion of form through very
considerable changes of temperature or of moisture in the atmosphere. In some
cases I can make my frame work of cast iron or any rigid material suitable for
the purpose on which is secured free to slide in a longitudinal direction an insulating
portion on which the measuring wire is mounted. It is of considerable importance
that the temperature co-efficient of lineal expansion of the wood or other material
forming this insulating part on which the slide wire is mounted should be the same
as that of the slide wire itself. For instance in cases where I use an alloy of
platino-iridium containing 17% of iridium I find that the temperature co-efficient
of lineal expansion of this alloy is so nearly equal to that of teak, mahogany and
box wood that these materials are very suitable for the purpose of the insulating
part frame or support. Instead of stretching or mounting the whole of my slide
wire in the manner usual in this kind of apparatus either over or on one side of my
framework carrying the corresponding scale, I do not find it necessary to mount

page_text: more than one fractional part of it in this manner, that is to say that if I choose to
employ a slide wire 10 metres in length and having a resistance of 100 ohms,
1 metre of it need only be applied to the measuring scale which for convenience
may be divided into 1000 parts, and the remaining 9 metres may be stretched
alongside the first portion which is over its scale by being wound alternately round
certain insulating pulleys placed at each end of and adjacent to the scale. In this
manner I can obtain the advantages of a scale and wire 10 metres long divided
into 10,000 parts. I prefer however to so arrange and place those portions of the
wire that are not above the scale that they in identical conditions as regards heat
radiation and connection with that portion applied to the scale, that is to say
that they are exposed as far as possible to the same influence of cooling air
currents.
In order to provide fixed contacts on this wire at the desired points on the
remainder of the wire I provide 10 knife edge contacts over which the wire passes
and against which it presses.
The arrangement that I prefer to maintain the wire in contact with these knife
edges is that of placing the centre line of the groove of the pulleys round which
the wire is wound slightly out of line of the level of the knife edges so that the
wire on being drawn tightly round the pulleys has a tendency to press against the
knife edges. In some cases I find it convenient to make the knife edges point
towards and so press the wire up against them. I find it convenient to make these
knife edges adjustable that is to say each of them mounted on a slide and actuated
by a micrometer screwed so that it can be traversed through a range amounting to
2 or 3% of the distance between the respective knife edges the object of this
being that the respective resistances of each of these portions of the wire may be
adjusted and made to agree with the corresponding length which is applied to or
stretched immediately over the scale itself. In place of the contact usually
adapted for slide wire measuring bridges I prefer to have sliding boxes to slide
longitudinally on a suitable part of the framework, such boxes contain a part
which forms a continuous rubbing contact against a conductor running from end to
end of the scale and parallel to it. In addition to this the sliding boxes themselves
are provided with a key contact consisting of a knife edge to make contact with
the slide wire which can be made to press itself with a definite amount of pressure
on the surface of the slide wire. I accomplish this in the following manner.
knife edge contact is mounted on a flat or spiral spring which when in action is of
sufficient strength to press this knife edge into contact with the wire to the
required degree. This knife edge contact and spring is held out of action by a
catch actuated by a vertical piston, this vertical piston is maintained in its normal
out of contact position by a second spring and when the pressure of the finger over-
comes that of this second spring and causes the piston to descend it thus releases
the knife edge which is pressed by its own spring into contact at the desired
pressure against the slide wire.
I prefer not to mount the knife edge contact with
its spring piston etcetera on the slide box itself but on a separate slide which can
have a longitudinal movement within the slide box. This slide can be made to
traverse longitudinally with reference to the slide box by means of a micrometer
screw.
The
In this way a fine adjustment of the position of the knife edge on the wire can
be made without moving the slide box which can be brought near the position
required and the final adjustment of the knife edge contact can be made by the
micrometer screw and slide.
In order to conveniently change the electrical connections from the first to any
one of the other 9 fixed knife edges I connect them to the various contacts of a
multiple switch, and so arranged that one terminal of the circuit leading from the
E.M.F. or difference of potential to be measured can be attached successively to
any one of these fixed knife edge contacts the other conductor being brought in
the usual way through the galvanometer to the moving knife edge contact.
Figure 1 shows in a diagramatic form the slide wire A B on which a constant.

page_text: potential is maintained by means of the accumulator cells C divided into 10 parts
of which one part only A to a is applied to the scale. a', a², a³ etcetera show the
10 contact knife edges. shows a multiple switch for connecting one terminal of
the galvanometer with any one of these knife edges. c shows multiple switch by
which any desired number of potential differences x and y etcetera can be switched
into series with the galvanometer. d is the sliding box which carries the contact
for the portion of wire over the scale.
Figure 2 shows the arrangement of the slide wire when wound round insulated
pulleys e, e etcetera, the other arrangements and same figures applying as in
Figure 1.
Figure 3 shows a third arrangement but in place of 9 of the wires being stretched
on the board they are wound into spirals of a form found very convenient for ready
and cheap adjustment. In this case in place of the knife edges the spirals (best
seen in the enlarged view Fig. 6) can be adjusted by one portion of them being
drawn backwards or forwards through the clamping terminals a, a¹, a² etcetera.
this case the same letters apply as in Figure 1.
In
Figure 4 shows a convenient arrangement of mounting the wire shown
diagramatically in Figure 2.
Figure 5 shows an enlarged arrangement of one of the knife edges a a¹ etcetera.
In this case the working surface of the knife edge a is protected by a strip of
platinum the position of the knife edge under the slide wire is adjusted by the
micrometer screw i working through the fixed nut k.
Figs. 7 and 8 show enlarged cross sectional views of the sliding contact box d.
Here is the contact itself mounted on the spring and held out of action by the
catch m actuated by the piston n. In Figure 7 the section is taken so as to show o
the slide of the micrometer adjusting arrangement.
Fig. 9 shows end cross section of a piece of contact box provided with spiral
springs instead of the flat springs used in Figs. 7 and 8.
Having now particularly described and ascertained the nature of my said inven-
tion, and in what manner the same is to be performed I declare that what I
-claim is :-
1. In a potentiometer a slide wire part of which only need be calibrated and
applied to the scale, the remaining part being strained over knife edge contacts
substantially as herein described.
2. In a potentiometer a calibrated slide wire and additional coils forming an
extension of the slide wire the said coils being wound into spirals in the form
substantially as shewn in Figure 3.
3. The form of contact box and knife edge shown in Figures 7 & 8.
2nd day of November 1892.
WM. BROOKES & SON,
55 & 56, Chancery Lane, London, Agents for the Applicant.