Improvements in Electrical Measuring Apparatus for Comparing Differences of Potential.
Year: 1896

page_text: Date of Application, 5th Dec., 1896
Complete Specification Left, 4th Sept., 1897-Accepted, 13th Nov., 1897
PROVISIONAL SPECIFICATION.
Improvements in Electrical Measuring Apparatus for Comparing
Differences of Potential.
We WILLIAM OLIVER SMITH and GEORGE KEITII BULLER ELPHINSTONE both
of 101 and 102 St. Martin's Lane in the County of London, Electrical Engineers
and Manufacturers and ARTHUR CECIL HEAP of 21 Park Road, Wimbledon
in the County of Surrey Electrical Engineer do hereby declare the nature of this
invention to be as follows:-
This invention relates to improvements in apparatus for comparing differences
of potential.
The apparatus consists essentially of a long wire of any suitable material,
preferably that known as Manganin. At about equal distances throughout the
whole length of this wire, which may be six hundred or more inches in length
small connection pieces are joined so that the Electrical resistance of each section
is equal. One of these sections of wire is arranged around a portion of the cir-
cumference of a circle, so that a moving contact carried on a radial arm can be
made to travel over this wire. A scale is provided by which the movements of
the contact over the wire may be noted; for reading these movements with great
accuracy, we provide a fixed mirror upon this moving contact arm which can be
made to reflect a beam of light on to a seale attached to or separate from the
instrument, so that very minute movements of the arm can be readily observed.
Upon the spindle which carries the above radial arm, a small worm wheel
is attached by means of friction spring, and engaging in this wheel is a worm
which has fixed to it a small milled head, so that very fine adjustments of position
of the contact on the wire can be obtained.
All of the small connection pieces placed along the wire referred to above, are
arranged around a circular insulated plate, and concentric with this plate is a
metal ring free to revolve around the edge of the insulated plate, and carrying
balls in grooves between the two edge surfaces; this ring carries a spring which
can make contact as the ring is revolved with any one of the large number of
small contact pieces on the plate; that is to say can travel over the whole length
of the wire in equal steps.
A convenient arrangement of the number of contacts, is to have this wire
divided into 150 parts, of which one part is arranged round the portion of a
circle, over which the moving contact can travel; and the other 149 are
symmetrically arranged round the circular plate about which the other contact
carried on wheel can rotate. Upon this wheel numbers are engraved which show
directly beneath a small window, or by means of an index pointer, the position at
which the contact is situated on the wire; that is to say, should the contact be on
the connection piece joining the exposed or slide portion of the wire, in the special
form above referred to the Fig. 149 would appear at the window, or at the index
rointer

page_text: In place of the engraving on the wheel as described above, we may gear the
wheel suitably for working any form of indicating counter.
Continuous with this divided wire is a Rheostat, or extra resistance which may
be made in two or more parts as desired; the first part consisting of a wire carried
round the circumference of a circle, so that a contact can be moved round to any
place upon it for fine adjustment; the further parts being arranged in any con-
venient manner as desired.
We provide a convenient form of double pole multiple switch with necessary
terminals, so that a number of electro-motive forces from different sources can be
connected to the instrument, and can in turn he compared with, and estimated by,
the fall of potential along any portion of the divided wire described above; by
placing a Galvanometer of any suitable form in a circuit joining the source of
E.M.F. to be measured, to such a point on the divided wire that no current flows
through the Galvanometer; the figures shown at the window or at the index
pointer and on the circular scale above mentioned, then give an indication of the
measurement required.
Keys and necessary connection terminals are provided so as to render the instru-
ment compact and complete in itself.
Dated this 5th day of December 1806
WILLIAM O. SMITH.
G. K. B. ELPHINSTONE.
A. C. HEAP.
Improvements in Electrical Measuring Apparatus for Comparing
Differences of Potential.
We WILLIAM OLIVER SMITH and GEORGE KEITH BULLER ELPHINSTONE both
of 101 and 102 St. Martin's Lane in the County of London Electrical Engineers
and Manufacturers and ARTHUR CECIL. HEAP of The Firs Tyler's Green in the
County of Buckinghamshire Electrical Engineer late of 21 Park Road,
Wimbledon 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:-
This invention relates to improvements in apparatus for comparing differences
of potential. The apparatus consists of a long wire of any suitable material prefer-
ably that known as Manganin at equal distances throughout the length of which
small connection pieces are joined so that the Electrical resistance of each section
is equal. The whole wire may be 600 or more inches in length. One of these
sections is arranged round a portion of the circumference of a circle, so that the
moving contact carried on a radial arm can be made to travel over this wire.
A scale is provided, by which the movements of this contact over the wire may
be noted, and for reading these movements with greater accuracy we may provide
a fixed mirror upon this moving contact arm which can he made to reflect a beam
of light on to a scale attached to or separate from the instrument, so that very
minute movements of the arm can be very readily observed. The whole of the
subdivided wire is placed in the inside of the case containing the apparatus with
the exception only of the small piece round which the contact travels.
In the accompanying drawings (Fig. 3) RRR represents the divided wire,
which although shown for convenience of drawing divided in fewer parts is
usually divided into 150 sections, 149 of which are placed inside the instrument.
The 150th portion KM is that over which the moving arm L can travel. The

page_text: contact J can travel over any one of the points 1 to 149 on the wire RR R. This
contact J is carried by the toothed wheel J (Fig. 2) which also carries a set of
numbers and divisions engraved on its upper surface, so disposed that they can
be seen at the opening in the small window J. Iminediately behind this window
is fixed the scale J3, over which the pointer attached to the arm L can travel.
The wheel J is carried through the sides of the case so as to afford an easy means
of setting it to any required position. Arm L is mounted on an axle which passes
through a worm wheel L2, in which engages a worm L, at one end of which is
a milled head L. The arm L is not, rigidly attached to the worm wheel, but
connected to it by means of a friction spring L, so that the arm L can be quickly
pushed to any desired position over the scale, and a fine adjustment subsequently
made by means of the milled head L, screw L and worm wheel L, without
having to adjust any clamps to make the worm wheel carry the worm.
At one extremity of the top of the instrument are fixed a number of pairs of
terminals, 6 being shown in the figure, T1, T2, T3 T4, T5, and T. Travelling
over flat contact plates attached to these pairs of terminals, is a switch V, which
carries two insulated, springs V, V, arranged so that one extremity of ono
spring V, rests on the fiat bar H, while its other extremity rests on the left hand
of the pair of terminals T (marked F in the nig.); and the other spring V2 is so
arranged that one end rests on the bar I; while its other end rests on the right
hand of the two terminals T (marked G in the fig.).
The terminals T, T &c. are for the attachment of the apparatus to any
different sources of Electromotive Force which is required to be compared by
means of the instrument. D and E in the fig. are the two terminals to which any
Galvanometer used in connection with the apparatus should be attached.
Terminal 1) is attached directly to the flat bar H of the multiple switch.
Terminal E is attached to a circular brass path Je, round which the contact carried
on the toothed brass wheel runs, the contact being insulated from the brass wheel
itself.
A short circuit key X of any convenient form, is mounted on the instrument,
and when the key is allowed to rest against its upper contact the two terminals IA
and E are then directly connected together.
From terminals A and B wires are led to a battery employed in connection
with the apparatus outside the instrument. A is joined preferably through a
small fuse C to the point M at one extremity of the portion of the divided
wire RR R, over which a contact L8 carried by the arm L can travel. From the
furthest extremity N of the divided wire a connection is led to the Rheostat O
for fine adjustments. From this Rheostat a wire is led to the arm P of the
Rheostat marked P. The proportions of the resistance between adjacent
contacts 1, P &c. on this theostat being such that any one section of it has a
slightly smaller resistance than the total resistance of Rheostat O. From the
extremity of Rheostat P a wire is led to the arm Q of the Rheostat Q, the
resistances of which are so proportioned that the resistance of any section Q, Q
c. is slightly less than the total resistance of the whole of theostat P. The
extremity of theostat Q being joined to the battery terminal B.
The bar I of the multiple switch V is connected to the spring key L and this
key can, when pressed down, make electrical contact with lever L-the spring
sey L being carried on the stem L which passes through an insulating tube L
which passes through the centre of worm wheel L².
From the above descriptions of the connections it will be seen that from A to B
here is a complete circuit; the whole of the wire It It I always being included in
he circuit and the total resistance of this cirouit heing variable owing to the
ositions into which Rhenstats 0, P and Q can be brought.
In place of the figured wheel J¹ we may gear the wheel J into a small
heel Js, and by convenient gearing J3 J4 motion can be imparted to a
ounter J as shown in Figs. 4 and 5.
Figs. 6 and 7 show the arrangement of a mirror L" carried upon arm L, so

page_text: that the movements of the arm can be magnified by observing the angle through
which L is turned by means of the telescope T, and the scale T¹.
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 what
we claim is:
1. In a potentiometer the dividing up of the wire RRR in a large number
of parts for the purpose of securing greater accuracy.
2. In a potentiometer the carrying of the slide portion of the wire RRR
around a portion of a circular arc for securing uniform and certain electrical
connections.
3. In a potentiometer the arrangement of a wheel carrying a moving contact
and device for determining its position substantially as described and illustrated.
4. The potentiometer substantially as described and illustrated.
Dated this 2nd day of September 1897.
WILLIAM O. SMITH.
G. K. B. ELPHINSTONE.
A. C. HEAP.