Improvements in Electrical Resistance Measuring Instruments.
Year: 1899

page_text: Date of Application, 20th Dec., 1899-Accepted, 24th Feb., 1900
COMPLETE SPECIFICATION.
Improvements in Electrical Resistance Measuring Instruments.
I, JOHN AMBROSE FLEMING, F.R.S., D.Sc., Professor of Electrical Engineering
in University College, Gower Street, in the Administrative County of London,
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 electrical resistance measuring instruments of the
Wheatstone's bridge type constructed in the form known as a "dial pattern
box bridge."
The main objections to the "dial pattern box bridge" form of the Wheat-
stone's bridge are that the brass blocks and the ebonite by being exposed to the
air soon get soiled, the ebonite loses its insulating qualities and dust and dirt
get into the plug holes, whilst the numerous plugs are sometimes lost by being
laid about.
My improvements have for their object the construction of a workshop form of
"dial pattern box bridge" in which there is no exposed ebonite or lacquered brass
work excepting the terminals for making connection respectively with the
battery, the galvanometer, and the resistance to be measured. Also my improve-
ments relate to means for preventing dust getting into the plug holes; to con-
venient arrangements for constructing the resistance coils so that they rapidly
get rid of the heat generated in them when electric currents are passing through
them; for giving convenient access to the interior of the box; to an improved
construction of the plugs to avoid the formation of shoulders thereon; to an
improved arrangement of the coils on the ratio arm; and to the general cou-
struction of the instrument.
In the accompanying drawings which are furnished in illustration of my
invention,
Fig. 1 is a plan of the box with the cover or lid and the connected operating
parts of the instrument removed;
Fig. 2 is part of an under plan and Fig. 3 a top plan of the cover or lid for the
box shown in Fig. 1, the operating parts of the instrument being secured to the
underside of said cover or lid;
Fig. 4 is an enlarged plan diagram showing my improved arrangement of the
resistance coils on the ratio arm;
Fig. 5 is a further enlarged under plan of one of the sets of coils of the
measuring arm of the bridge; and Fig. 6 is a section on the line 6-6 of Fig. 5,
showing the apparatus in an inverted position;
Fig. 7 is an enlarged view of one of my improved plugs;
Fig. 8 is an enlarged plan of my improved form of bobbin, Fig. 9 is a section
of same on the line 9-9; and Fig. 10 is a plan of the blank from which one
half of the bobbin shown in Figs. 8 and 9 is made.
Fig. 11 is a detached and reduced plan of the shutter for the measuring arm
shown in Fig. 5; and Fig. 12 is a section taken on the line 12-12 of Fig. 11.

page_text: The same letters of reference where they occur are used to denote the same
or corresponding parts in all the Figs. of the drawings.
The box A is made with a removeable bottom B and cover or lid C, both being
received on end battens D on, or secured to, the interior of the box A, and secured
in position by four turn-buttons E, one at each corner, engaging slots or keepers
made to receive them in the ends of the box, and operated by arms e rotatable in
a recessed ring F on the outside (see Fig. 3). G is a rack for receiving the
plugs when not in use.
The outside of the cover or lid C is fitted with three pairs of terminals c for
making connection with the battery, the galvanometer, and the resistance to be
measured, arranged in the ordinary way; and is also provided with the usual
number of dial faces H, with their plug apertures h; is furnished with two small
holes through which protrude ivory keys secured to spring contact keys h
secured on the underside of the cover or lid C, (see Fig. 2); and with an elon-
gated aperture h through which protrudes a knob or handle 5 secured to an
operative rod 6 (see Fig. 2). This rod h is mounted in suitable guides secured
to the underside of the cover or lid C, and is furnished with arms 8 which
engage projections jon shutters J normally closing over the inner side of the
plug holes in the cover or lid C.
On the underside of the cover or lid C of the box are attached three or more
shallow circular, ebonite trays or rings K, one of which is shown detached in
Figs. 5 and 6. These trays or rings K carry in their interior a brass disc or ring
surrounded by brass blocks and holes are bored through these blocks and
the disc, said holes being partly in the brass disc or ring and partly in the
blocks, which holes correspond to, and form continuations of, the plug holes /
in the dials H, the blocks corresponding in number to the plug holes, which
are usually ten. These plug holes h, as before stated, are normally covered by a
shutter J, shown detached in Figs. 11 and 12, which is provided with apertures
corresponding in number to the plug holes and is arranged as a hit and miss
valve with its apertures held out of coincidence with the plug holes by a suitable
spring or springs j. The shutter J is made of ebonite or other insulating
material and the ends of the spring or springs are secured to the shutter, and
to the ebonite tray or ring K, or to some other fixed part of the apparatus, so as
to keep the plug holes normally closed by the shutter. By operating the rod ho
through the medium of the knob or handle the shutter is drawn back against
the power of the spring or springs so that a plug may be inserted; and when
this is done, the shutter J is maintained by the plug in the opened position. In
Figs. 5 and 6, and 11 and 12, I have shown the shutter J fitted with two springs j
whose ends are connected to bolts passing through the tray or ring K and so
located as to normally hold the shutter in the closed position, the shutter being
made of an annular shape and rotating in an annular recess in the tray or
ring K. I have found this a convenient arrangement and construction of shutter,
but it will be apparent that many other forms of spring closed shutter may be
adopted, designed to effect the same result, without departing from this part of
my invention. Each of the three ebonite trays or rings K is constructed and
arranged in the manner above described and they differ only in being furnished
with coils of different resistances and are connected to their coils and together
in the ordinary manner.
г
Fig. 4 represents diagrammatically my improved arrangement of the resistance
coils in the ratio arm of the bridge. This is also arranged dial pattern and the
coils are respectively:
:-
Two of 0.1 ohm resistance,

page_text: Fig. 4. As both the half rings or discs 3 and their connections are similar, a
description of one of them will serve for both. Each coil is connected, as shown,
to two adjacent blocks so that all the coils are in series, and the first block of
the series is connected to one of the battery terminals, and, through the resist-
ance measuring coils of resistance to be measured, to one terminal of the galvano-
meter; the half ring or disc being connected to the other terminal of the
galvanometer. It will be obvious that by this arrangement the coils can be
brought into circuit progressively; for example, by plugging the first hole all
the resistances are short circuited, by plugging the second hole, the first coil only
is in circuit, by plugging the third hole, the first and second coils are in circuit,
and so on progressively. The ratio arm is fitted with a spring-closed shutter
similar in construction to that on the measuring arm and is opened by engaging
one of the plugs in a recess in the shutter, shown in Fig. 3.
In Figs. 8, 9, and 10, I have shown my improved resistance coil which I con-
struct in the following manner: A sheet of thin copper L of a suitable shape is
bent up so as to form a half cylinder with a flat side, see the lower part of
Figure 10, and a long tongue of copper 1, projects from the flat side. These half
cylinders are easily bent up out of a spade shaped piece of thin copper, such as
is shown in the upper part of Fig. 10. Two of these semi-cylinders are put
together with a thin slab of ebonite 12 between them to form the insulating
distance piece; the two half cylinders are then bound together with a thin silk
tape. The wire which is to form the resistance wire, and which is generally
formed of platinum-silver or platinoid, is covered with silk, and has its two ends
soldered respectively to the two copper semi-cylinders . A loop of wire is then
wound closely round the cylinder and tied or fastened in position with a silk
string or tape. The wire on the cylinders may afterwards be covered with
shellac varnish and baked.
To prevent the formation of a shoulder on the plug at the part where it wears
by coming into contact with the edge of the plug hole I make an annular
recess m on the conical shank of the plug as shown in Fig. 7.
From the foregoing description it will be seen that there is no exposed metal
work, and no exposed ebonite work in the instrument, and that when the box is
not in use there are no openings by which dust and dirt can get in to render the
electric contacts imperfect.
Having now particularly described and ascertained the nature of my said
invention and in what manner the same is to be performed I declare that what
I claim is
(1) An electrical resistance measuring instrument, comprising a box having a
removeable bottom, and a removeable cover or lid to the underside of which latter
the resistance coils, the insulated discs or trays on which they are mounted, and
the contact blocks and discs, and battery, galvanometer, and line wires are all
connected, substantially as described and for the purposes stated.
(2) An electrical resistance coil, comprising a bobbin made of two semi-cylin-
drical open-ended sections secured together with insulation between them, and a
length of resistance wire coiled on the bobbin with the ends secured to the half
sections of the bobbin, substantially as described and for the purpose stated.
(3) A Wheatstone's bridge plug of the form herein before described and shown
in Fig. of the accompanying drawings, for the purpose stated.
(4) In an electrical resistance measuring instrument, the connection of each
of the resistance coils of the ratio arm to two adjacent contact blocks and the
connection of the first block of the series and the central contact half ring or disc
to the battery, resistances, and galvanometer, so that the ratio resistances can
be progressively brought into circuit as required, substantially as described.
(5) In an electrical resistance measuring instrument, an insulated base tray or
ring, for the measuring arm coils, contact blocks and a central contact ring or
disc secured to said insulated base and each having half plug holes therein, all

page_text: arranged and combined to operate substantially as and for the purpose herein-
before described with reference to Figs. 3, 5 and 6 of the accompanying drawings.
(6) In an electrical resistance measuring instrument a spring closed shutter
for the plug holes in the insulated base, said shutter being opened against the
pressure of a spring or springs and closing automatically, substantially as and
for the purpose herein before described with reference to Figs. 5, 6, 11 and 12
of the accompanying drawings.
(i) An electrical resistance measuring instrument of the dial pattern box bridge
form of Wheatstone's bridge the parts being combined and arranged to operate
substantially as and for the purpose herein before set forth and illustrated in the
accompanying drawings.
Dated this 20th day of December 1800
F. W. GOLBY,
Patent Agent, 36, Chancery Lane, London, W.C.
Agent for the Applicant.