Improvements in Electrical Measuring Instruments.U
Year: 1885

page_text: :
Improvements in Electrical Measuring Instruments.
...
1
I, JAMES ROBERT PRATT Farquhar- House Farquhar Road Upper Norwood SE
Electrician do hereby declare the nature of my invention for IMPROVEMENTS IN,
ELECTRICAL MEASURING INSTRUMENTS to be as follows:-
1.
These improvements relate to that class of Electrical Instrument known as the
Wheatstone Bridge and the object of this invention is to produce a more rapid
means of measurement than can be effected by the existing forms.
The principle of the Wheatstone Bridge is when four electrical, conductora
are taken and joined in a continuous circuit and a galvanometer or current
Indicator be joined to two diagonally opposite junctions and a source of Electro
Motive Force be connected to the other two diagonally opposite junctions and
representing the resistance of the conductors by A, B, C and X if A: B:: X: 0
no current is observed to pass through the galvanometer.
In the general form of Wheatstone Bridge the ratio is kept constant or can be
altered by fixed amounts; and C is altered until no current is observed to pass
through the galvanometer, by this arrangement there is a great loss of time in the
operation and it is very hard to follow a variable resistance.
My invention obviates this, loss of time and a variable resistance can be followed
up with ease; it consists of the following modification of Wheatstones plan.
1
The ratio
B
is made variable either continuously increasing or decreasing at the
A
same time C is continuously increasing or decreasing with the ratio
The sum of A + B remains constant during an operation.
The following is a discription of the instrument designed to carry out my
invention.
5.
A revolving drum of hard wood ebonite or other suitable insulating material is
taken on which is coiled like a double threaded screw two Platinum-Silver wires
side by side one forming the ratio the other forming C.
To make the arrangement of the Bridge more sensitive and to increase the range
of the instrument proportional coils are connected to both ends of the ratio wire
and are fixed securely in the body of the drum also a suitable arrangement for
switching or plugging in or out these coils is placed at the end of the drum,

page_text: Extra resistance can be added to C by connecting it to the terminal on jockey
which forms the end of C and making the other connections at the end of this extra
resistance which can be placed in the stand of the instrument with a suitable
arrangement for plugging it in or out.
In connection with one end of the drum is an apparatus for recording the
number of revolutions and fraction of a revolution that the drum has passed
through from which the result is obtained.
Above the drum is mounted a square bar on which travels a jockey carrying two
insulated springs and at the end of these springs are fixed two Platinum (or other
suitable material) rolling contacts arranged to press against the two wires mentioned
above.
The insulated springs are connected to two terminals on the top of the jockey
one for each spring.
The jockey is made to travel up and down the bar by means of a projection piece
fitting into a groove cut in the drum of the same pitch as the wires or instead of a
square bar a round one may be substituted with a thread cut on it which is made
to revolve by means of suitable gearing from the main spindle of the drum and so
work the jockey up and down.
The outer connections with the wires on the drum are made through the bearings
and pass to terminals on the top of the instrument.
The manner of connecting up this instrument for measuring an unknown
resistance is as follows.
One end of the ratio wire is permanently connected to that end of the wire
forming C which is farthest from it on the drum this ensures C increasing or
decreasing with the ratio ✓ the other end of the wire forming C is left free, the
length of C in circuit is determined by the position of one of the rolling contacts
the other rolling contact determines the ratio.
One end of the ratio wire is connected to one bearing and the other end of the
ratio wire where it joins on to C is connected to the other bearing.
The two poles of the battery are connected to the two terminals on the jockey
the galvanometer to the two terminals on the top of the instrument in connection
with the bearings.
The unknown resistance is connected to one of the terminals on the top of the
instrument and one of the terminals on the top of the jockey, but it depends upon
which of the wires is taken as the ratio wire to which of the terminals the
unknown resistance is connected.
Another part of my invention is as follows
In the case of a variable resistance a strip of paper is made to pass under the
drum at a uniform rate by means of clockwork and an arm is fixed to the jockey
carrying a pencil which presses upon the paper and forms a diagram which corre-
sponds exactly with the movements of the jockey, from this diagram the average
resistance can be calculated for any length of time.
This instrument measures any resistance in the shortest possible time as by the
mechanical arrangement a handle has only to be turned and the galvanometer
watched until a balance is obtained, when the result can be read off by means of
the recording apparatus or by reference to the paper strip.

page_text: I, JAMES ROBERT PRATT of Farquhar House Farquhar Rd Upper Norwood
London S.E. do hereby declare the nature of my invention for IMPROVEMENTS
IN ELECTRICAL MEASURING INSTRUMENTS and in what manner the same is to be
performed, to be particularly described and ascertained in and by the following
statement:-
These improvements relate to that class of Electrical Instrument known as the
Wheatstone Bridge.
A
Whereas in the ordinary Wheatstone Bridge if A, B, C and X represent the four
arms in a continuous circuit the ratio is kept constant or can be altered by
fixed amounts while C is altered until a balance is obtained. I make the ratio
continuously increasing or decreasing at the same time C is also continuously
increasing or decreasing with the ratio, increasing when the ratio increases and
vice versa.
A
In order to do this, on a revolving drum of hard wood, ebonite or other suitable
insulating material is coiled like a double threaded screw, two platinum Silver
wires e and f (figure I) one forming the ratio the other C.
.range
To make the arrangement of the Bridge more sensitive and to increase the r
of the instrument, proportional coils are connected to both ends of the ratio wire
(fig. IV) and marked 30 ohms, 1 ohm and 1 ohm. These coils are fixed securely in
the body of the drum and an arrangement for." plugging" in or out these coils is
placed at the ends of the drum (fig. I) and acts by simply screwing up or down
three terminals similar to p.
Extra resistance is added to C (fig. IV marked 300) and is placed in the jockey
(not described yet) with a similar arrangement as P for "plugging" it in or out.
•
In fig. III is shown a simple method of recording the number of revolutions and
fraction of a revolution that the drum passes through from zero position until a
balance is obtained by means of which the result is obtained. Parallel with the
axis of the drum is mounted a bar on which travels a jockey (figs. I and II
marked D fig. II) carrying two insulated springs and at the end of these springs
are fixed two rolling contacts either platinum or faced with platinum which press
against the two platinum-Silver wires mentioned above.
The insulated springs are connected to terminals on jockey as shown (figure IV
the jockey is made to travel up and down the bar by means of a projection piece K
(figure II) fitting into a groove cut in the drum of the same pitch as the wires
(shown figure 1) or by an alternative method the jockey may be worked up and
down by having a thread cut on the bar which is actuated by suitable gearing from
the main spindle of the drum.
Figure IV is a general diagram of connections. The outer connections with the
wires on the drum are made through the bearings as shown in fig. IV and pass to
terminals on the stand of the instrument,

page_text: The manner of connecting up this instrument for measuring an unknown
resistance is as follows.
One end of the ratio wire (B figure IV) including its proportional coil is
permanently connected to that end of the wire forming C. which is farthest from
it on the drum. This ensures C. increasing or decreasing with the ratio // the
other end of the wire forming C is left free. One end of the ratio wire including
its proportional coils is connected to one bearing and the other end of the ratio
wire including its proportional coil is connected to the other bearing.
The battery and galvanometer are connected up in their usual places, in reference
to the other arms of the bridge as shown in figure IV.
The unknown resistance joins the ends of A. and C. and is connected as shown
in figure IV. In order to measure an unknown resistance the drum has only to
be turned by means of a handle and the galvanometer watched until a balance is
obtained when the result is obtained by observing how many revolutions the drum
has passed through from zero position and on reference to a pre-calculated list the
resistance required will be found opposite the figure expressing the number of
revolutions. The result is calculated from the well known formula X = C.
Another part of my invention is as follows. In the case of a variable-resistance a
strip of paper is made to pass under the drum at a uniform rate by means of
clock work and an arın fixed to the jockey carries a' pencil which presses upon the
paper forming a diagram which corresponds exactly with the movements of the
jockey, from this diagram the average resistance can be calculated for any length
of time.
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
A
B
I. Making the ratio increase or decrease continuously and at the same time
making C increase or decrease continuously, increasing when the ratio increases
and vice versa.
II. In connecting proportional coils to one or both ends (as may be desirable) of
a wire which forms the ratio wire.
III. Placing the proportional coils with their plugging in arrangement in or on
a revolving drum.
IV. Winding the two platinum-Silver wires forming the ratio wire and C. parallel
to one another (similar to a double threaded screw) on a revolving drum.
Dated this First day of August One thousand eight hundred and eighty five.