Improvements in Electrical Recording Instruments.
Year: 1897

page_text: PROVISIONAL SPECIFICATION.
Improvements in Electrical Recording Instruments.
I, HUGH LONGBOURNE CALLENDAR, Professor of Physics, 62, Hutchison Street,
of the City of Montreal, Province of Quebec, Dominion of Canada, do hereby
declare the nature of this invention to be as follows:-
"
In the so-called "Null Methods" of electrical measurement, the value of the
quantity to be measured, whether resistance E.M.F. or current, is often obtained
by finding the balance point" on a bridge-wire or other suitably subdivisible
resistance, at which when contact is made with one terminal of a properly con-
nected galvanometer or indicator, no deflection is observed. If the contact is
shifted to one side of the true balance point, a deflection in one direction may be
obtained, which is approximately proportional to the deviation of the contact
from the balance point. If the contact is shifted to the other side, a similar de-
flection in the opposite direction is obtained.
The object of the present invention is to apply these methods to electrical re-
cording instruments. It is evident that if a recording pen is attached to the
point of contact on the bridge-wire, in such a manner that the movements of the
pen are the same as those of the contact piece, and if the contact piece be moved
in such a manner as to be always at the balance point, a complete rectangular
record of the variations of the quantity to be measured may be obtained on a
drum revolved by clockwork in the usual manner.
In order to cause the contact piece to follow automatically the variations to
be recorded, the galvanometer is arranged so that the deflection completes an
electric circuit, which has the effect of setting in motion an electro magnetic
apparatus, which moves the contact piece in such a direction as to restore the
balance, and reduce the deflection of the galvanometer to zero.
For cases such as the recording of the variations of voltage or current on an
electric circuit, in which considerable power is available, the galvanometer may
consist simply of a polarised relay or other equivalent device, which by making
contact one way or the other, sets in motion a reversible electric motor for moving
the recording point.
For cases in which a much more delicate galvanometer is required, and the
force available for making contact is very slight, special precautions are necessary
with respect to the contact making device, and it is advisable to use clockwork
for supplying the power for moving the pen and the contact piece. In these
cases, the stop or stops with which the moving needle of the galvanometer makes
contact, are kept in continuous motion, to prevent the sticking of the needle,
and are also continuously scraped by suitable brushes so that the contact surfaces
may be perfectly clean. It is also in many cases advisable to impart a slight
motion of oscillation to the galvanometer needle itself, by means of a fan, or other
similar contrivance.
For moving the contact piece along the bridge-wire, it is most convenient to
use an endless cord attached to the contact piece and passing round a pulley
geared to the motor or clockwork, so as to turn at a convenient speed. For the

page_text: most delicate recording apparatus, the galvanometer or relay contact, is made
to actuate a pair of electromagnets or a polarised relay, controlling two brake
wheels connected through a differential gear to the spindle which moves the re-
cording point. The relay contact actuates an electro magnet which permits either
one brake wheel or the other to turn, according to the direction of the galvano-
meter deflection, and the spindle of the differential gear is turned either one
way or the other, according to the direction of motion of the brake wheel which
is released. The brake-wheels may be replaced by ratchet wheels worked directly
by the electromagnets, but it is in most cases more convenient to actuate the
brake wheels through a train of wheels by means of a spring or a weight in the
usual manner.
The method above described can be used to record the most delicate variations
of electric potential, and therefore indirectly of resistance and current. It has
also been applied to the recording of temperature by means of electrical resistance
thermometers and thermocouples, using the Wheatstone bridge and the potentio-
meter methods respectively. A galvanometer with a relay contact, constructed
as above described, can also be used for recording purposes and as a relay on
a submarine cable.
In case it is required, as in an electric meter, to record the time integral of the
current or power, an integrating wheel moving over the surface of a revolving
disc or cone may be attached to the bridge-wire contact in place of or in addition
to the pen. By connecting a set of counter dials to the integrating wheel, the
total amount may be indicated in the usual way.
In order to adjust the scale of the record to an exact value, such as one centi-
metre to the degree, or to the volt, the bridge wire may be shunted with an adjust-
able resistance of suitable magnitude. In order to prevent slipping of the cord
on the pulley which works the sliding contact piece on the bridge-wire, it is con-
venient to cut a screw thread on the surface of the pulley, in the groove of which
two or three turns of the cord are laid.
Dated this 14th day of Tu1, 1907
CARPMAEL & Co.,
Agents for the Applicant.
Improvements in Electrical Recording Instruments.
I, HUGH LONGBOURNE CALLENDAR, Professor of Physics, 62, Hutchison Street,
of the City of Montreal, Province of Quebec, Dominion of Canada, 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:—
a
The method of recording electrical quantities herein described is directly ap-
plicable to those cases in which the measurement may be effected by sliding
contact piece along a wire, or other uniformly subdivisible resistance forming
part of an electrical circuit, until a point is found at which the value of the electric
potential is the same as at some other given point. This equality of potential
is ascertained by connecting a galvanometer, or other suitable indicator, between
the points in question. The wire along which the readings are taken is called
a bridge-wire, and the point at which the current through the galvanometer is
reduced to zero, is called the balance-point. By attaching a pen or other re-
cording device, to the contact piece on the bridge-wire, and by moving the con-
tact in such a manner as to keep it always at the balance-point it is clear that
the variations of the quantity to be measured may be recorded. The improve-

page_text: ments herein described relate to the means by which the contact is automatically
maintained always at the balance-point, to the adjustment of the scale of the
record, and to the construction of the apparatus in such a manner as to make the
application as general and accurate as possible.
In order to maintain the balance automatically, the galvanometer, which is pre-
ferably one of the movable coil type, is provided with an arm, called the "relay-
arm," which actuates one or other of two electric circuits, according as the galvano-
meter deflects to one side or the other. The circuit so actuated starts a motor,
which tends to restore the contact piece to its position of balance. To prevent
sticking of the relay contacts, and to secure sufficient delicacy of action, the stop
or stops with which the relay arm makes contact are kept in a continual sliding
motion, the contact surfaces are also continually cleaned by the action of suitable
scrapers, and the relay arm itself is kept in a state of gentle oscillation by means
of a fan. The contact stop is preferably made in the form of a slightly eccentric
wheel, revolving by clockwork, and scraped on either side by a pair of light metal
springs. The relay arm A B in Figure I, is shown with a pair of contacts B', 13",
forming a fork on either side of the single contact wheel C. But it is equally
possible to use a pair of contact wheels on either side of a single contact point
on the relay arm. Scraping springs of suitable form are shown at D.
The bridge-wire shown at A B in Figure II, is soldered at the ends to short
pieces of thick wire, A and B, which are held by set screws in the insulating
blocks E G and F H. The bridge-wire itself is made of platinum-silver, or other
suitable material, having a resistance of one or two ohms per metre. Its resistance
is adjusted to an exact value by shunting it with a resistance S of similar material,
which is adjusted so that the scales of different instruments may be commensurable.
The thick wires A and B at either end, serve for making connections, and also for
adjusting the zero point of the wire with reference to the scale on the recording
sheets. When the bridge-wire is connected on the principle of the "potentio-
meter," as described below, it is frequently desirable that the zero point, corre-
sponding to zero difference of potential, should be at the centre of the wire. It
would not be easy or convenient to make a connection to the centre of the bridge-
wire itself. The same effect is secured in a more simple and exact manner by
making connection to the middle point of the shunt S. Connection is made
between the bridge-wire and the galvanometer, by means of a wire of similar
material, CD, which is called the galvanometer wire, and is stretched alongside
the bridge-wire. The sliding contact is preferably made by means of light
springs carrying short pieces or rollers of platinum-iridium, as shown at M N.
These springs serve also to maintain a suitable pressure of the point of the re-
cording pen P on the revolving drum. The pen and the contact springs are
attached to a sleeve K L which slides along a rod E F. The tension of the bridge-
wire and galvanometer wire is maintained uniform by means of the metallic
rods E F and G H, which are clamped in the insulating blocks E G, F H, but
free to slide through holes in the block R. T. The blocks E G, R T, are fixed
to the base of the instrument, but the block F H is free to slide. The travelling
sleeve is provided with stops K and L, which engage with small spring switches V
and W, at either end of the travel. The switches V and W serve to break either
electric circuit automatically and to prevent the recording point being carried
off the limits of the scale, while leaving it free to return under the action of the
opposite motor.
The mechanism for moving the contact piece on the bridge-wire in either
direction according to the deflection of the galvanometer, preferably consists of
a pair of clock motors connected by means of a suitable differential gear. The
relay contact operates an electro-magnet which releases a brake-wheel, and permits
the corresponding motor to turn, and to move the bridge-wire contact in the
required direction. The differential gear by means of which the contact piece is
moved, may
consist of the usual combination of gear-wheels, and the motion may
be transmitted to the contact piece by means of a cord and a pulley as shown in

page_text: Figure II. The same effect may also be produced by means of the differential
system of pulleys shown in Figure III. In this diagram, A and B represent
pulleys connected to the clock motors so as to turn with suitable speed and power
when the motors are released by the corresponding electromagnets. C and D
are the differential pulleys. E is an idle pulley attached to a spring for keeping
the necessary tension on the cord. An endless cord passes round the pulleys A, C,
B, D, in order as shown. The pulleys C and D are connected by a cord passing
round the pulley E. It is plain on inspection that if the motor A starts to turn,
the pulley D is moved to the right, and C to the left. But that if the motor B
starts in the same direction, the opposite takes place. The contact piece on the
bridge-wire may be connected to either of the pulleys C or D, provided that the
galvanometer connections are made to correspond.
Connected to one end of the bridge-wire are a series of auxiliary resist-
ances XYZ etc., which are adjusted to represent 10, 20, 40, etc., or other suitable
numbers of divisions of the scale of the bridge-wire. These resistances serve for
extending the scale of the bridge-wire in the usual manner, and are provided
with plugs or screw nuts by means of which they can be short-circuited. For
resistance measurements on the Wheatstone bridge method, a pair of ratio
coils, QJ, JU, is also added. The galvanometer is then connected at J and M,
and the resistance to be measured at I and U. The place of these resistances
may be supplied if preferred by ordinary commercial resistance boxes, they are
not essential parts of the apparatus, but it is generally convenient to include
them in the same box.
The recording apparatus above described may be used for a variety of very
different purposes, without any modification in its essential features, by connecting
it to different pieces of apparatus, or different electric circuits, according to
methods which are well understood.
If the bridge-wire is connected in series with auxiliary resistances and a suitable
rheostat to a constant battery, the instrument may be used as a potentiometer,
for recording variations of electric potential. The difference of electric potential
to be recorded, would then be applied to the zero point on the auxiliary resist-
ances or shunt, and to the free terminal of the galvanometer (one terminal being
already connected to the contact point M on the bridge-wire). The difference of
potential to be measured may be that produced by a thermocouple, in which case
the record represents variations of temperature; or may be some suitable fraction
of the electric pressure on a direct current lighting or power circuit, in which
case the record gives directly the variations of the electric pressure. Variations
of current may be similarly recorded by the usual method of measuring the dif-
ference of potential on a known resistance through which the current is passed.
In order to record rapid cyclical variations of potential or current in the same
manner, it is only necessary to include a periodic contact in the galvanometer
circuit, to complete the circuit at a definite point of
the cycle.
the point of the cycle corresponding to the position of the contact
brushes be slowly and uniformly shifted, the recording pen will trace
the curve of the cycle. The forms of alternating current and potential
curves of dynamo machines may be automatically traced in this manner. By
means of a suitable thermopile, the cyclical variations of temperature in the walls
of the cylinder of a steam or other heat engine, may be similarly recorded. In
recording cyclical variations of this nature, it is convenient to connect the dif-
ference of potential to the middle point of the shunt, in order that the zero may
be at the centre of the bridge-wire, as explained above.
If
A wide range of applications of this recording system is afforded by applying
the instrument to record variations of electrical resistance. For this purpose
the bridge-wire is connected on the Wheatstone bridge method as explained above.
A battery is connected to the points Q, U. This battery may conveniently be the
same as that used for actuating the relay circuit. If the resistance to be measured
consists of an electrical resistance thermometer the record will represent the vari-

page_text: ations of temperature of the thermometer, and may be made to cover any desired
range, or to suit any desired purpose, by choosing a thermometer of suitable
resistance and construction.
or
the
If an alternating difference of potential be simultaneously applied to the ther-
mometer, and the wire be sufficiently fine to be heated by the current, the recorded
thermometer
variations of temperature of the resistance
will give
variations of the alternating voltage. The variations of an alternating
current may be recorded in a similar manner, by passing the current,
some suitable fraction thereof, through the wire of the thermometer.
For this purpose it is convenient to make the thermometer consist
of two wires in parallel, as shown in Figure IV, and to connect the alternating
current terminals A, A, to the centre of each wire, so that the alternating current
will not pass through the other resistances forming the Wheatstone bridge to
which the thermometer is connected at B B¹. If the resistance consist of a fine
platinum wire in a vertical tube containing mercury, changes of level of the
mercury will alter the length of wire exposed, and may therefore be recorded by
the same apparatus. This application is useful for recording variations of baro-
metric or steam pressure, and variations of water level etc.
If a pair of differential electrical thermometers be connected on either side of
the bridge-wire, the record will give the difference of temperature. It is thus
possible to record in a single curve the difference of temperature for instance
between two stations at different altitudes, or between the wet and dry bulb, ther-
mometers. Again, if one of the wires be blackened, and both be exposed to radi-
ation, such as sunshine, or the radiation of a furnace, the record represents the
intensity of the sunshine, or of the furnace.
It should be observed that all these different records may be obtained by means
of one single instrument, by simple changes in the electrical connections and in
the apparatus with which it is associated.
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. An electrical recording apparatus, substantially as above described, in which
a pen or other recording device is directly attached to the sliding contact of one
terminal of a galvanometer or relay on a straight bridge-wire, and is automatically
moved in such a manner as to be maintained always at the balance-point, at which
the electrical potential is the same as that of the point to which the other
terminal of the galvanometer is connected.
2. In an electrical recording apparatus, the use of a relay in which the contacts
are kept in a state of sliding motion or oscillation, and are continually scraped
by suitable brushes.
3. In an electrical recording apparatus, the method of moving the recording
device in either direction by means of a pair of clock motors, controlled by a relay,
and connected to the recording point through a differential gear, substantially as
described.
4. In a
bridge-wire method of electrical measurement, the adjustment of the
scale of the bridge-wire to an exact value by means of a shunt of suitable resistance
connected as described.
5. Electrical recording apparatus substantially a
as described and shown in the
drawings.