An Improved Method of and Apparatus for Measuring Electric Currents.
Year: 1891

page_text: Date of Application, 26th May, 1891
Complete Specification Left, 26th Feb., 1892-Accepted, 23rd Apr., 1892
PROVISIONAL SPECIFICATION.
An Improved Method of and Apparatus for Measuring Electric
Currents.
I, ALBERT CAMPBELL 66 Patshull Road, Camden Road, London, N.W.
Student of Science, do hereby declare the nature of this invention to be as
follows:-
The object of this invention is the measurement of electric currents, and thence
lifferences of potential, by means of the expansion of a fluid partially contained in
a thin-walled hollow conductor, part of which conductor is heated by the passage
of the electric current.
In the simplest form, the hollow conductor is a long thin tube. For strong
currents, a number of long thin tubes, arranged in parallel, may be employed, and
by similar means a number of currents or potential differences may have their
values added or subtracted. One of the arrangements used is an ordinary
differential air thermometer, in which the two bulbs are each replaced by a thin
metallic tube or tubes, one or more of the tubes being heated by the current.
Dated this 25th day of May 1891
ALBERT CAMPBELL.
COMPLETE SPECIFICATION.
An Improved Method of and Apparatus for Measuring Electric
Currents.
I, ALBERT CAMPBELL of Ballynagard House, Londonderry, in the County of
Londonderry, late of 66 Patshull Road, London, Electrical Engineer, do hereby
declare the nature of this invention and in what manner the same is to be
performed, to be particularly described and ascertaid in and by the following
statement:—
This invention is designed for the measurement of electric currents and, as some
forms of the apparatus are remarkably free from inductive effects, it may be used
with particular advantage for measuring alternating currents.
The main physical principle made use of is the tendency to expansion of a fluid
either gaseous or liquid whose temperature is raised by receiving heat from a
conductor heated by the passage of the current to be measured.
This principle has already been employed in several instruments such as that
described by D'Arsonval in "La Lumière Electrique " October 18th, 1884. In
that and similar instruments promptness of action and precision of measurement
cannot be attained because of two defects. These defects are:-Firstly, the fluid
is contained in a vessel of glass or other material which, as well as the fluid, has to
be warmed up to a steady state of temperature before the indication of the instru-
ment becomes constant; Secondly, there is great difficulty in conveying the
current to a conductor inside the vessel containing fluid, for a thin leading-in wire
heats the joint and a thick one takes too long to attain a steady temperature. My
invention entirely removes these difficulties in the following manner :-
The fluid is contained wholly or partially in a hollow vessel and part or parts of
the vessel itself forms or form the conductor along which the current to be
measured passes. I generally make the vessel in the form of one or more metallic
tubes of small diameter and thin in the walls.
These tubes are put with their interiors in connection with one another and with
an arrangement for indicating the amount of change of volume or pressure (or of

page_text: The whole combination I shall call a hot-tube instrument.
There are several different ways of carrying out my invention according as the
fluid is gaseous or liquid and according to the gauge employed. I shall describe
four types of the apparatus, reference being had to the accompanying drawings and
the letters and figures marked thereon.
'
Like letters refer to like parts in all the figures.
Type 1 is represented in Figs. I., II., III., IV.
Of these,
Fig. 1. is a diagramatic sketch showing the connections (pneumatic and
electric).
Fig. II. is a front view.
Fig. III. is a side view with parts of the case and base removed.
Fig. IV. is a back view with the back and sides of case removed.
Figs. II., III, IV. are half full size.
B is the base of the instrument made of insulating material, A is a support of
wood to which is fastened the gauge g. This gauge consists of a bent glass tube,
one limb of which is finer in the bore and longer than the other. In some cases
the gauge has equal limbs. The upper ends of both limbs are bent back so as to
pass through holes in the board A. The lower part of the glass tube contains a
liquid such as petroleum, which has low vapour pressure at ordinary temperatures,
and does not act much on metals. If petroleum be used it should have stood over
sodium for some time. The liquid may be coloured by a suitable colouring
material.
Into the end of the glass tube at F is fastened, by a wax or platinum joint to be
described below, a fine metal tube leading to the system of thin metal tubes a₁, a
3. 4. connected with one another by the fine-bore glass tubes d₁, d₂, dy. I shall
henceforth call such a connected system a "tube-system." The tube system a: 2,
3. 4, contains air or other gaseous fluid.
The current to be measured is passed along the middle portions of the
tubes a, a, a, a, being brought to them by conductors in electrical connection
with five binding screws 1, 2, 3, 4, 5, symbolically shown by 1, 2, 3, 4, 5, in Fig. III.
The connections to the tubes are made by conducting strips or wires shown in the
figures by wavy lines like h. The current may be sent firstly from terminal 1 to
terminal 3 in which case, it will traverse the four tubes in series; secondly it may
enter by 1, 2 and 3 united, and emerge by 4 and 5 united, in which case the 4 tubes
will be in multiple-are and if the parts of the tubes traversed are of equal resistance
the sensitivity of the instrument will be reduced to one fourth of that in the first
case.
The end of the tube a, is connected by means of the stop cock E with another
system of similar thin tubes b₁ to b. These have approximately the same internal
volume as the system a₁, a₁₁ az, a. Likewise the volumes of air in the limbs of
the gauge should be equal. Sometimes I add to that system which has the slightly
lower volume a small tube in which an oiled screw can be screwed in or out in
such a way as to increase or diminish the total volume of the system. The tube b₁
is connected with J, the upper end of the gauge. The system to b₁ may have
the same arrangement of the terminals as the system a, to a, but these are only
necessary for a special purpose to be described below.
The two tube systems are screened by a case of thin sheet metal made in two
parts D and H.
At the top and bottom of the case are a number of small holes.
In some cases I add an outer case similar to that shown. The inside of the case
and the outside of the tubes are blackened.
G is a scale, the graduations being found by comparison with a standard
instrument.
The thin tubes may be made by folding a narrow strip of thin sheet metal round
a straight wire and then rolling between smooth surfaces. The mandrel is then

page_text: withdrawn and the resulting tube soldered along the seam. This gives uniformity
of bore in a set of tubes. The sheet metal which I employ is of thickness about
10 inch and upwards.
The joints where the metal and glass tubes are united may be made in either of
two ways: (a) As in Fig. X. (in section, four times full size) where the metal
tube is inserted in the widened end of the glass tube and the whole made air-tight
by means of a suitable cement. When petroleum is the liquid used I employ
common sealing-wax and over it soft wax. (b) As in Fig. XI. (in section, four
times full size) where a small cone or tube of platinum is fused to the glass at r
and is then soldered to the metal tube at s s.
Fig. IX. shows a convenient way of making wax-sealed joints between metal.
tubes. The wax is melted between the flanges and does not run into the tubes.
In Figs. II. II. IV. I show four tubes in each system but each system may
contain any convenient number of tubes (the same number for both) and these:
tubes may be grouped in different arrangements of series and parallel other than.
that shown above. Sometimes I replace all the glass pieces d₁, dg, dzy de, by
metal tubes with soldered joints. The tubes are not necessarily vertical. Another
way of uniting the tubes into systems is shown on an enlarged scale in Fig. XII..
In this the tubes are joined by the glass connections 1, 2, 3, to a branched inetal
tube t which leads at one end to the gauge and at the other to the opposite system
by way of the stop-cock.
When it is desired to reduce the effects of self and mutual induction to a
minimum either of two devices may be employed. These are shown in section in
Fig. XII. 1st. w is a conducting strip close to the tube to which it is joined at
the top when the tube is closed. The current enters at 6, goes up w, down the
thin tube and out at z. 2nd. An outer tube v surrounds the thin tube with an air
space between and is joined to it at the top where the thin tube is closed.
current, led in at 21, goes up the tube v, down the thin tube and out at .
The
The instrument acts as follows. 1st. When the terminals 1 and 3 only are, used..
The current enters by 1, passes up tube a, down tube a,, up tube a,, down tube α,
and leaves by terminal 3. The current raises the temperature of the tube-
system a, a, a, a, until a steady state of radiation; convection and conduction is
quickly arrived at. The metal case will rise slightly in temperature, but the
temperature of the tube-system b₁ to will rise by approximately the same amount..
Accordingly the rise of the liquid in the gauge will depend on the difference of
temperatures of the tubes a, a, a, and the surrounding enclosure. The action
is similar when a₁, a, a, a, are in multiple-arc.
As the temperature of the room rises there is a slight falling off in the sensitivity
of the air-expansion; but as this is of the same order of magnitude as the increase
of sensitivity due to increase of the resistance of the metal tubes when those are of
a pure metal like copper. by choosing a suitable metal or alloy for the tubes, the
error may be minimized.
A scheme of type 2 is shown in Fig. VIII. In this the upper parts M₁, M2, of
the glass tubes L₁ and L, contain air or other gas or vapour, and all the remainder
of the interior of the tubes (left unshaded in the diagram) is filled with a liquid
The scale can
similar to that used in type 1. The stop cock E is not essential.
slide up and down. The two tube-systems are cased in and have terminals exactly
like those in type 1. At each reading the zero of the scale must be set to the level
of the liquid which stands lowest.
Types 3 and 4 are shown in Figs. V. VI. and VII., Figs. V. and VI. are half
full size.
Fig. V. is a front view of the instrument with part of the scale removed to show-
the inside parts.
Fig. VI. is a sectional view on the line N N of Fig. V.
Fig. VII. gives enlarged views of the parts k, p, 71, 72.
Q is a brass case, closed air-tight. by the glass R, beld on by the metal ring S and
screws to the flange Y, a little wax being melted between the glass and the flange.

page_text: The screws T, T, are soldered into the side of the case Q, and a block W of
insulating material of the same depth as the case Q is screwed to it by the nuts
on T, and T. The block W carries (screwed to it) the case D similar to that in
Fig. 11. but with holes above and below. This contains two systems of tubes
similar to those in Figs. II. and IV. with the addition of Z, a light framework of
ebonite to support the ends of the tubes. The terminals are at 1; 2, 3, 4, 5.
A is an axle carried, in jewelled bearings, by the pieces n₁, and ng. This axle
carries a small cylinder p, partly round which are wrapped two exceedingly thin
metallic strips 1. and 9, one end of each strip being fastened to the cylinder as
shown in Fig. VII.
71
and
to
The other ends of 72 are fastened by means of spiral springs m₁
and ma
the upper ends of 21 and which are two thin flat curved tubes made of thin
elastic metal. These 1 shall call "gauge-tubes." The tubes y₁y, connect them
to the two systems of tubes in the case D. ₁, and y, pass through the side of
the case Q, the holes being soldered to keep airtight, and through holes in
block W.
21 and are held by the brackets, & /, screwed to the back of Q by nuts
behind, for purposes of adjustment All screws passing through the back of Qare
made air-tight with wax. If one of the gauge-tubes is more powerful than the
others, an adjustable compensating spring m may be fastened to it.
The pivot carries also the pointer i and the counterpoise j.
A small stop-cock may enter the case Q that the pressure of the internal air may
be adjusted.
In type 3 all the tubes and the gauge-tubes contain air (or other gas), which in
type 4 they are entirely filled with liquid (such as petroleum).
The mode of action of both types 3 and 4 is as follows:
The current to be measured enters one or more of the terminals 1, 2, 3, 4, 5,
passes along the tube-system in communication with the gauge-tube, raising the
temperature of the tube-system and causing the fluid to expand. This expansion
makes the gauge-tube unbend and the movement is transmitted to the light
pointer by means of the spring me, the strip g, and the cylinder p and axle k.
The case is made air-tight to avoid the action of barometric changes upon the
gauge-tubes.
Any of the various arrangements above described of the tube-systems may be
employed in types 3 and 4 as well as in types 1 and 2.
The instruments described may be used as voltmeters by making the tube-
systems of sufficiently high resistance and long enough. Nearly all the requisite
resistance must be supplied by a suitable resistance coil in series with the tube-
system.
These hot-tube instruments may also be used to obtain the sum of the mean
squares of a number of different currents and to subtract from it the sum of the
mean squares of another set of currents. For this purpose all the tubes are
arranged non-inductively as in Fig. XII. The first set of currents are sent
respectively through single tubes or sets of tubes belonging to one side of the
gauge and the second set of currents similarly through those of the other side of the
gauge. The deflection gives the difference required. Thus the instrument may be
conveniently used to measure the power taken by an inductive resistance in an
alternate current circuit after the manner of the "Three Dynamometer Method"
or of the "Three Voltmeter Method," and so can be employed as a Watt meter.
To protect against too high currents a safety-fuse may be added to the
instrument.
Having now particularly described and ascertained the nature of my said inven-
tion, and in what manner the same is to be performed, 1 declare that what I
claim is :-
1. In electrical measuring instruments an arrangement characterized by two
systems of hollow conductors containing an exnansible fluid or fluids the individual

page_text: conductors being connected into the systems by means of insulating tubes, con-
ducting leads whereby the electric current or currents to be measured can be
passed along a part or parts of some or all of the hollow conductors, an outer case
or cases surrounding the conductors, a differential gauge connected with the two
systems, which gauge serves to show the difference of volumes or of pressures of
the portions of the fluid contained in the two systems.
2. The use in connection with the above arrangement of a gauge consisting of an
airtight chamber containing two flat elastic tubes a₁ 2a, spring m, mg ms, strips 41 923
cylinder p, axle, pointer and scale substantially as described above.
3. In the same connection, the use of a gauge formed of a U-tube y containing
liquid in the lower part, with a scale behind the tube; with such a gauge a gaseous
fluid is used in the tube systems.
4. The use in arrangement in Claim 1 of a gauge consisting of two glass tubes
closed at their tops and containing gas or vapour above the liquid which fills the
rest of the interior of the two tube systems.
5. In instruments such as are described in Claim 1, the arrangement of the
hollow conductors in the form of very thin tubes with a view to promptness of
action.
6. In instruments such as are described in Claim 1, the arrangement of the
hollow conductors in multiple-are for the purpose of being thereby able to measure
large currents without undue heating and with promptness of action.
7. For the purpose of minimizing inductive effects in arrangements as above
described, the arrangement of the hollow conductors in the form of thin tubes to
each of which the current can be conveyed by another conducting tube surrounding
a part of that tube.
8. In above described instruments, the arrangement of the conductors non-
inductively as in Claim 7 and at such distances from one another as to be quite
independent of one another both electrically and thermally in such a way as to be
able to find the difference between the sums of the mean squares of sets of
currents; thus enabling the instrument to be used as a Watt meter (in some cases)
in which case there are two currents in one system and one in half of the other
system.
Dated this 25th day of Kahruary 1899
ALBERT CAMPBELL.