Improvements in or relating to the Method and Apparatus used in Electrostatically Measuring Electric Currents in a Working Circuit and Calibrating the Measuring Instruments so Employed
Year: 1899

page_text: Date of Application, 18th Oct., 1899-Accepted, 23rd Dec., 1899
Improvements in or relating to the Method and Apparatus used in
Electrostatically Measuring Electric Currents in a Working
Circuit and Calibrating the Measuring Instruments so Employed,
I, GEORGE LEONARD ADDENBROOKE, of 53, Victoria Street, Westminster, in the.
County of London, Electrical Engineer, do hereby declare the nature of this inven
tion and in what manner the same is to be performed, to be particularly described
and ascertained in and by the following statement:-
My invention relates to the method and apparatus used in electrostatically
measuring electric currents in a working circuit, and calibrating the instru-
ments so employed, the object being to avoid certain difficulties or drawbacks
met with in general practice when measuring the said currents, and to ensure
greater exactness in the results obtained.
Owing to the delicacy of construction and adjustment of the electrostatic instru-
ments used for making fine measurements of electricity, it often happens that
when the measurements are taken outside of the laboratory in which the instru-
ments were standarized or calibrated, the shocks sustained on the journey to the
place of use render them inaccurate. To ascertain if any change of adjustment
has taken place, the instruments are sometimes returned to where they were
originally calibrated, and thus a double error may be introduced, so that any
readings obtained are of doubtful accuracy.
By my invention, I am enabled to test all the instruments employed at the
place of use under similar and working conditions, and to immediately proceed
to take the measurements required without any danger of disturbing the standard
adjustments arrived at. This I am enabled to do with great facility by the aid
of an apparatus designed for this special purpose, and further to repeat the tests
immediately after the measurements are taken: consequently the results arrived at
are not only accurate in themselves, but all doubt as to their accuracy is eliminated.
The invention is applicable for use in measuring alternating or continuous
currents.
The apparatus referred to above, and the connections used, are indicated by
the diagrams shown in the accompanying drawings to which reference is made.
Fig. 1 is a diagram of a switchboard apparatus and connections used in calibrating
the instruments employed, and in measuring the factors or elements of an
alternating or 'continuous current. Fig. 2 is a diagramatic arrangement of
similar apparatus and connections for measuring the elements of an alternating
or continuous current, but having the four switches shown in Fig. 1 combined to
form two switches only; Fig. 3 is a similar diagram to Fig. 1, on an enlarged
scale, but showing certain modifications in the switches and their connections;
Fig. 4 is a general elevation of the apparatus employed, showing the instru-
ments, lamp, switchboard apparatus, and adjustable scales on which the readings
from the reflecting mirrors of the instruments are taken; and Fig. 5 is a
plan of Fig. 4.
In measuring alternating currents, if there is inductance or capacity in the

page_text: circuit, it is found that the current which flows is influenced by three factors.
1. The resistance of the circuit and the work done within it. 2. The current taken
to produce the magnetic field created by the flow of current itself, and 3. The
current required to charge the electrostatic capacity in connection with the
circuit.
The magnetic and electrostatic effects being exactly opposite in phase counter-
balance each other as far as may be the larger factor however, predominating,
and producing a resultant effect either 90 degrees behind or in advance of the
phase of the energy current, according as the self-induction or capacity pre-
dominate. This resultant, current, when further combined with the energy
current, produces the compound current actually flowing through the circuit,
which, evidently, will not be in phase with any of its components. Therefore,
to measure the components of such currents, it is necessary to adopt some prin-
ciple which will separate either the self-induction, or the capacity current, (which-
ever predominates), from the energy current either by direct measurement, or
by inference.
In considering this, three leading principles are found on which methods for
measuring alternating currents may be based, and these are termed :
1. The three voltmeter principle.
2. The three ammeter principle.
3. The wattmeter principle.
The theory of these principles being explained in leading text books, they need
not be described in detail here.
Con-
In practice, there are certain disadvantages connected with the first and second
principles named, so that, while they may sometimes be used, the third or watt-
meter principle is the most convenient to employ in taking measurements.
sequently, and while by slight and obvious modification my method and apparatus
may be used under all three of the principles named, the following description
especially refers to the method I have founded on the aforesaid third or watt-
meter principle. In employing this method, three measurements must be made,
being,
(1). The impressed volts.
(2). The apparent amperes.
(3). The effective amperes, or the effective watts. But if the effective watts
are measured, and knowing the effective volts, the effective amperes can be
deduced.
Knowing these qualities, we can, in addition, deduce the power factor, the angle
of lag, and the magnetising or capacity current or their resultant, if both are
present. Also, if we know the periodicity of the alternations, other character-
istics are also obtainable. Finally, if inductance and capacity are persent, by
measuring one of them, we can with the above data find the other.
To carry out this method in an accurate and expeditious manner, I have
devised the apparatus already referred to, by which the measuring instruments
employed may be readily compared with a standard, and calibrated in a few
moments at any time, without altering or disturbing the apparatus and its con-
nections otherwise than by moving plugs or switches on a switch-board. In con-
nection with this apparatus I employ three instruments of the electrostatic type;
a wattmeter, a voltmeter, and an ammeter; and when a high degree of accuracy
is required, the instruments are made reflecting to read off upon a suitable scale
or scales.
The apparatus is provided with a suitable set of switches and terminals, having
indicating marks thereon, arranged as described hereinafter with the necessary
connections from each of the instruments, and means for separately standard-
izing each instrument, as well as for connecting it to a circuit on which measure-
ments are to be made. In addition, the apparatus comprises a standard cell,
(preferably of the "Clarke" type): a resistance for varying the ratio of resistance
such as a potentiometer or Wheatstone" bridge, ranging say to 10,000 ohms. ;
"

page_text: a secondary battery, or other source of electro motive force, of suitable voltage;
and a set of adjustable non-inductive resistances, made of short lengths of wire
or strips of metal having a low temperature co-efficient. The resistance of these
lengths must be accurately measured, and (when sensitive instruments are used)
is conveniently such that one volt may be dissipated between the two ends without
much heating, when the normal current for which they are intended is passed
through them. An omnibus or common terminal bar, with suitable means for
connecting thereto, is provided; and the apparatus is ready for use as indicated
by the diagram Fig. 1.
In the diagram Fig. 1, A indicates an alternating current generating machine,
having a main circuit A', wherein is placed a non-inductive measured resist-
ance a proportioned to the sensitiveness of the instruments used. From the
point a, a connection is shown to an omnibus or common terminal bar B, to which
most of the circuits hereinafter mentioned are joined. These are conveniently
arranged and shown in the following order. A standard cell C, (such as a Clarke
cell) a battery (or other source of electro motive force) D, of sufficient power to
give the maximum degree of voltage required; a ratio resistance adapted to
supply any required voltage to the instruments; a short circuit connection F
between the bar B and a terminals in each of the switches indicated below,
whereby any of the instruments may be short circuited; a voltmeter G, provided
with a switch S an ammeter H, provided with a switch St and a wattmeter I,
having two switches, S2 and S. The voltmeter switch S has four terminals, c, d,
s and the terminals short circuiting the instrument as above stated. With
the switch in contact with the terminal, the instrument is connected to the
battery D and similarly, when in contact with the terminal e, the switch puts
the instrument in connection with the ratio resistance E, so that a desired part or
ratio of the whole resistance can be measured off and any required voltage within
the limit of the battery put upon the needle g of the voltmeter G, its quadrant
plates being connected to the bar B. By this means, the voltmeter can be
calibrated throughout the whole range of its scale, as referred to hereinafter. The
terminal serves to place the needle g in connection with the circuit K, on
which measurements are to be made.
The ammeter switch S is similarly provided with four terminals, indicated
by the reference letters e, e, s and ¹. The terminal e likewise connects the
needie ¹ of the instruments H with the ratio resistance E, to put any required
voltage on the needle, and thus enabling the ammeter also to be calibrated
throughout its entire scale. The terminal e is for connecting the needle
with the standard cell C, the quadrant plates being connected to the bar B,
the terminals is for short circuiting the instrument, and the remaining terminal bi
connects the ammeter to the main circuit A', at the end of the measured non-
inductive resistance ab. The ammeter H is also provided with a working key h
adapted to short circuit the instrument through high resistance 3 of (for
example) 20,000 ohms.
a
The wattmeter I has a double set of switch terminals, one set, e, d, s and k for
the switch S, being exactly the same and with the same circuit connections as
above described for the voltmeter G. The other set for the switch $3 consists of
the terminal e, connected by the switch to one pair of quadrant plates, and
used for putting them in connection with the ratio resistance E as above stated,
a terminal c, to connect them to standard cell C, exactly the same as for the
ammeter H a terminal s, for short circuiting the quadrants of the instrument;
and a terminal, having the same connection to the end of the measured
non-inductive resistance in the main circuit, as described above for the ammeter H.
The switch S connects the needle with the first set of terminals, e, d, s and k,
as indicated in Fig. 1; and the additional switch, S3, connects the wattmeter
quadrant plates through the second set of terminals e, c, s and b¹. A key i,
similar to the key h, used with the ammeter, is also employed in working to short

page_text: circuit the wattmeter quadrant plates & through a high resistance (for example)
20,000 ohms.
The method of using the apparatus is as follows:-The wattmeter I is first
standardized by connecting the positive pole of the battery D through the terminal d
to the needle of the instrument, and the negative pole to one pair of quadrants
and the case, the ratio resistance E being placed between the battery terminals
as a shunt, with (for example) 10,000 ohms. out. The positive pole of the Clarke
or other standard cell C is connected to the quadrants, and the negative pole
of the standard cell is connected to the other quadrants and the negative pole
of the battery. This will produce a deflection of the needle (with its refiecting
mirror) and the amount of this deflection should be denoted upon an adjustable
scale, such as are shown in Figs. 4 and 5, or by other suitable means convenient
in working the instrument. I find in practice that if the electro motive force
of the battery is 100 volts, and that of the standard cell is 144 volts, a convenient
deflection is about 144 millimeters on a scale at about 2 metres distance. My
arrangements provide for putting the scales used on a support (as described
hereinafter) with means for adjusting the same through a suitable range, and if
the instrument is also adjustable for sensitiveness so that its reflecting mirror
gives a desired range of deflection, by a few trials and the adjustment of the
scale the instrument becomes direct reading.
Having noted the first deflection upon the scale after making the adjustments,
the connection from the quadrants of the wattmeter 1 to the positive pole of the
standard cell C is removed, and put across a portion of the ratio resistance E by
which the battery D is shunted. This resistance between the quadrant leads,
forming a portion of the whole resistance between the battery terminals, is
then increased or decreased by adjustment until the deflection reading on the
scale produced by the wattmeter mirror is exactly the same as that produced
by the standard cell C. The total resistance across the battery Dis
then noted, as well as the resistance between the quadrant leads, and the ratio
of one to the other gives the electro motive force of the battery D in terms of the
standard cell C, from which the number of volts is readily found.
The electro motive force of the battery D having thus been ascertained, and
that of the standard cell C being known, the instrument I is calibrated as a
wattmeter by calculation according to recognised formula, or practically as
follows. Assuming the electro motive force of the battery 1 to be 90 volts,
and the standard cell 144, then 90 x 144 = 129 6. The operator should then
so adjust the scale that with the battery D connected to the needle of the instru-
ment I, the reading on the scale becomes 129'6 divisions. Then if the voltage
on the needle is increased to 100 volts, while the quadrants remain at 144, the
reading would become 144 degrees if the divisions were proportionate; and the
instrument becomes direct reading. It is desirable to note, if great accuracy is
required, that for very widely varying readings and when the voltage
on the
needle approaches more that between the quadrants, there is a progressive altera-
tion in the constant of electrostatic instruments when heterostatically connected,
so that, for low readings of this character it is desirable to calibrate the wattmeter
Under these
Eomewhere in the neighbourhood of the voltages to be used on it.
circumstances if the quadrants are connected to a resistance inserted in a
circuit to be tested, the reading must be divided by the value of the resistance
in ohms, when the reading on the scale will directly indicate watts. Furthermore;
having obtained the electro motive force of the battery D, it is obvious that by
reconnecting the wattmeter to the terminal e of the ratio resistance E, and the
needle to the battery, the operator can reproduce any deflecion of the mirror
through the whole scale as obtained when the instrument was connected to the
circuit to be tested, and thus get its absolute value, instead of reading by de-
flection.
For instance, suppose the electrometer (or wattmeter I) when joined up to the
circuit gives a deflection of 175 degrees. The switches S and S are then put

page_text: on to the terminals d and e respectively which puts in the battery D and ratio
resistance E. The ratio is adjusted until the deflection of 175 degrees is re-
produced, when, knowing the value of the two ends or ranges of the ratio resist-
ance, and the electro motive force of the battery, we directly obtain the electro
motive forces required to produce the said deflection. By shifting the scale back
to a greater distance from the instrument and thus making the deflection visible
over a wider range upon the scale, or the insertion of a higher non-inductive
resistance in the circuit, an extreme degree of accuracy can be obtained.
The wattmeter I being thus calibrated, the voltmeter G should next be put
across the battery terminals by the switch S, through the terminal d to obtain
a deflection on the scale, and the instrument and the scale adjusted to obtain a
reading corresponding to the voltage of the battery, which has already been
accurately ascertained. If, however, by trial, the instrument employed does not
give proportionate increments for increasing voltages, a curve of deflections should
be made by aid of the ratio resistance E, and used to correct the calculations when
making the deflection scale.
In applying the apparatus to standardizing the ammeter H, the standard cell C
should be inserted between its terminals by the switch S¹, through the terminal c,
and a reading taken from the scale. From this, a scale reading by the square,
can also be constructed, as before. To ascertain if the ammeter deflections are
proportionate throughout the scale, the switch should be turned to the terminal e
to put in the variable resistance E, which is then varied as described above in
référence to the wattmeter, and voltmeter.
Having thus calibrated the wattmeter I, the voltmeter G, and the ammeter H,
and arranged the scales for direct reading in the manner described, the connections
of each instrument are now transferred to the circuit to be tested, by means of
the respective switch devices. It is to be understood that the ammeter H is
directly connected across the terminals of the resistance a-b, and the quadrant
plates of the wattmeter I also, the needle of the latter being connected to the
other side of the circuit. If, however, the instruments being arranged for direct
reading, the resistance should not be 1 ohm, then the readings of the ammeter and
wattmeter must be divided by the actual resistance in ohms, or parts of an ohm,
to give the proper ratios in amperes, and watts.
In testing a circuit where the lag of the current is very great, and the watts
are small, or vice versa, it is desireable in some cases, for greater accuracy, to
have two separate resistances in the circuit; one for the ammeter, and one for the
wattmeter, but this is seldom necessary except for laboratory purposes.
In changing the instruments from one set of connections over to the other, it
is very desirable to have a means of separately short circuiting the ammeter and
the wattmeter quadrants through a high resistance of about 20,000 ohms while
the operation is performed, otherwise the quadrants must be left unconnected for
a period of time which might lead to violent deflections of the needles, from
charges on the ebonite or other adjacent surfaces, otherwise difficult to avoid.
This is provided for by the ammeter key and resistance h³, and by the wattmeter
key and resistance, as shown in Fig. 1; or by the intermediate contacts as
in Fig. 3.
'
In testing circuits of high tension, say above 250 volts, differing modes of pro-
cedure can be followed. In one, a wattmeter and voltmeter arranged for high
pressure readings may be adopted, or a high non-inductive resistance may be
inserted across the circuit, and the two instruments adapted for the usual range
of (say) 100 to 200 volts. can then be connected across a portion of this resistance.
Or else, a set of small condensers in "cascade" may be arranged across the circuit
instead of the resistance, and the instruments connected across one or more of them.
Either of the plans are feasible with suitable instruments, and such are described
in my specification for "Improvements in electrometers and in keys for operating
the same," dated 6th December, 1898, No. 25752, but on account of the difficulty
of standardizing high voltage instruments, and the convenience of always keeping

page_text: instruments at the same degree of sensitiveness, I prefer for most purposes to use
the resistance, or condensers, aforesaid.
Con-
It will be noticed that in Fig. 1, the connections to the wattmeter comprise a
combination of the several connections of the voltmeter and ammeter.
sequently, advantage is taken of this, in some cases, to reduce the number of
connections and bring them to the terminals of two switches only, as is shown in
Fig. 2. In this case, it is better to attach a separate connection between the volt-
meter G and the ratio resistance 1, so that when the former is being calibrated at
the higher potentials it requires, the ammeter (being adapted for much lower
potentials) may not be switched in accidentally or else the ammeter connections
should be uncoupled while the voltmeter is being calibrated. It is further to be
observed that the two switches shown in this Fig. 2, having an equal number
of terminals, may be so combined as to be operated by one handle in carrying out
the necessary operations. Furthermore, by spacing the contacts already referred
to, and inserting between them other contacts, Fig. 3, connected in multiple
arc to the high resistances, and, described in relation to each of the keys h
and, the function of the latter will be retained and performed automatically
without the keys, in preventing the quadrants from being insulated while the
switch is passing from one contact point to another.
Although Fig. 2 is the simplest arrangement, for general purposes I prefer to
employ four switches, with their connections, as shown in Fig. 1, each switch
adapted to make contact with either one of four contact terminals, in addition to
the ammeter key h, and the wattmeter key i, or the alternative arrangement
Fig. 3. This enables each instrument to be operated independently, and not
only standardized but calibrated throughout its whole scale by the contact
terminal e and connection to the ratio resistance E.
If it is desired, the instruments may be connected to a circuit from which
measurements are to be taken, and when the deflections are noted, the switches
are respectively moved to the contact e while the ratio resistance E in shunt
with the battery is adjusted until the exact deflection of each instrument is re-
produced, from which the exact value of each deflection is easily found. By
the insertion of the high resistance contact (, Fig. 3) between the several
terminals of the ammeter and wattmeter, the keys and (in Fig. 1) may in this
case also be dispensed with as aforesaid, and their function automatically per-
formed, in preventing the insulation of the quadrants while the switch is passing
from one contact point to another.
In Figs. 4 and 5 a convenient arrangement of the general apparatus employed is
shown, as I prefer it in making electrostatic measurements of electric currents,
although it will be understood that modifications therein, or omissions of or
changes in the parts, may be made without departing from the nature of my inven-
tion. In these figures a suitable lamp L is arranged to throw a beam of light
upon the respective reflecting mirrors suspended with the needles of the instru-
ments G, H and I, and those mirrors are so placed that their respective reflections
of the beam of light are cast upon the corresponding scales G¹, H' and I. For
convenience, the scales vary in height, and each have means provided as shown both
for vertical and transverse adjustments to meet the line of the reflected light from
its mirror from the zero point to the extreme range of its angular movement when
under deflection. In addition to this, however, the supporting base of each of
the scales G, H¹ and I, are adapted to slide in grooves in the platform Mon
which they are mounted, so that the scales can be moved towards or from the
reflecting mirrors, in order to vary the range of the light beam on each scale,
and adjust it for direct reading or otherwise. The scales, however, may be
mounted in a similar manner upon a side wall or screen, if found more con-
venient or dispensed with if the instruments denote deflections by a pointer. My
improved apparatus is conveniently arranged in a box or case N, in which are
placed the standard cell C, (Fig. 5), battery D, the four switches S to $3 and the
resistances employed as denoted by R. Suitable provision is made for joining

page_text: up the various connections to the terminals, as will be understood by electricians
or instrument makers.
It is to be understood that the high resistance E referred to above as the ratio
resistance is so adapted that a third connection can be made to it at any desired.
point within its range across a corresponding number of ohms, so that when a
battery such as D) is connected across the extreme terminals of the resistance, a
desired or corresponding voltage is obtained by the said third connection, and
applied to an electrostatic instrument in the manner and for the purpose above
described.
Having now particularly described and ascertained the nature of the said
invention, and in what manner the same is to be performed, I declare that what
I claim is:—
1. The herein described method of electrostatically measuring electric currents
in a working circuit, which comprises the setting up of an electrostatic instrument
with suitable connections to the said circuit, testing, comparing by standard,
and calibrating the said instrument and taking a deflection reading therefrom
upon a suitable adjusted scale; then making measurements of the current in
the said circuit under working conditions by a deflection reading or readings upon
the said scale as adjusted; and finally re-testing or calibrating the instrument by
a further reading or readings upon the said scale as first adjusted, substantially
as and for the purpose herein described.
2. The herein described method of electrostatically measuring electric currents
in a working circuit, which comprises the setting up of an electrostatic instru-
ment with suitable connections to the said circuit: then taking and noting one
or more deflection readings upon a suitable scale of the current from the said
circuit under working conditions; and finally ascertaining the value of the said
deflection readings by adjustments of the ratio of resistance in shunt with the
battery until the exact deflection of the readings under working conditions are
reproduced, substantially as and for the purpose herein described.
3. The herein described method of electrostatically measuring electric currents
in a working circuit by first testing or calibrating an electrostatic wattmeter,
voltmeter, and ammeter by means of a suitable standard cell, ratio resistance, and
battery, and taking deflection readings therefrom upon suitably adjusted scales;
then connecting the said instruments to the circuit to be tested, and taking
measurements of the current therefrom under working conditions by deflection
readings upon the said adjusted scales, substantially in the manner and for the
purpose hereinbefore described.
4. The herein described method of electrostatically measuring electric currents in
a working circuit by first testing or calibrating an electrostatic wattmeter, volt-
meter, and ammeter by means of a suitable standard cell, ratio resistance, and
battery, taking deflection readings therefrom upon scales suitably adjusted for
direct reading; then connecting the said instruments to the circuit to be tested
and taking measurements of the current therefrom under working conditions by
deflection direct readings upon the said scales so adjusted, as and for the purpose
hereinbefore described.
5. The herein described method of electrostatically measuring electric currents
in a working circuit by connecting an electrostatic wattmeter, voltmeter and
ammeter to the said circuit and taking deflection readings therefrom under
working conditions upon suitable scales: then calibrating the said instruments
by means of a suitable standard cell, ratio resistance, battery, and readings upon
the said scales suitably adjusted for direct reading and finally reproducing the
original deflections upon the said scales so adjusted and ascertaining their value
directly as and for the purpose hereinbefore described.
G. An apparatus for measuring electric currents in a working circuit under
working conditions by one or more electrostatic instruments, and for calibrating
the said instrument or instruments at the place and time of taking the said

page_text: measurements under the same or similar conditions, substantially as and for the
purpose hereinbefore described, and indicated by the diagrams Figs. 1, 2 and 3.
7. A switchboard apparatus and connections combined therewith for electro-
statically measuring the currents in a working circuit under working conditions
comprising one or more electrostatic instruments, a standard cell, battery, ratio
resistance or potentiometer, one or more switch devices or equivalent thereto
adapted to put the said instrument or instruments in or out of circuit with the
said connections, a non-inductive resistance, short circuit connection, and common
terminal, substantially as and for the purpose hereinbefore described and indicated
by the diagrams Figs. 1, 2 and 3.
8. A switchboard apparatus for calibrating electrostatic instruments and
measuring electric currents in a working circuit having in combination one or
more electrostatic instruments, a standard cell, battery, ratio resistance, one or
more switch devices adapted to put the said instrument or instruments in or out
of circuit with the said working circuit or the said cell, battery, or ratio resistance,
a non-inductive resistance, and suitable circuit connections substantially as and
for the purpose hereinbefore specified and indicated by the diagrams Figs. 1, 2
and 3.
9. A switchboard apparatus for calibrating instruments and measuring electric
currents in a working circuit having in combination an electrostatic voltmeter,
ammeter and wattmeter, a non-inductive resistance, standard cell, battery or
source of electro motive force, ratio resistance, short circuit, common terminal,
and four switch devices with terminal contacts adapted to place the said instru-
ments in electrical connection with the aforesaid resistances, cell, battery, short
circuit, and the working circuit respectively; substantially as and for the purpose
herein before described and indicated by the diagram Fig. 1.
10. A switchboard apparatus for calibrating instruments and measuring electric
currents in a working circuit having in combination an electrostatic voltmeter,
ammeter and wattmeter, a non-inductive resistance, standard cell, battery or
source of electro motive force, ratio resistance short circuit, common terminal, and.
two switch devices with terminal contacts adapted to place the said instruments
in electrical connection with the aforesaid resistances, cell, battery, and short
circuit, and the working circuit respectively; substantially as and for the purpose
herein before described and indicated by the diagram Fig. 2.
11. A switchboard apparatus for calibrating instruments and measuring electric
currents in a working circuit having in combination an electrostatic voltmeter,
ammeter and wattmeter, a non-inductive resistance, standard cell, battery or
source of electro motive force, ratio resistance, short circuit, common terminal,
and four switch devices with terminal contacts adapted to place the said instru-
ments in electrical connection with the aforesaid resistances, cell, battery, short
circuit, and the working circuit respectively, and high resistance contacts placed
between the said switch terminal contacts, substantially as and for the purpose
herein before described and indicated by the diagram Fig. 3.
12. A switchboard apparatus for measuring the current in a working circuit
under working conditions, and for calibrating or testing the instruments employed
in the said measurement having in combination suitable switch devices, a non-
inductive resistance, standard cell, battery, ratio resistance, short circuit, common
terminal and suitable connections adapted and operating by means of the said
switch devices to calibrate the said instruments, and then to take measurements
of the current in the said working circuit, and then again (if required) to re-test
the said instruments under identical conditions; substantially as and for the
purpose hereinbefore described and illustrated by the drawings.
Dal 1011
O 1000
FAIRFAX & WETTER,
37 & 39, Essex Street, Strand, London, Agents for the Applicant.