An Improved Instrument for Measuring Currents of Electricity.
Year: 1889

page_text: An Improved Instrument for Measuring Currents of Electricity.
Sir CHARLES STEWART FORBES Baronet 23 Thurloe Square London S.W. 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:—
To
The object of my invention is to so utilize the thermal or heating effect of the direct
or alternate current passing through a measuring instrument as to obtain a step by
step motion suitable for driving a hand or pointer over a dial, or a train of wheels or
equivalent device for indicating the value in current or energy taken by the user.
this end I sometimes employ the main current passing, or to obtain an extended range
I may use a part of the same, regulated by a special shunt device with, or without
resistance. To enable my invention to be better understood reference is made to the
drawings in which :-
Fig. 1, is a front view of the instrument. Fig. 2. is a side elevation on line X Y,
part being in section. Fig. 3. a plan on line A B Fig. 1. Fig. 4. a plan on line C D.
Fig. 5. front view of dial. Fig. 6. plan on line EF. Fig. 7. Diagram shewing
arrangement of shunt device. Fig. 8. Diagram showing rubbing resistance contact.
M, Fig. 1 is a terminal or binding screw to which the main lead or cable conveying
the current is attached. A wire m, shewn in dotted lines, connects this terminal with
the mercury cup m², mounted on rocking armature A, which is pivoted at b, both
ends t, t, of armature A, are so formed as to dip alternately into the mercury
cups m, m, which are connected preferably by thick copper wires c¹, c¹¹ with the
metal clamps d, d¹. Two wires e, e¹ of german silver, platinum or other metal of high
resistance suitably proportioned to the current to measured are clamped therein,
and carried through the movable upper contacts fif, which dip into the murcury
cups n, n¹ these are connected by strip or wire c¹11, which is then secured to terminal MI,
a rocking contact g, is adjustably pivoted at 611, so as to rest upon wires, e, e¹. Two
adjustable contacts h, h', are so mounted as to engage with the rocking contact
One contact h, is connected to the ingoing wire upon electro magnet E¹, the other ',,
being connected to the ingoing wire upon E", the outgoing ends of the wire upon
E', E, being coupled together and connected to terminal 3, Terminal 4, being
9.

page_text: connected to the rocking contact g. Mounted upon the mercury cup upon pivoted
armature A, is the ring Ŕ. See also Figs. 2. 4 this ring is fitted with two spring
pawls p, p' and adjustable stops w, which engage with the ratchet wheel », pivoted to
bridges, and the case. The pivot, may be connected to a pointer or equivalent
moving over a dial or may be connected to a train of wheels suitably arranged to
shew either upon a dial or dials, or upon their periphery, ampere hours, standard
units, or any other predetermined value in current, or electro motive force, and
time, See fig. 2. 5. The electromagnets E¹, E are preferably wound to high
resistance, and their circuits are operated either by the battery X, which may be of any
form, or a shunt wire may be taken from the mains and connected to terminals 3 and 4.
To enable the instrument to work over a wider range, I sometimes employ the
shunt device shewn in figs. 7. 8. The cable conveying the main current is con-
nected to terminal M, and passes through two thermal wires of german silver, or
platinum e, e¹ these are mounted in clamps of insulating material, or the separated
clamps as previously described for figs. 1, 2, the other ends are secured to a bond
or loop of metal V. The current after traversing both wires, led by a suitable
conductor to outgoing terminal M. Resting upon the bridge, bond, or loop V, is a
lever u, pivoted at Z, the other end of this lever is connected to a spiral of any high
resistance wire, or it may consist of a rod of the same or any suitable substance.
The rod spiral or other termination of the lover, dips in a mercury cup O, or it
may be arranged to rub over blocks so as to throw in resistances as in Fig. 8.
When the shunt device is used in conjunction with the instrument forming my
improvements a wire connects pivot Z, with terminal M upon the same, and another
wire connects terminal M, on shunt device, and M, on the instrument, which has
thermal wires of suitable area for producing the desired effect with a small current.
The operation of the shunt device is as follows:-The main current in passing
through wires e, e', heats them, and causes them to expand, thereby raising the pivoted
lever u, and spiral, rod, or equivalent out of the mercury so permitting a less flow
of current in the thermal wires e e¹, in the instrument coupled up to the shunt device,
the record will therefore be strictly proportional to the main current passing.
The operation of the measuring instrument described under figs. 1. 2. 3. 4. 5. 6; is as
follows:-
The
The current being switched on passes from terminal M, to mercury cup m², and
thence to mercury cups m", ml, by armature A. The bends tt¹, of armature A,
are arranged so as to both dip into the mercury in cups m", m, sufficiently to
permit the transfer of current from one to the other without abruptly breaking the
circuit and so causing a spark. In the drawing the bend t, is shewn raised out of
the mercury in cup m", and the circuit is therefore broken through wire e.
current can now only pass from the mercury cup m¹, to wire e', which is heated
thereby and expanded or lengthened sufficiently to tilt the rocking contact g, against
adjustable contact h, closing the circuit operated by battery or shunt X, round the
coil of electromagnet E', which is energized thereby, and pulls down armature A,
shunting the main current thereby from m¹ to m¹, and thermal wire e, which now
expands tilting the rocking contact g, against contact h¹, and closing the battery or
shunt circuit through electromagnet ", which again attracts armature A, and
shunts the current through el, and mercury cup ml as before. The rocking action
of armature A, to which the ring R, is fastened causes the pawls p,p', to alternately
engage with and advance the ratchet wheel r, one step for each transfer, and the
rotary motion imparted thereby to the spindle or pivot ¹, connected to a pointer
moving over a dial or dials or to a train of wheels y, see fig. 24, serves to indicate
the volume of current or energy supplied to the user; the rate of rotation of the
ratchet depending upon the volume of current passing through the thermal wires e, e¹,
and the rapidity of expansion induced thereby. I so proportion the size of the
wires e, e¹, as to cover an useful range of current or energy; the instrument being
calibrated in each case by standard tests made to determine the practical expansive
capacity of the thermal wires employed. Since mains conveying current are regulated
by a fixed potential, instruments constructed as herein before described, can be used to
1

page_text: determine standard units, current time, or time voltaic. It is obvious also that my
improvements can be used in conjunction with dials driven by springs or equivalent,
or electrically controlled.
I am aware that electrical measuring instruments have been constructed in which
wires of high resistance rigidly fixed at both ends have been employed to indicate
by the dip caused by their expansion current and electro motive force, and that bi-
metallic strips heated by the current have also been employed for this purpose; to
such I lay no claim; I sometimes replace the mercury cups with springs or equivalent
devices capable of producing the same result, and mount upon the same instrument
the shunt device described, or separate sets of thermal wires for giving an extended
range.
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
1st. The particular combination hereinbefore described for measuring currents of
electricity or electrical energy in the way and manner described.
2nd. The combination in or with an instrument for measuring electric currents or
energy, of the two thermal wires e, e¹, alternately heated by the passage of the main
or shunted current passing through them, said wires being secured at one end by
clamps or equivalent d, d'. The rocking contact g, pivoted as described, resting upon
free ends thereof and actuated by the alternate expansion of said wires. The
contacts ff¹, fastened adjustably to said wires, and dipping into cups n, n¹. The electro-
magnets E¹, E, operated by the rocking contact 9, adjustable contacts h h¹, and
primary or secondary battery or shunt circuit, for transferring the main or shunt
current by means of pivoted armature A, having bends t, t¹ dipping in cups ml m¹¹.
The mercury cup m², mounted upon said armature, ring R, attached thereto
operating ratchet r, by pawls pp¹, and stops w, for the purposes and in the manner
set forth.
3rd. The combination of a series of thermal wires, each pair having a specific
range, within or upon one instrument, with the separate electrical, mechanical and
indicating devices described for effecting the purposes set forth.
4th. The particular construction and arrangement of shunt device described under
figs. 7. 8 whereby any fractional portion of the main current may be caused to actuate
the measuring instrument as described.
5th. The combination with a shunt device as described of the rod or spiral resistance
dipping in mercury, or the rubbing resistance as in fig. 8.
Dated February 25th 1889
FRANCIS M. ROGERS,
Agent for Applicant.