Improvements in Apparatus for Measuring Electric Currents.
Year: 1899

page_text: Date of Application, 2nd May, 1899-Accepted, 20th Jan,, 1909
Improvements in Apparatus for Measuring Electric Currents.
We, THE BRITISH THOMSON-HOUSTON COMPANY, LIMITED, of 83, Cannon Street,
in the City of London, Electrical Engineers, do hereby declare the nature of this
invention (which has been communicated to us from abroad by Walter C. Fish,
residing at Lynn, in the County of Essex, State of Massachusetts, United States
of America, Electrical Engineer), and in what manner the same is to be performed
to be particularly described and ascertained in and by the following statement: -
The present invention relates to the construction of a new type of the now
well known maximum demand instruments. It has for its object to substitute
for the class of such instruments which uses thermal effects and the displace
ment of liquids, one which is entirely mechanical and is therefore easily kept in
order and not liable to the inconvenience attendant upon the use of the old
types. While different plans have been proposed for the construction of maximum
demand meters, there are objections to very many of them. One of the principal
objections to proposed types has been the absence of any time constant":
that is to say, in many kinds of such meters an instantaneous flow of current far
larger than any normal or usual demand to be supplied by the branch circuit
would effect the instrument and give the inequitable basis of charge. For
instance, a momentary short-circuit in a lamp socket, or other incident of that
kind may for a moment cause current to flow far beyond the capacity of the lines
to furnish with safety; it is manifestly improper to take such a current flow as a
basis for charge.
It is
A most serious defect in instruments of the thermal class is that they cannot be
made standard and yet calibrated for the variations of atmospheric temperature
to which they must be subjected Such variations are very large, 50° or 60° C.,
being not uncommon; this may exist either between two meters of the same type
installed in different situations or at the same meter at different times.
manifest that the only way for compensating for this great defect is by a calcula-
tion of more or less intricacy, which will furnish a correction for temperature,
and that even this is impossible in some cases. Since all measuring devices are
looked upon with suspicion by consumers, they should at least be direct-reading,
so that the computation of the charge can be readily tested by persons of ordinary
business acquirements.
The arrangement of this invention obviates some defects, and at the same time
furnishes a simple and convenient maximum demand instrument which may be
connected in the circuit with the ordinary recording watt meter or with any other
preferred form of electrical measuring instrument, so that by inspection at one
time the meterman may get both readings and by a readily effected manipulation
adjust the maximum demand instrument to zero. In its essence the instrument
of our invention consists of an indicating ammeter or wattmeter and means for
recording the greatest reading existing for a certain predetermined time. The
preferred method of accomplishing this is to cause the hand of the ammeter to
make contact with the recording hand used to indicate the maximum demand,
thus completing a circuit which so long as it is maintained causes a clock to run

page_text: and move the recording hand. By adjusting the rate of the clock we may thus
vary the time constant of the instrument to any reasonable extent. As soon as
the recording hand moves away from the hand of the ammeter the clock is stopped.
The preferred method of doing this is by including a small magnet in series in
5
a circuit, in such a manner that, when the circuit opens, a detent previously held
up by the magnet is allowed to fall in the path of some moving part of the clock.
The old type of maximum demand instrument which involves the moving of
liquid by increase of heat of a thermal device included in the circuit embodies a
very variable time effect. For instance, if a certain current flow has caused the
displacement of part of the liquid into the receiving chamber commonly employed,
a very much higher current flow occurring later will not cause any part of the
liquid to enter the chamber until the whole volume of the latter is heated suffi-
ciently to fill the space left vacant by the removal of a part of it and the subsequent
cooling of the remainder. In the meantime the maximum flow might change
materially, or even fall within the previous recorded limit: thus a substantial
flow of current for an appreciable time may go unrecorded. This difficulty, it is
manifest, is obviated by our invention, because the movement of the ammeter
needle to its stop on the recording hand is practically instantaneous, the time
constant being the same for all flows of current within range of the instrument:
and, as already pointed out, this may be adjusted to be anything required. Of
course the difficulty pointed out increases in the old construction with the range
of the instrument, whereas this is not the case with our invention. The accom--
panying drawings show a demand meter constructed according to the invention.
Figs. 1 and 2 are respectively a front elevation and a side elevation, with parts
in section, of such an instrument: Fig. 3 is an enlarged detail of a detent for
the clock Fig. 4 is a diagram of the circuits.
In Figs. 1 and 2, A is the base of the instrument, and B is the cover. For
purposes of illustration the now well-known inclined-coil measuring instrument
is selected, but the present invention is not limited to this selection as other types
could equally well be chosen. C is the coil through which flows the current to
be measured. Of course the instrument may be either a series or shunt ammeter:
in the latter case only a minute part of the total current supplied to the circuit
would be required to run the device, and in circuits of large capacity this would
be the preferred form. In the present case the series type is illustrated. 1), D¹ are
two discs or plates of iron also in an inclined position upon the shaft of the
instrument, by the displacement of which the movement of the needle F is
effected. This needle is shorter than the ordinary needle of the ammeter, and
bears against the pin in the recording hand H of the instrument, when suffi-
cient current passes through it to cause the required displacement. A clockwork G
drives the needle H over the scale E. The clock is of the ordinary balance-wheel
or marine escapement type, which is preferred because it is self-starting under
ordinary conditions of service. It is manifest that a fly might be substituted for
the escapement, although, as this would require a higher gearing, it is not
desirable. The anchor K of the escapement is carried upon and operated by the
usual lever L. The detail of this is better understood from Fig. 3. In this,
as in the other figures, I is a magnet the circuit of which will presently be
described. The armature M¹ of the magnet operates a pin M which when the
armature is not attracted, bears against the lever L, and thus stops the clock,
the weight of the armature being made sufficient for this purpose..
The operation of the device as a whole will be best understood from the diagram
of the circuits in Fig. 4. In this diagram ab are the mains from which a branch
carrying the load is taken. The usual wattmeter WM is indicated, the circuits
of which it is unnecessary to describe. The resistance R is of course in series
with the armature circuit of the wattmeter. The parts of the maximum demand
instrument are shown in the lower part of this diagram. Here it will be seen
that a circuit through the magnet coil I from across the mains or from any other
suitable source is completed whenever the hand F of the ammeter is brought in

page_text: contact with the pin h in the recording hand H of the instrument; as will be
understood from Fig. 3 in connection with this diagram, the magnet I is then
energized, the armature M¹ is lifted and the lever L is released. The clock then
continues to run until it has moved the hand H over a sufficient arc to break the
contact at h, when the pawl or armature M drops, stopping the clock.
It is manifest that, should the measured current flow sufficiently diminish, so
that the hand of the ammeter takes a different position, the contact at h will be
broken and the clock stop. Upon the occurrence of a great load persisting for an
appreciable time, the ammeter needle will again establish the circuit through the
magnet and the clock will again start.
Having now particularly described and ascertained the nature of our said
invention (as communicated to us from abroad) and in what manner the same is
to be performed we declare that what we claim is :-
1. The combination of an electro-magnetic ammeter with an indicator, and
means for advancing the indicator progressively as current is indicated and for
stopping its advance when the current indication falls below the maximum already
recorded.
2. The combination with an electric measuring instrument, of a recording
mechanism, and means actuated by the measuring instrument for starting the
recording mechanism, and for stopping it when the electrical indication falls
below the maximum already recorded.
3. The combination of an electro-magnetic ammeter with a clock and an
indicator moved by the clock, with means for starting the clock when the current
indication exceeds that recorded by the indicator, and for stopping it when the
current indication falls below the record.
4. The combination of an electro-magnetic ammeter, with a clock, and an
indicator moved by the clock, a circuit completed by a contact operated by both
the clock and the ammeter, and a magnet in the circuit operating a detent for
the clock whereby when the ammeter needle and the clock indicator complete
the circuit, the clock starts and when the circuit is broken, the clock is stopped.
5. In a combination, an electro-magnetic amnieter, a clock having a marine
escapement, and a circuit completed by the clock hand and the animeter needle,
including a magnet and a detent operated by the magnet and acting upon the
escapement.
D1 1
1 1
1000
THE BRITISH THOMSON-HOUSTON CO. LTD.
A. R. MONKS,
W. MARTINDALE,
Directors.
ALFRED CLEMENTS,
Assistant Secretary.