meters or apparatus for measuring electric
Year: 1887

page_text: Date of Application, 20th Sept., 1887
Complete Specification Left, 19th June, 1888
Complete Specification Accepted, 24th Aug. 1888
I, Professor GEORGE FORBES of 34 Great George Street Westminster, Engineer
do hereby declare the nature of this invention to be as follows:-
This invention has for its object improvements in electric meters or apparatus for
measuring electricity.
My method of measuring electric currents is founded on the fact that an electric
current passing through a conductor of electricity generates heat. This heat I apply
to evaporate a liquid and from the quantity of the liquid so evaporated I estimate the
quantity of electricity which has passed through the conductor.
Preferably so arrange the apparatus that the quantity evaporated is directly
proportional to the current flowing in the conductor.
Dated this 10th day of Santombor 1887

page_text: Improvements in Electric Meters or Apparatus for Measuring
Electricity.
I, Professor GEORGE FORBES, of 34 Great George Street, Westminster, Engineer,
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:
The subject of this invention is apparatus for measuring the total quantity of
electricity which has passed through a conductor in any time. There are two distinct
and different conditions under which meters of this kind can be used. In the first,
the quantity of electricity passing per unit of time when the current is being used, is
always approximately constant. In the second, the current which is being used at any
time is a variable amount, and the indications of the meter while in action must be
proportional, not only to the time, but also to the strength of the current. For
clearness of description, I call the first a time meter for electricity, and the second a
proportional meter for electricity.
In constructing meters of both kinds, I pass the electric current which is to be
measured through a conductor, whose resistance is small, but perceptible, and I place
this conductor in the neighbourhood of, a liquid which is evaporated by the heat
generated in the conductor. The liquid is condensed in another part of the
apparatus, collected and measured and it indicates upon a scale the total quantity of
electricity which has passed through the conductor.
When the current in use is constant the construction of the instrument is more easy
than when variations in current strength are required to affect in a truly proportional
manner the indications of the meter. But in either case as is indicated in Figure 1
of the annexed drawings the liquid is contained in a closed glass or other vessel A
communicating with the condensing chamber D. These form one piece of apparatus,
which preferably is exhausted of air. The conductor E is placed either in the liquid
or near it (the drawing indicates the latter) it evaporates the liquid when the current
is in use. The liquid condenses in the condensing part D of the apparatus and it
accumulates in the tube the quantity is read on the scale F and this gives the indication
of the meter.
In the case of proportional meters, it is essential, that the quantity of liquid
transferred from the evaporating to the condensing part of the apparatus, should be
proportional, at any rate approximately, to the strength of the current of electricity,
in spite of the fact that the heat generated in the conductor is proportional to the
square of the electric current. I am able to do this by suitably proportioning the parts
of the apparatus.
In Figure 1, the glass bulb A is blown on the end of a glass tube which is bent
twice at right angles at B and C and passes on to the measuring apparatus. A
conductor, in the form of a strip of metal E is wound round the horizontal arm B, C,

page_text: this being connected at the terminals with the circuit, the current in which is to be
measured. I make this conductor of German silver, the width of the ribbon being
about inch, and the thickness about inch, one and a half turns of the ribbon
closely surround the tube. The vessel A is partially filled with alcohol, as shown.
The bulb diameter is 1 inches and the exterior diameter of the tube is inch.
t
The air in the tube is expelled by heating the bulb A before sealing the end of the
tube. It is sometimes desirable to replace the bulb A by a larger reservoir of liquid,
into the neck of which the end of the tube may be cemented. In this and other forms
of the meter I prefer to enclose the apparatus in a case chiefly composed of metal,
so as to equalize the natural temperature in the inside. A double case of metal is
even more advantageous for this purpose.
In another form of the apparatus, Figure 2, I employ a reservoir A, filled with
alcohol. A glass tube BC, lies nearly horizontally, and has attached to it at one end
the thin tube CD, or other apparatus for measuring the quantity of liquid distilled;
and also has a tube E at its side passing through the stopper of the reservoir A; the
open end B of the tube is closed by a stopper F, through which the terminal
wires G G pass, these being connected inside the tube by a wire H KL, preferably
bent into the "M" shape shown in the figure; the stoppers at E and Fare carefully
cemented to prevent evaporation to the external air; the wire H K L, is covered with
cotton thread or other absorbent of the liquid, and this thread at the ends H and L
passes through the tube E, and dips into the liquid in the reservoir A. I prefer to
incline the tube B C slightly, to allow the condensed liquid to flow downwards, but I
keep the wire HK L, horizontal. Capillary attraction causes the cotton thread to be
moist, and the heat developed by the electric current in the wire H K L evaporates
the liquid from its surface which, is condensed in the tube BC and runs down into the
tube C, D. Since the heat developed is proportional to the square of the electric
current, it might be supposed, that the quantity of liquid condensed would be
proportional to the square of the current; this is not so, because, except with the
smallest currents, part of the covering of the wire is dried up, and thus the stronger
the current the less moist surface is there from which evaporation can take place. I
assist this action by winding the cotton in a very open spiral from K for a certain
distance along the wire, I make the turns closer as we golalong, and as we approach
the ends Hand L put several layers of cotton on the wire.
I generally find that I can increase the range of accurate readings by a final
adjustment, which consists in moving the stopper F farther in or out of the tube B C,
thus allowing more or less of the liquid evaporated from the ends H and L with
powerful currents, to be condensed in the tube E, and consequently not to register in
the measuring part of the apparatus. In order to hasten the moistening of the wire
which has been dried by a strong current after the current is stopped, I sometimes
pass a thin thread from the point K of the wire, and through the tube E to the liquid
in the reservoir A.
I do not necessarily employ a wire H K L of uniform section, I sometimes thicken
it rapidly towards the ends H and L, by this means I am able to make the instrument
work over a larger range.
The following description gives the details of an instrument adjusted to indicate
truly over a wide range. Figure 3 shows the apparatus fitted up. Figure 4 gives
the dimensions of apparatus. The diameters of the tubes are inside measurements.
The conductor G G is formed of No. 18 B.W.G. Cu. wire, with a loop at each
end, bent at right angles to it (see Figure 5). A piece of No. 12 B.W.G. copper
wire H is put through each loop and firmly soldered. The conductor measures
8 inches between loop and loop.
The conductor is bent as in Figure 6 in the shape of the letter M. The distance
between the bends of the conductor is about inch to inch. The distance from
end of loop in the middle to the end of conductor is inch. The conductor must
be bent so that it is horizontal, when the tube is on the incline. The conductor is
wound with threads of various thicknesses, consisting of different numbers of strands,
each strand being the same thickness in every instance.

page_text: That part of the conductor marked a to b, (see Fig. 7) is wound with a thread of
Istrand of cotton. There are 120 turns and about 4 turns to cach inch of
conductor. From a to c, which is inch, from the extreme end, is wound with a
thread consisting of 2 strands, and of 35 turns. At the beginning of the winding at c
there are 4 turns to the inch, increasing to 6 turns per inch at a.
From e to e
inch, there are 3 turns of a thread of 4 strands. Then from e to
end are 4 turns of the same thread as above, wound with no space between turns.
Then this thread is wound for 2 turns over the above layer, near the extreme end,
forming a second layer, and is then tied off. The different threads are joined by a
single knot and cut off close.
The threads with more than one strand are wound with the strands not twisted, but
parallel with one another as in Figure 7". (The windings on the other end of the
conductor are exactly the same).
The conducting wick N is a thread composed of 16 strands of cotton, half of which
are connected to each end of the conductor. Connection is made by taking one turn
round the extreme end of the last layer of cotton wick and then tieing with a double
knot.
The two halves of the conducting wick, after leaving the conductor, are twisted
together and brought down into bottle.
The spirit in bottle must be above the bottom of the conducting tube which
surrounds the conducting wick.
Arrangement must be made so that the conducting tube is either large enough, or
is widened at the bottom, so that no appreciable difference in the height of the fluid
in the conducting tube in the bottle will occur, due to the expansion and contraction
of the air in the upper part of apparatus. No part of the conductor or conducting
wick must touch the sides of the tube.
The thick wires for conducting the current into the apparatus must be well sealed,
and the whole apparatus is air-tight.
The beginning of the conductor is between and over the hole in conducting
tube as in Figure 8. The exact position of this is a final adjustment for high
currents.
My meter may also be made in a vertical form.
Figure gives the outside appearance of the apparatus and also its dimensions.
The diameters given are inside measurements.
The branch tube leading into bottle must be opposite the ends of conductor so that
when the conducting wick is brought down it may be in the centre of the tube without
touching the sides of it.
The tube leading into bottle must slant slightly downwards at the bend as the fluid
would be likely to dribble over from the conducting wick where it touched the bend
into the condensing tube c.
The opening where the thick conducting wires are sealed in must be so made that
the conductor can be put into tube through it. The opening must be sealed airtight
and the tube leading into bottle must be below the surface of the fluid in bottle. It
is well to have the conducting tube t leading into bottle, widened at its lower part as
the drawing indicates.
The conductor is formed of inches of No. 18 B.W.G. Copper wire with a loop at
each end as in Figure 10 and a piece of No. 12 B.W.G. copper wire firmly soldered
in the middle of the loop. First 8 complete turns of Copper wire 035 inch diameter
are wound on commencing from the extreme end of conductor. Then 12 turns of
Cu. wire 027 inch diameter are wound on commencing where the S turns of the
former wire left off.
Then 2 layers of Copper wire 007 in diameter are wound on for a length of inch,
the turns being wound as close as possible.
Then one layer of the above wire wound on for an inch.
The whole of the turns are soldered together, the solder nearly filling up the spaces
between the turns.
The conductor is wound with a thread of the same thickness throughout composed

page_text: from d to d there were 69 turns of cotton wick or in all 154 turns on the whole
length of the wire.
The cotton wick on conductor is tied off not on the last thickness of conductor but
at the point just before it commences.
The conducting wick is formed of 2 strands slightly thicker than that used on
conductor tied on to the end of conductor just at the point p.
There is also a 2 stranded thread as a conducting wick on the other end of the
conductor.
The 2 threads are twisted together and brought down through the bent tube leading
into bottle.
The conductor is bent back on itself at the middle (see Fig. 11). The space s
between the loop is
inch.
The conducting wick is 2 inches long from the conductor to where it reaches the
surface of the fluid.
No part of the conductor must touch the sides of the tube, and no portion of the
conducting wick must touch the conducting tube, level with or above the lower part
of opening in conducting tube, where it branches from the apparatus.
The cotton thread used in the conductor of this form of meter and of the horizontal
form is such that 10 strands 125 inches long weigh 8 grains.
The cotton thread used in the conducting wick is such that 1 strand 177 inches
long weighs 2 grains.
Having now particularly described and ascertained the nature of my said
invention and in what manner the same is to be performed I declare that what
I claim is
1. In an instrument for measuring electricity, the combination substantially as
described of a conductor heated by a current to be measured a vessel containing a
liquid which is evaporated by the heat, a surface serving as a conductor, a receiver in
which the liquid when recondensed is collected and a scale on which the quantity of
recondensed liquid is read to give the indication required.
2. In an instrument for measuring electricity the combination substantially as
described of a conductor heated by a current to be measured a vessel containing a
liquid an evaporating surface moistened by the said liquid and so adjusted that the
evaporation therefrom induced by the heat in the conductor is approximately propor-
tional to the current passing therein; a surface serving as a condenser; and a
receiver for the liquid resulting from such condensation.
3. Electric meters or apparatus for measuring electricity substantially as herein
described and illustrated by the annexed drawings.
"
Dated this 15th dox of Tuna 1000