Improvements in Protecting Static Volt Meters.
Year: 1898

page_text: Date of Application, 20th Oct., 1898-Accepted, 26th Nov., 1898
COMPLETE SPECIFICATION.
Improvements in Protecting Static Volt Meters.
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 Edward M.
Hewlett, residing at Schenectady, in the County of Schenectady, State of New
York, 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 concerns electro-static measuring instruments, the best
example of which is the electro-static voltmeter, which, by a modified construc-
tion, is also used as a ground detector. This is a well-known form of instrument
applicable particularly to alternating currents, and is particularly useful with
high voltage circuits. But such an instrument must, necessarily, form a short
circuit across the line, except for the air gap, which must be made very small
in order to secure sensitiveness, and if for any reason the potential should rise
so far as to break down the air barrier, a considerable amount of damage might
be done. As the instrument operates entirely by charge, and normally carries
no current, it has been found that it can be efficiently protected by including
between it and the lines resistance in series sufficient in amount to cut down
the current flow so that no destructive arc could be maintained between the
opposed surfaces of the instrument, even if the air gap should break down. This
resistance may be inductive or non-inductive in character; for some reasons it
is desirable that it should be inductive. It is not desired however to limit the
invention in this regard, and in the accompanying drawing, which shows an
embodiment of the invention, there is illustrated a resistance of nonconductive
character upon each side of the instrument. This is a convenient disposition,
but it is not imperative that the resistance be divided. It is however, desirable
because it affords protection to both sides of the circuit in case of an accidental
ground.
In the drawing A, B are high potential lines of an alternating current system
and C, C are the resistances each connected to one of these lines. Each of these
resistances is connected to a terminal of the voltmeter D, which is shown as of
the usual type, provided with stationary quadrants, (as they are commonly
called) E, E, which are extended surfaces of conducting metal. The vanes F, Fi
are attached to the needle f, which moves over the scale G. A counterpoise F"¹
is provided as usual. The scale G is electrically connected to the needle, so that
the vanes F, F and the scale form one side of the circuit, while the quadrants
which are electrically connected together, form the other.
The particular type of measuring instrument is however unimportant, and
there is simply described a well-known form for which any other might be
substituted.
Not only does the present improvement protect the instrument against the
destructive effect of an accidental are following the spark passing from one side

page_text: of the circuit to the other, but it also permits the distance between the moving
and stationary parts of the voltmeter to be safely reduced so as to amplify its
readings and make it very sensitive to small differences of potential.
The operation of the resistance affording the results pointed out may be briefly
described as follows:-Since the instrument works by charge and not by current
flow, so long as the insulation is perfect, the potential between its opposed
surfaces will be the same, irrespective of the resistance that may be in circuit
with it. If a spark, however, should puncture the air insulation, under ordinary
circumstances, à destructive arc would be formed, but as soon as current begins
to flow, the high resistance causes such a drop in voltage that the difference of
potential can no longer inaintain the arc between the opposed surfaces, and it is
extinguished without harm to the apparatus.
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, with an electro-static measuring instrument, of high
resistance in series between the supply main and the instrument.
2-The combination, with an alternating current circuit and an electro-static
measuring instrument connected across the circuit, of a resistance between each
main and the instrument sufficient in amount to substantially prevent current
flow, even if the instrument breaks down.
Dated this 18th day of October 1898.
THE BRITISH THOMSON-HOUSTON CO., LTD.
W. MARTINDALE,
A. R. MONKS,
Directors.
ALFRED CLEMENTS,
Assistant Secretary.