Differential Hot Wire Voltmeter.
Year: 1899

page_text: Date of Application, 25th Mar., 1899-Accepted, 6th May, 1899
Differential Hot Wire Voltmeter.
I, ALEXANDER MELVILLE CLARK, of the Firm of A. M. & Wm. Clark, of 53,
Chancery Lane, in the County of Middlesex, Fellow of the Chartered Institute
of Patent Agents, do hereby declare the nature of this invention (a communication
to me by the Firm of Hartmann and Braun, of 9, Obere Königstrasse, Bocken-
heim, Frankfort-on-the-Main, Germany,) and in what manner the same is to be
performed to be particularly described and ascertained in and by the following
statement:
This invention relates to a differential hot wire voltmeter for measuring the
difference of two electric tensions, and will be described with reference to the
accompanying drawing wherein and (Figure 1) represent two fine wires
which are fixed by their ends at p c and, c, respectively and are connected
from centre to centre by a flexible thread ƒ wound around the axle & of an
index z the thread being at right angles to the wires and by its mechanical ten-
sion straining the wires and deflecting them from the straight form as shewn,
the clamping screws c c admitting of adjustment whereby the mechanical ten-
sions of the wires and the position of the index may be regulated at will. As
the elasticity of the wires d and dis sufficient to impart to the index direct
rotary motion when they are sufficiently deflected the provision of a special
actuating spring or weight is unnecessary. By reason of their symmetry or
identity of arrangement, the two wires when deflected or expanded to a certain
degree will retain the equality of mechanical tension imparted by the two adjust-
ing screws and c¹. If however, for example, the heat of the wire d, as com-
pared with that of ¹ (Figure 1), increases in consequence of a greater difference
of potential at its ends the mechanical tension of will be greater and will cause
the wire d to be bent or be deflected to a higher degree and at same time cause
the index spindle to be rotated in the one direction, the action being converse
if the heat of wire is of a higher degree, that is to say the index will be de-
flected in the opposite direction. The index moves over a scale by which the
difference between the electric tensions may be directly read off.
In case of one member of the system remaining entirely without current in
consequence of there being no electric tension, the other member is caused as
before, to display its mechanical tension, the total electric tension of the said
system being then shewn by the index. The apparatus therefore acts in this
case as an ordinary voltmeter.
In order to increase the elasticity of the hot wire systems and the sensitive-
ness of the apparatus, a compound arrangement may be adopted such as is shewn
in Figure 2, wherein to each hot wire dd, a wire t is attached at right angles,
each wire being made fast to a fixed point and the two wires being strained and
deflected by the thread f operating the index as before.
Having now particularly described and ascertained the nature of the said in-
vention and in what manner the same is to be performed I declare that what I
claim is:.
:―
A measuring instrument of the hot wire type in which the measuring is effected

page_text: by the compensation of the different tensions of elastic wires as produced by the
electric current or currents substantially as specified.
Dated this 25th day of March 1899.
A. M. & WM. CLARK,
Chartered Patent Agents, 53, Chancery Lane, London.