A Safety Device for Electric Circuits.
Year: 1890

page_text: Date of Application, 28th Apr., 1890
Complete Specification Left, 27th Jan., 1891-Accepted, 28th Feb., 1891
PROVISIONAL SPECIFICATION.
A Safety Device for Electric Circuits.
We, WILLIAM LOWRIE, Electrical Engineer, and CHARLES JAMES HALL,
Electrician, both of 117, Bishopsgate Strect, in the City of London, do hereby
declare the nature of this invention to be as follows:-
Our invention relates to safety devices for electric circuits, its object being to
prevent a dangerous electrical pressure existing latent in any circuit, by providing
an outlet to earth should, a given limit be excluded.
For this purpose, we employ a pair of conductors placed within a closed vessel
containing a rarefied gas. The conductors tend to come into contact; but are
normally separated by a material which will become carbonised or destroyed by
the electric current should the electrical pressure exceed a given limit, thus
allowing the conductors to come into electrical contact with each other and so close
the circuit to earth.
The vessel may form one of the conductors. One of these conductors is per-
manently connected to earth, and the other to the circuit or system of circuits to
be protected. An indicator to show when the circuit to earth has been closed may
be attached or combined with this device, actuated magnetically by the passage of
the current when the circuit to earth is closed, or mechanically when the separating
material is destroyed.
Dated this 28th day of Anvil 1890
FAIRFAX & WETTER,
433, Strand, London, Agents for the Applicants..
COMPLETE SPECIFICATION.
A Safety Device for Electric Circuits.
We, WILLIAM LOWRIE, Electrical Engineer, and CHARLES JAMES HALL,
Electrician, both of 117, Bishopsgate Street, in the City of London, 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 -
Our invention relates to safety devices for electric circuits, its object being to
prevent a dangerous electrical pressure existing latent in any circuit by providing
an outlet to carth, should a given limit be exceeded.
For this purpose, we employ a pair of conductors placed within a closed vessel
containing a rarefied gas. The conductors tend to come into contact; but are
normally separated by a material which will become carbonised or destroyed by the
electric current should the electrical pressure exceed a given limit, thus allowing
the conductors to come into electrical contact with each other and so close the
circuit to earth.
The vessel may form one of the conductors.
One of these conductors is per-
manently connected to earth, and the other to the circuit or system of circuits to
be protected. An indicator to show when the circuit to earth has been closed may
be attached or combined with this device, actuated magnetically by the passage of
the current when the circuit to earth is closed, or mechanically when the separating
material is destroyed.
In the accompanying drawings,
Fig. 1 is an elevation showing a safety device as made according to our
invention.

page_text: Fig. 2 is a diagram showing an electrical circuit with which is connected a safety
device having one pole put to earth.
Figs. 3, 4, 5, 6, 7 and 8 are various forms or arrangements of the safety device.
Similar letters, throughout the figures denote similar parts or parts having the
same function.
In Fig. 1 the safety device consists of the exhausted and sealed glass bulb a
containing a pair of conductors or fingers, made with a spring which tends to
bring the lower ends or points into contact, but they are separated (when in the
normal condition) by a material or substance e which will become carbonised or
destroyed by the electric current should the electrical pressure exceed the required
limit. The conductors, have terminals of platinum wire passing through and
sealed in the glass similar to the method adopted in making incandescent lamps,
and they are formed into loops at the outer extremity by which they are attached
to the insulated circuit terminals d, d' mounted within a suitable frame e of a case
having a glass front for inspection. The case is secured against the wall or in any
other convenient manner, and is locked or sealed to prevent tampering therewith.
In Fig. 2, the safety device a is shown as connected by the terminals d, d'
to the conductors the former leading from the secondary coils g of the
primary coily of the working circuit, while the conductor f¹ is led to earth, E.
In Fig. 3, the fingers are made straight, but inclined towards each other at the
bottom, and only separated by the interposed substance e as before.
Fig. 4 is the same arrangement as that shown by Fig. 1 but supplemented by a
spring acting to bring the conductors and 6 together, but insulated therefrom.
In Fig. 5 the outer receptacle a is a glass tube with the ends sealed, and in this
two spiral springs, and bare secured at their outer ends to platinum wire
terminals scaled in the glass, and their inner ends are flattened and arranged to
engage with each other by the tension of the springs, but are kept apart by the
substance e as before.
Figs. 6 and 7 are examples of spring fingers and so formed as to come
together at their inner extremities, the former figure in the direction of their length,
and the latter in a direction transverse to their length, but kept apart in each case
by the interposed substance c.
Fig. 8 has a short, rigid and flattened finger 6 while the other finger &¹ is a spiral
spring tending to lengthen, and its flattened end is kept from making contact with
the rigid finger by the interposed substance c.
In some cases it was found necessary to increase the thickness of the platinum
terminals to resist the bending action produced by the spring fingers and b¹, and
as this added unduly to the cost, an insulating loop or bar of glass i is employed to
take the strain, and to give the required resistance to the spring fingers. The
interposed substance c may be paper, cotton, or other suitable substance, and it is
sometimes weighted in any convenient manner to keep the material in position and to
assist in removing the same from between the fingers, when carbonised by an excess
of current.
Having now particularly described and ascertained the nature of our said inven-
tion, and in what manner the same is to be performed, we declare that what we
claim is :-
1. A safety device for electric circuits, substantially as and for the
shown and described.
purpose
herein
2. The combination of two current conducting fingers arranged or formed to
make contact at one point, with an interposed substance capable of being destroyed
or carbonised by an excess of current substantially as and for the purpose
described.
3. The combination of two current conductors, a substance capable of being
destroyed or changed by an excess of current, and an insulating material adapted
to keep the said current conductors in their relative positions, substantially as and
for the purpose herein described.

page_text: 4. A safety device for relieving electric circuits of an excess of current strain
comprising two conductors having an interposed substance capable of being destroyed
by an excess of current, a circuit connection with one of the said conductors, and an
earth connection with the other conductor, operating substantially as and for the
purpose herein described.
5. A safety device for electric circuits comprising an exhausted glass vessel
containing two conductors kept apart by an interposed substance capable of
being destroyed by heat, platinum terminals connected to the said conductors
within and passing through the glass, a circuit connection to one platinum ter-
minal, and an earth connection to the other, substantially as and for the purpose
herein described.
Dated this 26th day of January 1891.
FAIRFAX & WETTER,
433, Strand, London, Agents for the Applicants.