Improvements in Electrical Cut-outs or Safety-fuses.
Year: 1891

page_text: Date of Application, 13th Jan.. 1891-Accepted, 21st Feb., 1891
COMPLETE SPECIFICATION.
Improvements in Electrical Cut-outs or Safety-fuses.
I, JOHANN PULUJ, Professor at the Technical High School, Prague, in the
Empire of Austria, 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 :-
In order to protect electrical conductors if they accidentally become short
circuited from becoming red-hot, cut-outs of lead or tin-foil are used at present in
different construction, the leading idea is that lead and tin-foil on account of their
comparatively low melting point will be melted by an increase in the current
through the conductor and therefore will break the current before the conductor
becomes red-hot. By a suitable thickness and length of leaden or tin-foil strips
the said purpose is attained but the danger of fire is by no means entirely excluded
for even a small spark from the melting conductor might cause an explosion in
places filled with combustible gases or explosive dust-clouds. Numerous experi-
ments which I have made with different cut-outs of lead or tin-foil have persuaded
me that even a well-constructed and well-closed cut-out may prove dangerous after
repeated melting of the protecting wire because the precipitate from the metallic
vapours may form a connecting bridge between the terminals and give rise to the
formation of a voltaic arc. From this therefore results the necessity of not only
hermetically closing up the protecting wire of a cut-out but also of removing the
precipitate from the metallic vapours resulting from the fusion of the protecting
wire.
The construction of the cut-outs described here however is based besides upon
another.idea. If with a certain distance between the terminals a wire is melted
and evaporated the electric arc between the terminals can only be formed and
maintained for an appreciable time if there prevails a suitable difference of potential
at the ends of the wire and if there is a certain quantity of glowing hot metallic
vapour of sufficient conductivity. If the quantity of vapour be a very small one
the voltaic arc cannot be formed. To exemplify this in practice for 3 incandescent
lamps of 0.54 amperes and 120 volts a leaden wire 1" thick is used as a cut-out.
But for the same purpose a copper wire only 0.1 mm thick will suffice without
getting particularly warm. However if such a leaden wire is melted within a
distance of the terminals from each other of only 10 m/ and a tension of 120 volts
the voltaic arc will be formed and will melt the terminals whereas a copper wire
of the above mentioned thickness melted off under the same conditions will not
give rise to the formation of the voltaic arc because the quantity of vapour
arising from the fusion of the copper wire is about 100 times less than that of the
leaden one.
The formation of the voltaic arc is likewise prevented by the high melting point
of the copper wire because the metallic vapours formed must be condensed the
more quickly the greater the difference is between the temperature of the vapours
and the environs. From this it is easily to be understood that metals with a high
melting point and great conductivity are better adapted to securing the conductors
than the leaden wires and tin-foil strips hitherto employed. To such metals of high
melting point belong in the first place copper and silver, but also platinum, iron,
brass, German silver and other alloys of a high melting temperature can be used
for the purpose just mentioned. Platinum may besides be introduced into a small
glass tube closed at either end and a platinum wire sealed in this manner forms an
absolutely fire-proof cut-out for electric conductors which may be used even in the
midst of explosive gases without any danger whatever.

page_text: "
So-called " Lyons thread used for decorating Christmas trees is also very
suitable also carbon filaments coated with metal of a high melting point.
In Fig. 1 is represented a platinum wire melted into a small glass tube and
Fig. 1 shows a copper wire cemented into another glass tube at both its ends.
Fig. 1° represents a small glass tube with platinum wires introduced into it to each
of the ends of which wires a thin copper wire is soldered in the interior of the
tube. In Fig. 1d a finished fuse is represented. Each of the wires is bent and
projects out of the tube and is soldered at m to a small copper cap H, these caps
are cemented on the ends of the small glass tube. Fig. 1 shows another sort of
safety fuse. A strip made of a good incombustible insulator and slit or forked at
each end is furnished with a protecting wire the ends of which are drawn through
the slits and wound round the ends of the insulator as in Fig. 1. In order to
prevent its being damaged the safety-wire may be placed between two slips by
which also the condensation of the metallic vapours inside the fuse case is attained.
In Fig. 18 is shown a fuse constructed in this manner and having wound on its ends
a piece of thin copper plate or tin-foil by which the contact with the safety wire
is effected.
Fig. 2 shows the construction of a cut-out of oblong shape into which the safety
fuse P is inserted from above between two contact springs F F. There are two
binding screws L L to fasten the conducting wires. The contact springs are
mounted on a small square and oblong plate or base S S made of insulating
material such as serpentine, china, glass, wood or the like. This plate may be
fastened to a wall by means of screws K K. Fig. 2" shows the cross section of the
contact springs F and fuse P.
Fig. 3 represents the safety-fuse P mounted on a plate of serpentine or other
insulating material. The ends of this safety fuse are pressed by two springs F F
against two brass plates or strips to which the conducting wires may be screwed by
means of binding screws L L. This plate is covered in by a cap of enamelled
iron or other insulated or insulating material. This cap is fastened to the plate
by a screw at M. Fig. 3b shows the cross section of the plate and of one of the
contact springs.
Figs. 4 and 4' represent a cut-out with contact springs F F between which the
fuse P may be inserted from above.
Figs. 5 and 5 show another construction of oblong shape where the fuse may be
inserted sideways.
Fig. 6 represents a concise construction of cylindric form. The fuse P is
enclosed in a small glass-tube closed at both ends by metallic caps which may be
directly used as terminals for the conducting wires. The cap H is cemented to the
glass-tube the other end of the tube however is fitted with a ring on which the
second cap H1 may be screwed. A spiral spring F ensures the contact between
the safety fuse P and the caps H and H¹.
All these before described constructions of cut-outs for electric conductors
afford the advantage that the precipitate from the metallic vapours may be removed
together with the glass tubes out of the safety fuse case which according to the
experiences above mentioned is of great importance if these safeguards are to be
relied upon. Numerous experiments were made with the safety-tubes here described
by putting them in gun-cotton or in illuminating gas and melting the safety-
wires by a sufficient current. They melt almost instantaneously and noiselessly
without bursting the glass-tube and without setting on fire the gun-cotton or the
illuminating gas.
These fuses may therefore justly be denoted as absolutely safe.
Having now particularly described and ascertained the nature of my said inven-
tion, and in what manner the same is to be performed, I declare that what I
claim is :-
:-
1. For the protection of electrical conductors in case of their becoming short-
circuited the insertion of safety fuses consisting of metal wires or nine of a high

page_text: melting point or of carbon threads or filaments coated with a galvanic deposit of
any suitable metal of a high melting point confined in closed tubes of any suitable
insulating material substantially as described.
2. For the purpose of readily switching in and exchanging safety fuses the
securing of them between spring controlled contacts substantially as described and
illustrated in the accompanying drawings.
3. For the protection of electrical conductors in case of their becoming short
circuited the employment of copper silver platinum iron brass German silver and
other alloys the melting point of which is higher than that of lead and tin-foil
substantially as described.
4. The construction of safety fuses of metals with a high melting temperature
in order to protect electric conductors substantially as described.
5. The combination with a safety fuse of two contact springs by means of which
it is inserted into the circuit substantially as described and illustrated in the accom-
panying drawings.
6. The various forms of cut-outs or safety fuses substantially as described and
illustrated in the accompanying drawings.
Dated this 13th day of Januar 1801
W. P. THOMPSON & BOULT,
Agents for the Applicant.