IMPROVED SAFETY DEVICES FOR USE WITH ELECTRIC APPARATUS OR CONDUCTORS, DESIGNED CHIEFLY FOR PREVENTING OR DIMINISHING FIRE RISKS
Year: 1882

page_text: PROVISIONAL SPECIFICATION left by Silvanus Phillips Thompson at the
Office of the Commissioners of Patents on the 21st August 1882.
SILVANUS PHILLIPS. THOMPSON D.Sc. University College, Bristol in the County
of the City of Bristol "IMPROVED SAFETY DEVICES FOR USE WITH ELECTRIC
APPARATUS OR CONDUCTORS, DESIGNED CHIEFLY FOR PREVENTING OR DIMINISHING
FIRE RISKS."
The objects of this invention are means or devices designed chiefly for ensuring
greater safety from fire than has been heretofore attainable in the use of electric
currents for lighting telegraphing and for kindred purposes.
The first part of my invention relates to the devices known as "safety catches"
which serve to obviate the injury that might arise to incandescent lamps, telephones
and other apparatus, by the passage of a stronger current than such apparatus are
capable of bearing. To this end heretofore a strip or wire of lead or other easily
fused metal has been inserted in the path of the current, the current being com-
pelled to pass through said strip or wire. Now when the electro motive force was
much beyond the normal limit, the excess of energy being converted into heat in
the soft metal caused its fusion and produced a consequent interruption of the
current.
This among other disadvantages, besides totally interrupting the connection, gave
rise to troublesome repairs. I obviate or diminish these difficulties as follows.
My improved safety catch instead of being a piece of soft easily fusible metal, is
a piece or strip, or a wire, of iron, platinum or similar material which, whilst being
easily heated to a considerable degree by a strong current will sufficiently resist
fusion thereby. By the heat produced in the said strip or piece, I cause fusion of an
easily melted substance which normally is in contact with the strip or wire and I
utilize this indirectly caused fusion for either interrupting or short circuiting
the current, or for neutralizing the excess current by the insertion of a suitable
resistance into the circuit.
The fusible material may either be a conductor such as lead, tin or an alloy, or
preferably a non-conductor, wax or resin for instance
One advantage gained by the use of this part of my invention is, that the limit
at which the current will be interrupted or diminished (relatively to the lamp or
Price A1

page_text: telephone &c) can be more accurately predetermined whilst the chief advantage to
be derived therefrom is, that I am enabled to use for this device, in connection with
iron, platinum or the like, materials fusible at a much lower temperature than those
previously applied to the same object, and I thereby considerably lossen the dan ger
of ignition to surrounding objects.
Another form of improved safety catch consists of a strip or bar either straight,
curved helicoidal, or of any other suitable shape and made of such conducting
material or inaterials that it shall when heated by the passage of too strong a current
through it, bend, or expend, or twist or contract, in such a manner as by its move-
ment to interrupt or short circuit the. current, or regulate it by introducing
additional resistances into the circuit. Such thermoscopic safety-catches are in
some cases dependent upon the simple expansion and contracting of the heated
conductor; in other cases they depend upon the difference of expansion of two
different materials forming the strip or bar, either or both of the said materials
being conductors of the current Such thermoscopic safety catches can be made
to act either directly by breaking or short circuiting or regulating the current or
indirectly by actuating levers or other mechanical devices whose office is to break
or short circuit, or regulate the current.
The second part of my invention relates to means for preventing or diminishing
the sparking produced at the commutators, collectors, switches and other contact
points of electric machinery and apparatus at the moment when the path of
the current is changed or ruptured.
4
I employ for this purpose what I term an "Induction shunt." The induction
shunt consists of a coil or coils of insulated wire of sufficiently high resistance,
with or without an internal iron core. Said induction shunt is placed across the
contact points where there is liability to sparking, the coil and its core, if any, being
so chosen and constructed (as to length, number of coils, materials &c) that the
ratio of its co-efficient of self-induction to its resistance shall be equal to the ratio of
the co-efficient of induction to the resistance of the whole of the working circuit, or
the branch thereof across which the induction shunt is placed, as will be readily.
understood by electricians.
The point of equality in inductive action is the essential one, because without
this equality the extra-current sparks due to unbalanced induction of the circuit
on itself are certain to appear. The induction shunt may be made of high or
low resistance according to the purpose to which it is to be applied. For
example for use as a shunt-cut-out for an arc lamp which has an electro magnetic
regulator, the induction shunt should have a resistance equal to that of the
lamp, whereas an induction shunt for preventing sparking on breaking the main
circuit of a dynamo generator may have a very high resistance.
Induction shunts of the type herein before described are applicable to other
purposes besides those hereinbefore set forth; being useful for diminishing the
effects of induction in general at the moment of breaking or making the circuit
irrespective of the question of the production of sparks at the switch or commutator
I also utilize the second part of my invention in constructing an improved safety
switch vie:
My improved safety switch has a threefold action that is to say, previous to
breaking circuit in a main leading wire it makes contact through an induction
shunt of bigh resistance across the point where the main circuit will be broken; it
then breaks the main circuit preventing sparking by the previous application of the
inducting shunt and lastly it interrupts the current through the induction shunt, at
this last step a spark will be produced, but if the resistance of the induction shunt
be high enough the spark will be too minute to be in any way dangerous
In some cases where an induction shunt of sufficiently high resistance is used the
first of the describod three actions can be dispensed with that is to say the induc-
tion shunt may if desired connect the two ends of the line before the switch is
turned as in such case the very trifling current that may leak through the induction
shunt will have no appreciable effect in heating it.

page_text: When the switch is reversed its action may be such that the main line will be
completed before contact through the induction shunt is made.
My improved safety switch is applicable to main circuits and generally in all
cases where switches are used.
The use of my induction shunt, whether independent of or embodied in the
switch, by preventing or greatly diminishing sparking also reduces the fire risks
attendant upon the use, especially of very powerful, electric currents for the purposes
hereinabove indicated.
It is obvious that either both parts of my invention jointly or one or the other
part thereof alone may be used with an electric apparatus, as described.