SAFETY FUSES FOR ELECTRIC CIRCUITS
Year: 1884

page_text: We, Sir WILLIAM THOMSON, Knight, of Glasgow College, Doctor of Laws and
Professor of Natural Philosophy in the University and College of Glasgow in the
County of Lanark, North Britain, and JAMES THOMSON BOTTOMLEY, also of the
University and College of Glasgow aforesaid, Master of Arts do hereby declare
the nature of the said invention for "SAFETY FUSES FOR ELECTRIC CIRCUITS " to
be as follows:-
The object of this Invention is to secure that the current shall be stopped by
the action of the fuse, in as short time as possible after it has surpassed the greatest
permissible degree. For the substance in which the heat, which melts the fuse, is
generated we prefer silver or high-conductivity Copper, because the rate at which
temperature commences to rise in a wire, on the commencement, or sudden
augmentation, of a current through it, is inversely proportional to the product of
the electric resistance into the specific heat, and because either of these metals is
more suitable to give definite and constant results, and is also more convenient for
making the connections required to put it into circuit than the lead, or the soft
fusible alloys which have been most frequently used hitherto for safety fuses.
When the permanent current is large enough, for example when it is more
than 8, or 10, ampéres, we use, a copper wire in two parts and united by an easily
fusible solder at its middle-To secure that the contact shall be broken when 'the
solder is melted, we make the end contacts in such a manner that the parts on the
two sides of the soldered middle interface shall be pulled asunder when the solder
melts. This we do, either by hooking the ends of the wire upon two pegs or rings
so as to give it longitudinal pull, or by fixing the ends into holes or slots so as to
produce lateral forces tending to separate the two parts on the two sides of the
middle interface, either the wire itself, or the supports of the two end pieces in the
circuit, being so shaped and placed as to give by their elasticity the required pull,
and at the same time to maintain firm enough electric contacts at the two ends.-
One very easy way by which we do this is as follows:-We make the two halves
of the wire of the fuse of two spiral springs, or portions of spiral springs, with
the hooks and the central parts in the axis of the springs or nearly so, and with
the middle interface perpendicular to the axis. Thus when the solder melts, the
two portions are pulled. directly asunder from the interface.-Another plan by
which we obtain the same result, and which though it requires somewhat more of
workmanship, is preferable because it requires somewhat less of metal in the fuse
and therefore less waste of energy in the use of it, and because it is better adapted
to give good end contacts, is as follows:-the wire to be used for fuse is bent into
the shape of an oblique letter Z, with its end branches inclined, the upper one*
upwards and the lower one downwards from the line of printing supposing the Z,
to be placed in a horizontal printed line.-The soldered interface is oblique to the

page_text: _line of the wire in such a manner as to be perpendicular to the line of printing in
the supposed printed line. To avoid circumlocution, let us suppose this to be the
line which is exactly horizontal in the use of the fuse-An arrangement which
though not necessary will be most convenient in many cases. The lower end
branch of the Z, is pressed into a horizontal hole, into which it fits easily; and
after that, by turning the Z round the axis of this hole, the upper branch is pushed
into a slot between two solid projecting pieces, this slot being wide enough to allow
the branch of the Z to lie obliquely in it. Then by a screw in the upper projecting
piece, the branch of the Z is forced into nearly a horizontal position. The upper and
Tower branches of the Z being thus forced to be nearly horizontal, the whole takes
nearly the form of a regular letter Z, but with rounded corners. As an alternative
we sometimes give to the wire, to be used for fuse, the form of the letter V, with
the soldered interface.at the middle of the bend. In this case the fuse is held by
a screw, or screws, forcing it to take the form of the letter U.-The transmission
of stress through the bent wire in both the Z and U shaped forms is clearly, in
virtue of the syminetry, such that the two parts on the two sides of the vertical
middle interface, are by it urged horizontally from one another. Thus when the
solder melts they spring asunder directly from the middle interface.-We prefer to
make the Z or U shaped fuses of wire of square section, in order to give facility
for clamping the end or ends thereof.-When the current to be provided for is so
small that the wire of the fuse is very thin and flexible, and sufficient force to make
the end contacts cannot be given by its own resilience, we use elastically mounted
end pins, and loops on the ends of the wire by which it is stretched between these
pins; or we fix weights to the two ends of the wire, and hang it over the two pins
- laid in grooves to prevent it slipping off-keeping it stretched by the weights.
In cases where it is convenient that the two pins should be in the same horizontal
line, the wire is simply laid over them, passing a quarter turn round each and kept
stretched by the weights;-or the wire may have one or more turns round each
pin, in any case in which this seems desirable, to secure good contacts.-When one
pin is vertically over the other, the wire must be at least half a turn round the
upper pin, and once round the lower pin, in grooves as, when the pins are
horizontal; but it may, besides, be taken several times round each pin if desirable:―
When the current to be provided for is not more than one or two ampéres, or
a fraction of an ampère, we use very fine copper wire, and in some cases dispense
with the soldering in the middle, and allow the fuse to act simply by melting the
copper wire itself. In every case we keep the wire away from contact with any
solid matter, in order that only itself, and the air surrounding it, shall have to be
heated in order to bring it into action; and we enclose it in a case or box of
suitable material to diminish the waste of heat from currents of air, and to prevent
irregular action from irregular currents of air.-We also prefer to make one side of
the case or box of glass, and to place it so that the action of the fuse may be
readily seen, without opening or touching any part of it. The glass front is
secured by lock and key, or by seal, or by stamped paper, or by paper certified by
handwriting, in cases in which unauthorised tampering with, or replacement of, the
fuse is to be guarded against.-The outer sides of the box or case may be of wood,
when there is no possibility of a sufficiently strong current passing to injure the
wood by its heat; otherwise the box must be made of, or lined with, stone or slate,
or earthenware, or other material not liable to be injured by any temperature that
may be reached..

page_text: COMPLETE SPECIFICATION.
We, Sir WILLIAM THOMSON, Knight, of Glasgow College, Doctor of Laws, and
Professor of Natural Philosophy in the University and College of Glasgow, in the
County of Lanark, North Britain, and JAMES THOMSON BOTTOMLEY, also of the
University and College of Glasgow aforesaid, Master of Arts do hereby declare the
nature of our said invention for SAFETY FUSES FOR ELECTRIC CIRCUITS and in
what manner the same is to be performed, to be particularly described and
ascertained in and by the following statement:—
The object of this Invention is in the first place to secure that the current shall
be stopped by the action of the safety fuse, in as short time as possible after it has
surpassed the greatest permissible degree, and that the amount of energy consumed
in the permanent use of the fuse shall be as small as possible. For the substance
in which the heat, which melts the fusible conductor is generated, we prefer silver
or high-conductivity copper, because the rate at which temperature commences to
rise in a conductor, on the commencement, or sudden augmentation, of a current
through it, is inversely proportional to the product of the electric resistance into
the specific heat, and because either of these metals is more suitable to give definite
and constant results, and is also more convenient for making the connections
required to put it into circuit, than the lead, or the soft fusible alloys, which have
been most frequently used hitherto for safety fuses.
i
When the permanent current is large enough, for example when it is more than
eight or ten amperes. We use a copper wire in two parts and united by an easily
fusible solder at its middle. To secure that the contact shall be broken when the
solder is melted, we make the end contacts in such a manner that the parts on the
two sides of the soldered middle interface shall be pulled asunder when the solder
melts. This we do by booking the end of the wire upon two pegs or rings so as
to give it longitudinal pull, or by fixing the ends into holes or slots or upon properly
inclined metallic surfaces so as to produce lateral forces tending to separate the two
parts on the two sides of the middle interface; either the wire itself, or the support
of the two end pieces in the circuit, being so shaped and placed as to give by their
elasticity the required pull, and at the same time to maintain firm enough electric
contacts at the two ends. One very easy way by which we may do this is as
follows: We make the two halves of the wire of the fuse of two spiral springs or
portions of spiral springs with the hooks and the central parts in the axis of the
springs or nearly so, and with the middle interface perpendicular to the axis. Thus
when the solder melts, the two portions are pulled directly asunder from the
interface.
A form of fuse embodying the features hereinbefore set forth is represented in
Figures 1, 2, 3 and 4 of the annexed Drawings of which Figure 1 is a front
elevation, Figure 2, a side elevation, Figure 3 a back elevation and Figure 4 a
plan.
The fuse-wire a, which is represented at Figures 1 and 2, consists of two straight
portions of wire connected together in line by a fusible metal soldering b. Each
end of the fuse-wire a, so formed is fitted with a comparatively heavy link c, of

page_text: copper or other metal of good conductivity. The peg d, with which the upper
link c, of the fuse-wire is in contact, is fixed and is in metallic connection with one
side of the cut or break in the electric main provided for the insertion of the fuse.
The peg e, with which the lower link c, of the fuse-wire is in contact is formed by
the projecting arm of the spiral spring f, which is in metallic connection with the
other side of the cut or break in the electric main. The two pegs d, and e, are
approximately in the same vertical plane, and have the upper and lower surfaces
respectively slightly concave (as represented more particularly at Figure 2) so as to
prevent the links c, of the fuse-wire a, from slipping off the said pegs by reason of
any vibration in the support to which the fuse is fixed. The drawing indicates
by dotted lines at Figure 1, the position taken by the projecting arm of the spiral
spring f, when there is no fuse-wire in the circuit, hence, by virtue of the resilience
of the said spring f, the peg e, exerts on the fuse-wire a, placed in the circuit, a pull
acting in the direction of the axis of the wire. By virtue of this pull the two
portions of the fuse-wire a, are pulled asunder and the circuit broken when the
current increases beyond the normal amount and has thereby melted the fusible
soldering b, A portion of the fuse-wire a, so detached is represented in dotted lines
at Figure 1. The two screw terminals g. at the back of the fuse-box h, are placed'
in good electric connection with the two ends of the cut or break in the electric
main. To place a fuse-wire a, in the electric circuit all that is necessary is to hook
one of the links c, on the peg d, press upwards the projecting arm forming the
peg e, and hook the other link c, of the fuse-wire a, upon it.
i In place of the spring f, herein before described, we may employ a spiral or
equivalent. spring having its axis in line with the fuse-wire a, and thereby
producing the necessary tension in the said fuse-wire a.
We may also obtain the foregoing results by another method of applying the
resilience of the fuse-wire.. The wire to be used for the fuse is in this application,
bent into the shape somewhat resembling an oblique letter Z, with its end branches
inclined, the upper one upwards and the lower one downwards from the line of
printing Supposing the Z to be placed in a horizontal printed line. The soldered
interface is oblique to the line of the wire in such a manner as to be perpendicular
to the line of printing in the supposed printed line. To avoid circumlocution, let
us suppose this line of printing" to be the line which is exactly horizontal in the
use of the fuse, an arrangement which though not necessary will be most
convenient in many cases. The lower end branch of the Z, may be pressed into a
horizontal hole, into which it fits easily; and after that, by turning the Z round
the axis of this hole, its upper branch is pushed into a slot between two solid
projecting pieces, this slot being wide enough to allow the upper branch of the Z
to lie obliquely in it. By a screw in the upper projecting piece, the branch of
the Z, is forced into nearly a horizontal position. The upper and lower branches of
the Z being thus forced into a nearly horizontal position the whole takes
approximately the form of a letter Z, having rounded corners. As an alternative
we sometimes give to the fuse-wire the V-form that is to say, a letter U with its
arms bent outwards, and having the soldered interface at the middle of the bend.
In this form of fuse, the fuse wire employed, is connected by screws to fixed end
pieces causing it, when in the circuit, to have the form of the letter U. The
transmission of stress through the fuse-wire in both the Z-shaped and the
U-shaped forms is clearly, in virtue of the symmetry, such that the two parts on
the two sides of the vertical middle interface, are by it, urged apart horizontally
or vertically from one another. Thus, when the solder melts, the two parts of the
fuse-wire spring asunder directly from the middle interface. We sometimes make
the Z or U-shaped fuses of wire of square section, in order to give facility for
clainping the fuse-wire to the fuse connections.
The application of the Z-form of fuse which we prefer to employ is represented
at Figures 5, 6, 7 and 8, Figures 5 and 6 being respectively a front and a side
elevation, Figure 7 a transverse vertical section, and figure 8 a plan. The fuse-
い

page_text: T
wire 1, in this application is in two parts united by fusible, soldering ), at about the
middle of its length, and with one end bent into, the form of a hook as represented
at k upon the annexed drawings. It is important, for the proper action of this
form of fuse, that the two ends of wire at the soldering be not connected in the
form of a butt joint, but in the form of an overlapping joint, and that the plane
passing through the overlapping ends of wire at the joint be perpendicular to the
plane of the hook on the one end of the fuse-wire. We further arrange, by the
construction of the fuse-wire i, and by the manner of placing it in circuit as
hereinafter described, that on the melting of the fusible solder j, the two ends at
the soldering will separate in virtue of the resilience in the two parts of the fuse-
wire i, and in doing so will not require to slip past one another. The form and the
relative position of the two end supports l, and m, under one application of
the Z fuse, are shewn more particularly at Figure 7, and it will be seen by
reference to this Figure that when the fuse-wire i, is in the electric circuit, with
its ends secured each in electric contact with its respective support, it will, when
looked at from a point of view at one side of the fuse-box n, take a form
approximating to that of the letter Z. As an alternative, we sometimes make the
end supports for receiving the fuse-wire i, with their contact surfaces in the same
vertical plane, and obtain the necessary resilience in the said fuse-wire 2, by giving
to it a slight bend when placing it in the electric circuit within the fuse-box n,
according to the manner hereinafter set forth. A fuse-wire similar to that shewn
in Figures 5 and 7, is placed in circuit within the fuse-box n, by hooking it from
the left-hand side on the pinching screw o, of the upper support, and thereby
securing that end of the fuse-wire, and then pushing in and securing by the
screw p, on the lower support m, the other end thereof. In the case of the
modified form of this fuse herein before referred to, the fuse-wire with its hook
turned to the lower end, is hooked on from the left-hand side and secured on the
lower support, then the finger is applied" behind the fuse-wire close to the lower
support, to bend the fuse-wire slightly out from the vertical, next apply pressure
at the top end of the fuse-wire to bring it into position for securing on the top.
support where it is fixed by tightening the screw.
..:
"
The application of the U-form of fuse for employing the resilience of the fuse-
wire to effect the separation of the fusible joint is shewn at Figures 9, 10 and 11,
Figures 9 and 10 being respectively front and side elevations and Figure 11 a back
elevation thereof.
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The fuse-wire q, for this form of fuse consists of two pieces of wire soldered
together in a line, similar to the fuse-wires herein before described. "In this case
however the fuse-wire q, is bent into the form of the letter U with the soldered
joint in the middle of the bend, the soldered connection being preferably a butt
joint. At the top of one of the branches of the fuse-wire q, a hook is formed in a
plane at right angles to the plane of the U. The two supports or terminal blocks ?,
with their screws 8, are in metallic connection each with one side of the cut
break in the main respectively. It will be seen with reference to Figure 9 of the
annexed drawings, that the two contact surfaces of the supports r, for the ends of
the fuse-wire q, are so inclined to each other that when a fuse-wire is secured
the circuit its two ends are pressed inwards from their normal relative position
(represented in dotted lines, at Figure 9) and the resilience of the wire so brought
into play tends to pull apart in an approximately horizontal line, the two ends at
the soldered joint, thus effecting their separation when the solder melts. In placing
in circuit a fuse-wire of the U-form herein last described the hook of the U is
passed over one of the screws 8, and by it' secured to the support r, then the finger
is applied at the top of the other branch of the U to bring it into position for
clamping to the other support. We sometimes make a hook on each branch of
the U and in that case, in placing it in circuit' we simply unscrew each terminal
screw 8, sufficiently to receive the hooks of the U-fuse-wire without bending inwards
the branches and then screw up the terminal screws so as to fix the ends thereof.
...
71
5

page_text: In order to secure that the two parts of, the fuse-wire shall separate sufficiently
when the solder melts, we sometimes use a round bar mounted eccentrically or
a bar rotating on its axis and carrying an eccentric disc, so that after the fuse-wire
is placed in the circuit a half-turn of the bar, or bar carrying the eccentric disc,
applies a bending force to one end of the fuse-wire, which force aids the resilience
otherwise imparted to the fuse-wire for the purpose of effecting the separation of
its parts when the solder melts. To avoid circumlocution we have described and
represented upon, the annexed drawings the afore-mentioned fuses as constructed
for a fuse-wire in a vertical position only, but each of the fuses could effectively be
placed so that the fuse-wire would be. in a horizontal, an inclined or in an
inverted position.
When the current to be provided for is so small that the wire to be used for the
fuse-wire is necessarily very thin and flexible, and sufficient force to make the end
contacts cannot be given by its own resilience, we use elastically mounted end
pins, and loops or links on the ends of the fuse-wire by .which it is stretched
between these pins; or we fix weights to the two ends of the fuse-wire and hang
the said fuse-wire over two fixed pins and in grooves formed therein in order to
prevent it slipping off-In cases where it is convenient that the two pins should
be in the same horizontal line, the fuse-wire is simply laid over them, passing
a quarter turn round each and kept stretched by the weights.
Should it be found desirable in order to secure good contacts, the fuse-wire may
have one or more turns round each pin. When one pin is vertically over the
other, the wire must be at least half a turn round the upper pin and once round
the lower pin, in grooves as when the pins are horizontal, but it may if desired for
good electrical contact be taken several times round each pin.
A fuse constructed according to the manner hereinlast described is represented
at Figures 12 and 13 respectively in front and side elevations. The two contact
pins t, are each fitted with a screw terminal u, by which they are put in metallic
connection respectively with the two ends of the cut or break in the electric main
and the fuse wire v, which has the weights w, secured one to each end, is laid over
the said contact pins t, in the grooves . When the current to be provided for is
not more than one or two amperes, or a fraction of an ampere, we use very fine
copper wire, and in some cases dispense with the soldering in the middle, and allow
the fuse to act simply by melting the copper wire itself...
In every form of fuse constructed in accordance with our present. Invention, we
keep the fuse-wire away from contact with any solid matter, in order that only
itself, and the air surrounding it, shall have to be heated in order to bring it into
action; and we enclose it in a case or box of suitable material to diminish the
waste of heat from currents of air, and to prevent irregular action from irregular
currents of air. We also prefer to make one side of the case or box of glass, and
to place it in such position that the action of the fuse may be readily seen, without
opening or touching any part of it. The glass front is or may be secured by lock
and key, or by seal, or by stamped paper, or by paper certified by handwriting, in
cases in which unauthorised tampering with, or replacement of the fuse is to be
guarded against. The other sides of the case or box may be of wood when there
is no possibility of the current passing through the fuse wire becoming sufficiently
strong to scorch or ignite the wood by the heat generated, otherwise the box must
be made of, or lined with stone, slate or earthenware, or other material not liable
to be injured by any temperature likely to be reached.
Having now particularly described and ascertained the nature of our said
Invention and in what manner the same is to be performed, we declare that what
we claim is
First Fuses for electric circuits wherein are provided such arrangements that
the heat generated in the fuse conductor shall permit the circuit to be broken by
the melting of a metal of lower fusing point than the fuse conductor substantially
as hereinbefore described with reference to the annexed drawings.

page_text: Second. The arrangement of resilience of substance of the fuse conductor, or of
the main conductor or either or both sides of the fuse, or of a spring not conveying
any part of the current or of an equivalent weight, by which when the fusible
metal of the fuse melts the force required to break the circuit acts substantially as
herein before described.