IMPROVED ELECTRICAL APPARATUS FOR INDICATING THE EXIST- ENCE OF INJURIES OR DEFECTS IN ROOFS, BRIDGES, AND OTHER STRUCTURES
Year: 1877

page_text: PROVISIONAL SPECIFICATION left by Fred Barber at the Office of the
Commissioners of Patents on the 22nd October 1877. A communication from
abroad by John Forbes, of Dartmouth, Nova Scotia.
FRED BARBER, of Sylvester Works, Sheffield, in the County of York, Cutlery
Manufacturer. "IMPROVED ELECTRICAL APPARATUS FOR INDICATING THE EXIST-
ENCE OF INJURIES OR DEFECTS IN ROOFS, BRIDGES, AND OTHER STRUCTURES." [A
communication from abroad by John Forbes, of Dartmouth, Nova Scotia.]
This Invention relates to means for obtaining a prompt indication of any defect
or insecurity in bridges, roofs, and other structures, when such defect or insecurity
is caused by overloading, or undue or excessive strain of any part or of the whole
structure, or by derangement of parts at joints or connections, or by any change of
condition, either sudden or gradual, from that under which the structure was
designed to serve.
In the case of a bridge, for instance, the various parts of which are subjected to
either tensile or compressive strain, the several proportions of such parts are so
adjusted as to afford a determined margin between the ultimate breaking load or
strain, and the greatest load or strain to which such part is intended to be exposed
under traffic or use.
This margin may be reduced by various causes, such as the passage of an
excessive load, a sudden jar produced by slight obstructions to wheels, breaking of
flanges, or by other causes, or within the structure itself, such as loosening of rivets,
gradual weakening of the part under frequent repetitions of the load, imperfect
workmanship, flaws in the material, or errors of calculation not apparent at the
time of erection.
.
Such lessening of the margin of safety need not be fatal to the structure, pro-
vided it can be detected, and suitable remedial measures promptly adopted, and
the originally designed margin of safety obtained.
In the case of parts exposed to tensile or compressive strain no permanent injury
will result until the limit of elasticity of material is exceeded. Indication of over-
loading will be recorded before this limit has been reached.
In the case of dislocation or derangement of parts, as the slipping out of position or
shutting by of butted joints, as in upper chords or vertical posts, or oblique struts,
in all such, and in all similar or analogous cases, it is the object of my Invention

page_text: to furnish a ready means for obtaining a direct indication, that such overstrain or
derangement has taken place, although no permanent set or immediately apparent
evidence may have been left upon the part or structure itself.
My said Invention consists in attaching upon each part of the structure an
insulated wire or conductor, so arranged that an electric circuit may be made or
broken by the abnormal condition resulting from the excessive strain or dislocation
of parts, from whatever cause it may arise, through which a motion, either in the
substance of the part itself or between contiguous and adjoining parts, has taken
place, the closing or rupture of the circuit to be indicated by an annunciator
operated by the electric current from a battery through electro-magnets suitably
arranged, the armature of the magnets being so adjusted, that as in the case of call
bells a number or indicator shall be exposed, said number to indicate a particular
part of the structure.
I propose to employ terminals so arranged that in the case of tensile parts
the sum of the extra extension caused by overstrain shall unite the terminals and
complete the circuit, and in the case of compression parts the terminals to be so
arranged that the sum of the extra compression caused by overstrain shall in
that case also unite the terminals and complete the circuit; and thus in either case
produce an indication. The same result, if under any circumstances it should
be more convenient, may also be produced by the rupture of a closed circuit.
In the case of joints or any similar portions of a structure upon the integrity of
which the margin of safety depends, the same arrangement as above described may
be made to indicate the part in which dislocation or derangement of parts has taken
place.
I
The several return wires may be all separately connected with the battery, or
they may be all joined, and a single wire pass from their junction, and a suitable
switch provided in it, so that only a permanent derangement, or set, or dislocation,
may be indicated, and that only when a connection shall be made with the battery
on the occasion of the visit of the inspector to the annunciator, which may be done
previous to the entry of a train, or at stated intervals.
The insulated conductors may in practice be secured upon the various parts in
any convenient manner, notably by small screws through lugs, or by pinching
bands or the like, the separated terminals constituting the open circuit system.
Eyes at the terminals having an inside diameter as much larger than the diameter
of the material of the other terminal as will admit of certain latitude of motion
corresponding with the margin of motion decided upon as admissible before indi-
cation is required, I consider, as a convenient and well adapted shape in many cases,
and other forms of terminals may be used.
The terminals may be gilt, nickeled, or platinized, in order to ensure proper con-
ductivity at point of contact.