Improvements relating to Electrical Resistance-boxes and like Apparatus and to Switches therefor.
Year: 1899

page_text: Date of Application, 24th Mar., 1899-Accepted, 13th May, 1899
Improvements relating to Electrical Resistance-boxes and like
Apparatus and to Switches therefor.
We, CHARLES THOMAS GAMBRELL, and THOMAS EDWARD GAMBRELI, both ot
17, Giltspur Street, in the City of London, Electricians and Manufacturers of
Scientific Instruments, do hereby declare the nature of this invention, and
what manner the same is to be performed, to be particularly described and ascer-
tained in and by the following statement:-
--
Our invention relates to electrical resistance-boxes and like apparatus and to
switches therefor.
Our said invention is chiefly designed to provide a substitute for the plug-
switches heretofore in general use for Wheatstone bridges, resistance-boxes,
condensers and like apparatus.
In the plug-switches heretofore employed, there is a certain amount of varia-
tion in the contact resistance between the plugs and the blocks between which
they are inserted, this being due to particles of dust lodging on the contact sur-
faces and also to oxidation or other atmospheric influences. Moreover dust,
moisture and other matter are liable to collect on the surface of the ebonite or
other insulating plate on which the said blocks are mounted and thus impair
the insulation resistance between them. Another disadvantage incidental to the
use of the ordinary plug device, in cases where linear series of contact blocks
with plugs between them are used, is that the removal of one or more plugs is
liable to alter the contact resistance of the remaining plug-contacts.
Now by our invention, we entirely obviate the above-mentioned disadvantages.
An important feature of our said invention is the use of a conducting bar
arranged in proximity to each series or group of fixed contact-pieces and pro-
vided with a sliding contact-block which, by moving it along the said bar in
one or the other direction, can be brought into engagement with either of the
fixed contact-pieces in the said series or group. We prefer to provide suitable
means whereby the contact-pressure between the sliding contact-block and the
fixed contact pieces can be decreased during adjustment of the resistance and
consequently increased when the final test or tests are to be made.
We find it advantageous to arrange the fixed contact-pieces in rows or linear
series on a plate of ebonite or other suitable insulating material and to make
the contact surfaces thereof and of the sliding contact blocks of circular or arc
shape in transverse section, so that the sliding contact blocks will make good
contact with the fixed pieces notwithstanding oscillations of the said blocks."
To provide for the adjustment of the resistance whilst the pontact surfaces are
entirely enclosed and protected from dust and moisture, we sometimes attach to
the said sliding contact-blocks, bars or rods which extend out through a protective
casing and are provided with handles to permit of their being actuated from the
exterior of the said casing.
Our said invention, moreover, comprises other improvements hereinafter set
forth.

page_text: In the accompanying drawing, we have shown how our said invention may be
conveniently and advantageously carried into practice.
Figure 1 is a plan, with the lid removed,
Figure 2 a section on the line -, Figure 1, and
Figure 3 a section on the line, Figure 1, showing a resistance--box
having our improvements applied thereto.
Figure 4 is a diagram illustrating the connections employed in the said
apparatus.
Like letters indicate corresponding parts throughout the drawing.
" is a base of ebonite or other suitable insulating material on which are fixed
contact-pieces barranged in rows or rectilinear groups. The resistance coils
are connected to rods & secured to the underside of the contact pieces b, which
rods, as in the arrangements heretofore employed, serve for securing the said
contact-pieces to the base a, for holding the coils c in position and also for making
the necessary connections. If desired, however, other suitable arrangements may
be employed for supporting the bobbins.
'
A conducting bare is arranged above and parallel with each row of contact-
pieces b, and on each conducting bar is mounted a sliding contact-block f of
suitable length to engage with one contact-piece band having its under surface
shaped so as to fit the upper surface of the block f and make a good contact there-
with. We prefer to make the contact-surfaces of the pieces 6 of semicircular
farm in cross-section so as to ensure good contact between the said contact-pieces b
and the sliding block f, even should the said block be tilted slightly to one or
the other side.
Each conducting hare is provided at both ends with a rod y extending at right
angles thereto through holes in the base a. These rods pass through washers k
arranged above the said holes in the base plate a. and, in the case of the balancing
resistance, are provided beneath the said base-plate with screw-threaded discs
or nuts and with springs k, which bear against the underside of the base a and
against the said dises j, and tend to draw the conducting bar e down towards
the contact-pieces and thereby press the sliding contact-blocks f against the
said contact-pieces with force enough to ensure cleaning of the contact surfaces
during the motion of the sliding blocks f, and also to provide a sufficiently good
contact between the said surfaces to enable the value of the balancing resistance
for each particular case to be determined approximately. The rods are also
provided at their ends with nuts / adapted to engage with transverse bars m of
ebonite or other suitable insulating material, stiffened laterally, if necessary,
by brass strips. These bars m are provided with holes through which the said
rods pass. Above these transverse bars are arranged cams of ebonite, metal
or other suitable material, mounted on a shaft turning in bearings on the
base a and provided with a detachable handle whereby it can be readily turned
about its axis so as to cause the said cams to force the bars m and e downward
and press the contact-blocks f firmly against the contact-pieces and thus ensure
an efficient contact between them and enable a final test or tests to be made.
Each pair of bars m controls the motion of four longitudinal conducting bars
so that, by a single motion of the shaft p, the whole of the contacts necessary
for determining the value of the balancing resistance are made secure. It is
obvious that, if desired, there may be any desired number of rows of coils and
that there may be more than one cam-shaft. In cases where there is more than
one cam-shaft, it is sometimes advantageous to connect the said shafts together
by suitable gearing so as to permit of their being operated simultaneously by a
single handle.
In the case of the proportional, or "ratio" coils, the arms of which are arranged
in the same row, a separate conducting bar el having rods g is provided for each
arm, these bars a being drawn down solely by the action of the springs which,
in this case, act on screw-nuts and are of sufficient strength to ensure good

page_text: contact between the sliding-blocks f and the fixed contact-pieces without
employing cams as in the arrangement herein before described with reference to
the balancing resistance. If desired, these can also be provided with cams or
equivalent devices for the purpose above specified.
In some cases, moreover, we find it advantageous to employ weights to act on
the rods g or bars e, e' in place of the springs k.
The contacts b, f are covered by means of a casings having a lid s' which is
made dust-proof and of such size as to leave the terminals exposed. To permit
of adjusting the sliding contact-blocks f from the exterior, rods are secured to
the said blocks respectively by screws or other suitable means and extend through
bushed holes in ebonite or other insulating plates forming the ends of the
casing these rods are provided with handles whereby they can be moved,
and with graduations for indicating the particular contact-piece on which the
sliding block f is resting at any time.
"
The resistance-coils in the balancing arm are connected together as indicated
in Figure 4, in a similar manner to the corresponding coils of a dial-pattern
Wheatstone bridge, the bars e corresponding to the central dise of each dial and
the contact-pieces to the contact pieces arranged around sush central dise.
The proportional, coils are similarly connected except that, in each arm, one end
of each coil is connected to a common conductor e leading to one of the diagonally
opposite terminals of the quadrilateral circuit, for example, to the galvanometer
terminals G, GL as shown whilst the bars el are both connected by a conductor
to one of the other terminals, for example, to the battery terminal B. The re-
sistance X to be measured can be connected, as usual, to the terminals (+ L, BL;
the battery and its key between the terminals B, B L; and the galvanometer
and its key between the terminals G. GL. INF represents a metal strap or
conductor which can be removed when it is desired to insert a practically infinite
resistance into the balancing resistance arm, i.e. to interrupt the circuit
It is obvious that the apparatus can be connected, up for other measurements
as required.
Another advantage obtained by our improved connecting or switching device
is that the circuit through the balancing resistance arm of the bridge is not
broken during the adjustment of the resistances in the said arm as is the case in
the plug form of the dial-pattern bridge, and thus no violent kicks of the gulva-
nometer needle will be produced if the keys be inadvertently kept depressed
during adjustment of the said resistances.
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:--
1. A conducting bar held in proximity to a series or group of fixed contact-
pieces and provided with a sliding contact-block which, by moving it along the
said bar, can be brought into engagement with either of the said contact-pieces,
substantially as hereinbefore described and for the purposes specified.
2. A switch for resistance-boxes and like apparatus, comprising a series or
group of fixed contact pieces, a conducting har arranged parallel with the said
series or group, a contact-block movable along the said bar and adapted to engage
with either of the said contact-pieces and means for pressing the said movabl
contact-block against the contact-piece, substantially as, and for the purposes,
hereinbefore described.
3. The combination, with a row or series of fixed contact-pieces, of a conducting
bar arranged parallel therewith, a sliding contact-block on the said bar, and
springs or weights tending to draw the sail bar towards the fixed contact-pipes,
for the purposes above specified.
4. The combinat.on, with a row or series of fixed contact-pieces, of a conducting
bar arranged parallel therewith, a sliding contact-block on the said bar, and cams

page_text: or equivalent devices for pressing the said contact-block forcibly against one or
other of the said fixed contact-pieces, substantially as, and for the purposes, here-
inbefore described.
5. A switch such as is hereinLefere described, wherein the contact surfaces of
the fixed and sliding contact-blocks are circular or arc-shaped in transverse
section, substantially as, and for the purpose, above specified.
6. The combination, with the sliding contact-blocks in our improved switch,
of rods attached thereto and extending through a protective casing enclosing the
contact-surfaces, substantially as, and for the purposes, hereinbefore described.
7. A resistance-Dox wherein a conducting bar is arranged parallel with each
row or series of fixed contact-pieces and is provided with a sliding contact-block
adapted to engage with either of the fixed contact-pieces in the said series and
with means for drawing it towards the said contact-pieces, for the purposes above
specified.
8. A resistance-box having our improvements applied thereto and constructed
substantially as described with reference to the accompanying drawing.
Dated this 24th day of March 1899
D. YOUNG & Co.,
11 & 12, Southampton Buildings, London, W.C.,
Agents for the Applicants.