Improvements in Electrical Resistances.
Year: 1896

page_text: Improvements in Electrical Resistances.
I ROOKES EVELYN BELL CROMPTON, Electrical Engineer of Mansion House
Buildings London E.C., do hereby declare the nature of this invention to be as
follows:-
My invention relates to improvements in electrical resistances with which other
resistances can be accurately compared.
Hitherto it has been necessary when such comparisons are made to accurately
note the temperature not only of the unknown resistance but also the standard
resistances themselves and the accurate observation of these temperatures and
the maintaining of them constant for a sufficient time to enable the electrical com-
parisons to be made is always a matter of expense and difficulty.
By my improvements I supply standard resistances the temperature of which
does not require to be observed. I carry this out by providing two wires of equal
resistance arranged to be coupled in series so that the current passed through them
for measuring purposes will pass through both of them in series; these wires must
be so arranged or wound so that they lie side by side in close proximity and there-
fore under identical conditions as regards temperature. Usually I make one of
these wires of a pure metal which has a comparatively high co-efficient of
increase of resistance for increase of temperature, and the other of an alloy which
has a low co-efficient of increase of resistance for increase of temperature. As
for instance for the one wire I can use platinum and for the second wire I can use
a standard alloy of platinum silver which is known to remain very constant and
which was adopted by the British Association as the best alloy for such purposes:
but I do not confine myself to these two materials as I can employ any pure metal
which sufficiently protects itself by change from oxidation for the one wire and
any other pure metal for the other wire which has a coefficient of
increase of resistance for increase of temperature differing sufficiently
from that of the first wire as for instance I can use platinum for
one wire, and nickel for the other, and other combinations of two
wires each composed of pure metals can be used. In the preparation of my
improved standards all that is necessary is to make the resistance of each of the
two wires exactly equal to one another and to a third which must be a carefully
selected standard at a certain temperature and as it is always an easy matter to
raise the temperature of the apparatus above that of the laboratory in which the
tests are made, I prefer to fix this which I call the calibration temperature at a
point rather above the highest summer temperature of the laboratory or test room
in which such comparisons are likely to be made and I obtain this calibration
temperature by means of an electrical heating coil which I prefer to place in close
proximity to my standard wires in order that I may raise the temperature of these
and maintain them, exactly at this calibration temperature.
It will be readily understood that once my improved coil is calibrated as above
it will always give the exact value only at the time when the resistance of the two
wires are equal to one another which will be at the calibration temperature and

page_text: it is therefore unnecessary to take note of the temperature when the standards
are used for comparing other resistances with them. It is not absolutely necessary
to obtain this calibration temperature by means of the supplementary coil as in
some cases this may be done by increasing the current through the coils them-
selves but the separate heating coil is a convenient and accurate method of
obtaining the desired calibration temperature...
It will be seen that my method of obtaining the calibration temperature enables
me to make my standard resistances of two pure metals which can be more easily
obtained of constant resistance than any alloy..
Did the 2nd dox of December 1896.
WM. BROOKES & SON,
55 & 56 Chancery Lane, London, Agents for the Applicant.
Improvements in Electrical Resistances.
I ROOKES EVELYN BELL CROMPTON, Electrical Engineer, of Mansion House 1
Buildings, London, E.C., 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:-
My invention relates to improvements in electrical resistances with which other
resistances can be accurately compared. Hitherto it has been necessary at the
time when such accurate comparisons are made to note the temperature not only
of the wire of the unknown resistance but also that of the wire of the standard
resistance itself.
The accurate observation of these temperatures and the maintaining of them
constant.for extended periods of time has always been a matter of delay, expense
and difficulty. By my improvements I am able to manufacture and supply
standard resistances the temperature of which does not require to be observed. I
carry this out by providing two wires of equal resistance arranged to be coupled
in series circuit so that the current which passes through them for measuring
purposes would pass through both of them in series. Figure 1. shows in elevation
and Figure 2. in plan view these two wires wound around a central cylindrical
pillar made of insulating material. Figure 3. shows plan view with central part
emoved. Figure 4. shows plan view of central portion separately. Figure 5.
show: sectional view of the base. A is the base which may be made of shellacked
wood or of ebonite or of other suitable material and to the centre of this is fixed
the cylindrical pillar B on which is cut a three threaded spiral, on to. this spiral
are wound three wires the first of them being of platinum silver, the second of
them being of pure platinum, and the third one of any suitable material but pre-
ferably of nickel or platinum, this latter wire being used solely as a heating coil
for varying the temperature of the other two wires. The platinum silver wire and
the pure platinum wire are connected together at the top of the spiral by the
terminals shown at t and their lower ends are connected electrically to each of a
pair of the terminals shown on the plan view Figure 2 at B1 and B2.
electrical resistance of each of these wires is arranged to be such that the standard
resistance of 1 ohm, or 10 ohms etc. is obtained at a point very low down on the
pillar by means of an adjustable knife edge contact pressing against the surfaces
of the wires at or near the point where the wire leaves the pillar to be connected
to, the termins!. This adjustable knife edge contact piece is shown in side view
Figure 6 and in plan view Figure 7. The knife edge contact which may be of
The

page_text: platinum or platino-iridium or other non-corrodible conducting material is held
at the point. The adjusting screws as shown in Figures 6 and 7 enable adjust-
ments to be given by sliding the whole block within the fixing screws and the final
adjustments can be given by the two adjusting screws. Two terminals H¹, H
are also provided for connecting with the third or heating wire above mentioned
so that the current can be passed through this wire for the purpose of raising the
temperature of the two other wires to the desired degree. In this case the tem-
perature co-efficient of increase of resistance for increase of temperature of the pure
platinum wire is nearly twelve times that of the platinum silver alloy wire but
any other two non-oxidisable metals can be used provided these two metals differ
sufficiently in their co-efficient increase of resistance for increase of temperature.
To calibrate these resistances if I desire to produce a standard of one ohm I prefer
to first adjust one of the wires, preferably the platinum, silver wire, in a room at a
fixed temperature which should be a high temperature i.e., several degrees in
excess of that of any laboratory or testing room where it is intended that this
standard resistance should be used. After the platinum silver wire has been care-
fully adjusted to 1 ohm at this higher temperature say 25 degrees centigrade I
then adjust the other or pure platinum wire which is coupled in series with it
until it also is exactly one ohm at the same temperature; it follows therefore that
the resistance of these two wires if adjusted equal to one another at the temperature
at which this adjustment is made namely twenty-five degrees centigrade are
equal to one another at this temperature only and at any temperature in excess of
twenty-five degrees centigrade the resistance of the platinum wire will be greater
than that of the platinum silver wire, and conversely at temperatures below
twenty-five degrees centigrade the resistance of the platinum wire will be lower
than that of the platinum silver wire. In using the instrument therefore as a
standard all that is necessary is to raise the temperature of the instrument until
the resistance of both wires is equal as this will be only at the temperature of
twenty-five degrees centigrade, both of them at this temperature will be exactly
one ohm and hence no observation of temperature is necessary. Although it is
convenient to use a third supplementary wire as herein described to raise the
temperature of the other two wires until the desired equality between them is
obtained this is not absolutely necessary. Another method of obtaining the same
result is to make the central pillar hollow and provide the means of heating it
by a gas jet or by the introduction of a piece of heated metal or by other convenient
means. Another but not so convenient method is to utilise the two wires them-
selves as the heating coil but this although it can be done once the instrument is
eriibrated cannot easily be done at the time the instrument is being compared
vith another standard as it is usual in this case to pass the same current through
hoch standards.
Although I have described one convenient arrangement of mounting these wires
so as to obtain my desired standard resistance yet I do not confine myself to this
special arrangement or to the arrangement of the knife edge contact shown which
although they are convenient for potentiometer methods of comparison are not
so convenient for bridge methods but I have described one of the arrangements for
mounting, these resistance wires and have described the method by which I am
able to use them as standard resistances without the necessity of temperature
observations.
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. In electrical resistances used as standards of resistance the employment of
two or more wires made of metals or alloys which differ from each other in their
co-efficient for change of resistance for change of temperature and which are
made equal to the desired standard resistance value only at the ane temperature
at which they are calibrated,

page_text: 2. The construction and use of electrical resistances, as standards of resistance
or as resistances with which other resistances can be accurately compared, as and
for the purpose set forth, and illustrated by the accompanying drawings.
Date this 2nd day of September 1897.
WM. BROOKES & SON,
55 & 56 Chancery Lane, London, Agents for the Applicant.