Improvements in Slide Resistances for Electrical Purposes.
Year: 1892

page_text: Date of Application, 14th Mar., 1892-Accepted, 16th Apr., 1892
COMPLETE SPECIFICATION.
Improvements in Slide Resistances for Electrical Purposes.
1, ALEXANDER MUIRHEAD, of Cowley Street, in the City of Westminster,.
Doctor of Science, Electrical Engineer, do hereby declare the nature of this inven--
tion and in what manner the same is to be performed, to be particularly described
and ascertained in and by the following statement :-
This invention relates to an improvement in the construction of what is known
as Thomson and Varley's Slide Resistance. Slide resistances as at present made
for electrical testing and like purposes consist of two series of coils of wire of equal
resistances a primary series and a secondary series. The primary series consists
of 101 coils each with a resistance of 1,000 ohms; the secondary series consists of
100 coils each with a resistance of 20 ohms. The two ends of each series of coils
are connected to two terminals on the instrument and the junctions of the successive
coils are connected to a series of studs. Along the primary series of studs slide
two arms or contact-pieces insulated from one another and at such a distance
apart that when one arm rests on one stud the other arm rests on the next stud
but one one end of the secondary series of coils is connected to the one arm of
the primary series and the other end of the secondary series is connected to the
other arm of the primary series. Along the studs of the secondary series of coils.
moves an arm or sliding contact-piece called the index.
The construction of such a set of slides involves the accurate adjustment of"
201 separate coils and the expense of a corresponding number of studs tipped with
platinum. According to this invention a saving in cost is effected by reason of
the diminished number of coils to be adjusted and in the manufacture of the contact-
studs.
The annexed drawing (which is partly diagrammatic but mainly a plan of
the under side of the upper part of my improved apparatus) illustrates my
invention.
In carrying out the invention I prefer to construct the set of slides of four series
of coils, three series of eleven coils each and one series of ten coils. These coils,
illustrated diagrammatically on the drawing and shown at, a, are mounted. on
ebonite or other suitable platforms, -6, capable of being turned to a limited
extent on their axes from the upper part of the apparatus by milled-edged finger-
pieces, c.
The seven contact-springs, d, tipped with platinum or gold alloy, are fixed to the
frame of instrument and planted so as to bear against the platinum-tipped studs, e,.
mounted on the edge of the platforms which carry the coils. A device consisting
of a stout bent spring, f, bearing against pins, g, on the platforms is attached to
the frame of the instrument in such position that the spring falls between the pins.
whenever the contact-springs, d, are exactly over the corresponding studs, e.
When the total resistance between the two terminals of the instrument, a, and, y,
is to be 100,000 ohms the coils on platform, b', are adjusted to 10,000 ohms each,.
2,000 ohms each, of 68 400 ohms each, and of 6' 80 ohms each.
of
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 :-
In the construction of slide resistances the combination of three or more series.
of coils in the manner above described.