IMPROVEMENTS IN THE MANUFACTURE OF RESISTANCE COILS FOR ELECTRICAL PURPOSES
Year: 1882

page_text: PROVISIONAL SPECIFICATION left by John Henry Johnson at the Office
of the Commissioners of Patents on the 20th September 1882. A communi-
cation to him from abroad by Pierre Uzel of Paris in the Republic of France.
JOHN HENRY JOHNSON of 47 Lincolns Inn Fields in the County of Middlesex
Gentleman "IMPROVEMENTS IN THE MANUFACTURE OF RESISTANCE COILS FOR
ELECTRICAL PURPOSES a communication to me from abroad by Pierre Uzel of
Paris in the Republic of France
37
Resistance coils have heretofore been gauged and adjusted in one piece that is
to say that when it is required to make a coil say of one hundred ohms it has been
the practice to make this coil itself directly equal to another coil of one hundred
ohms termed the standard coil-A drawback to this being properly effected is
that the heat of the hands which were necessarily brought near the coil in order
to lengthen or shorten the wire affected its resistance to such an extent that at the
moment when the comparison or measurement was being made the coil presented a
certain amount of resistance but when after a certain lapse of time it had returned
to its normal temperature it assumed another value or power as a resistance
Hence the necessity for repeated trials with the same coils in order to obtain a
satisfactory result.
Before describing the invention it should be observed that the influence of
temperature upon the resistance increases in proportion as the resistance increases.
Therefore the greater the resistance of the coil the greater is the difficulty in
regulating it. The object of this invention is to reduce the adjustment of a coil of
great resistance to that of a coil of small resistance.
For example suppose a coil of one hundred ohms is required. A coil is prepared
in the first place having resistance inferior to one hundred ohms but preferably
only slightly inferior-For this purpose the method usually adopted is employed
but the wire to be coiled is divided into two parts.
Two of the ends are attached to binding screws on the coil. The wire is then
wound in the coil and the other two ends are attached to a second pair of screws.
The first named screws are then connected together by a mass of metal having no
noticeable resistance. The coil is so arranged as to present a lower resistance than

page_text: the resistance which it is desired to obtain as a final result. The wire is then
soldered to the screws and the coil treated with paraffin in the usual manner.
The coil is then left without being touched for a certain period say twelve
hours. The resistance of the coil is then carefully measured and is found to be
say ninety-nine ohms and ninety three hundredths so that there is a deficiency of
seven hundredths of an ohm. A coil of a small resistance corresponding to the
deficiency is very easy to make as the temperature has comparatively little
influence on so small a resistance Moreover an approximation which would not
be very close for a coil of seven hundredths of an ohm will be a very close
approximation for a coil of one hundred ohms. Such a small coil is prepared and
combined with the coil first mentioned.
Suppose an error of one hundredth of an ohm is made in the small coil so that
instead of seven hundredths there are only six hundredths the finished coil will then
correspond to ninety-nine ohms and ninety-three hundredths plus six hundredths or
ninety-nine ohms and nine nine hundredths. This is a very close approximation
which may be rendered still closer by taking more care with the small
supplementary coil.
The small supplementary coil employed is preferably fixed over the large coil
and in order to facilitate the adjustment the supplementary coil is made of much
thicker wire than that of the large coil.