KINETIC ENERGY OF ELECTRICITY IN MOTION [I]
15
placed by an equal ballast resistance, the magnitude of which
could be very exactly adjusted in the bridge.
When a current was passed through the branch thus
detached, and reversed in a suitable manner, the current in-
duced by this branch in the other could be measured. The
quantity P' was then the mutual inductance of the one branch
on the other. Of course the extra-current might also have
been used with the current flowing the same way through
both branches of the spirals, but this was too large compared
with that obtained from the spirals with their branches
traversed oppositely to be accurately observable under like
conditions.
We have first to calculate the numerical values of P and
P'. P may be determined with a sufficient accuracy from the
geometrical relations of the spirals and the calculation will be
performed immediately; but P' can in this way be found only
by means of simplifying assumptions, which introduce a con-
siderable error. Hence I preferred to determine it directly
by experiment by comparing it with the known inductance of
straight wires.
Determination of P.-The following assumptions are made
as regards the arrangement of the wires and are very nearly
correct:-
:-
1. In one and the same layer wires traversed positively
and negatively alternate with each other: the distances
between their central axes are equal to each other and to
the mean distance, which is got by dividing the length of the
spiral by the number of turns. 2. Two neighbouring layers
are laterally displaced relatively to each other through half
the distance between two centres. These assumptions com-
pletely determine the geometrical position of the wires; but
whether the extreme wires near the ends of the spiral are all
traversed in the like direction or in opposite directions alter-
nately, it was impossible to decide in the case of the inner
layers. For this reason and because of the unavoidable
irregularities an accurate calculation of the inductance is
not possible; we can only determine limits between which it
must lie and we shall see that these limits may be drawn
rather closely. In calculating the inductance of one layer we
may without appreciable error cut it open, develop it on a
COIENOK ENGINEEDING