26
KINETIC ENERGY OF ELECTRICITY IN MOTION [I]
of the laboratory and connected by vertical wires with the
experimental tables. In form it was a rectangle 7229 mm.
long by 946 mm. wide. Its sides were formed of two parallel
wires close together, each of which formed one branch of the
system. A commutator enabled the currents to flow through
them in the same or in opposite directions. The wire used
was hard copper wire, its diameter was determined at several
places by microscope and micrometer screw, and, with small
deviations, was found in the mean = 0.4104 mm. To keep
the distance between the two branches everywhere exactly the
same, the wires were passed over wooden supports with nicks
cut in them and fitting exactly. These supports were pre-
pared by the aid of two brass stencil plates. The distance
between the wires was measured on these latter by microscope
and micrometer screw and thus found to be in the mean
2.628 mm. from centre to centre. The wires were covered
throughout their length with a layer of cotton wool, so as
to guard them from rapid temperature changes due to air-
currents.
The inductance of the whole arrangement, which con-
sisted partly of parallel, partly of perpendicular wires, could be
easily calculated with accuracy by the formulæ already given.
It was found that—
1. When the current flowed the same way through both
branches
1972,400 mm.;
2. When the currents flowed in opposite directions
P=193,160 mm.
Hence we find the ratio of the strengths of the extra-
currents to be expected in the two cases
P' - P
= 5.330.
P-P
In this calculation only the action of the commutator, of
the movable arrangement used to adjust the bridge, and of the
external circuit on the parts of the bridge has been neglected.
These actions have only a very small effect, and the error