140
KINETIC ENERGY OF ELECTRICITY IN MOTION [II]
IV
transition-resistance; and the disturbances due to heating were
only just perceptible, and small compared with other unavoid-
able ones. The spindle was rotated by a cord, which con-
nected it with the quickest spindle of a Becquerel's phosphor-
oscope, so that it revolved at double the speed of the latter.
The crank of the phosphoroscope was turned by hand, one
turn of it corresponding to 290 revolutions of the spindle.
As the whole apparatus was built as lightly as possible, even
large velocities could be rapidly generated and again annulled.
The galvanometer used was of Siemens' pattern, with an
astatic system of two bell magnets and four coils, with a total
resistance of about 7 Siemens units. By aid of external
magnets the arrangement could be made as astatic as desired;
in the final experiments the sensitiveness was such that a
difference of potential of one-millionth of a Daniell between
the points D and C gave a deflection of 32 scale divisions.
The motion of the needle was aperiodic; a second position of
rest was reached in about 8 seconds with an accuracy sufficient
for the experiments described. The current was supplied from
a Daniell cell and measured by a common tangent galvan-
ometer. A commutator was placed in the connections to both
galvanometer and battery.
After the current had been allowed to flow through the
plate until no further heating took place, the needle was
brought nearly to its natural position of rest by adjusting the
external resistances between A, C, and B. Then the crank of
the phosphoroscope was made to turn once round as uniformly
as possible, an operation which on the average required 8 to 9
seconds, and was terminated by an automatic catch. But
after the rotation ceased the needle hardly ever returned to
the original position of rest, but to a new position of rest.
As soon as this was attained (i.e. after 6 to 8 seconds) it was
read off. The deviation from the original position of rest I
shall call the permanent deflection; by the instantaneous de-
flection will be meant the distance of the needle at the end of
the rotation from the mean of the initial and final resting
points. We regard the instantaneous deflection as a measure
of the current whose causes act ouly during the rotation, e.g.
the influence of inert mass; while we ascribe the permanent
deflection to disturbances which continue to act after rotation