XI
213
HOT-WIRE AMMETER
are 01 to 0.2 mm. in diameter, and 25 mm. long. It is now
clear that any warming of the silver wire must tend to
untwist the wires ac and bd and cause the wire ab to turn
around its axis; by means of a mirror attached to the axis this
motion is read off through a telescope on a scale at a distance
of about 2 metres. In order to prevent any deflection of the
mirror through a general change of temperature, the screws A
and B are not fixed directly upon the wooden frame, but upon
a strong strap of brass (from which they are of course
insulated). Since brass and silver have very nearly the same
expansion, changes of temperature of the whole apparatus
have but a very slight effect upon the position of rest. The
instrument is protected from air-currents by a case, which
is not shown in the figure. The apparatus can either stand
on a table or hang by a hook from a wall; in the former case
levelling-screws are unnecessary.
If we suppose the wire to be warmed 1° above its
surroundings, its expansion would amount to 19 millionths of
its length, so that each half of it would expand by 760
millionths of a millimetre. On the scale this expansion
appears magnified in the ratio of 2 x 2000/04:1 10,000:1,
and therefore causes a deviation of 7.6 mm. Hence an
elevation of temperature of C. would correspond to a
deviation of about mm. which should be clearly perceptible.
The following are the results of my observations :-
O
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1. The resistance of the instrument is 0.85 S.U.
2. The instrument can be used in any position and requires
no special care in adjustment. The image of the scale remains
perfectly quiet, even in a place where a delicate galvanometer or
dynamometer keeps continually moving on account of ground-
tremors. When the mirror is thrown into vibration, the
vibrations are so rapid that the motion of the image of the
scale cannot be followed: but the air-damping is sufficient to
bring the image completely to rest in a second or less.
3. When a current of suitable strength is passed through
the silver wire, the image moves with a jerk into its new
position of rest, and the latter can be read off after 1 or 2
seconds. When the current is stopped the image jerks back
again to its first position of rest. If the deflection is large,
there remains a certain amount of after-effect, but this appears
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