XI
215
HOT-WIRE AMMETER
only limited by the accuracy with which the rotation of the
axis could be read off. I therefore made experiments with
the object of rendering visible even smaller extensions of the
wire by further magnification. This was done partly by
applying to the axis of the instrument a lever which rotated
other axes; and partly by quite different arrangements of the
stretched wire. In this way I succeeded in obtaining
deflections ten times as large as those given above: but I
cannot recommend those modifications, because they do not
admit of the same ease in handling and the same certainty
of adjustment. The sensitiveness is best increased by using a
thinner silver wire, diminishing the diameter of the axis ab,
and increasing the length of the silver wire; for it is rarely
that one requires a dynamometer of such small resistance as
the one here described.
If we further investigate the theory of the instrument,
assuming that ceteris paribus the amount of heat emitted by
the wire is proportional to its surface but approximately
independent of the nature of the metal, we obtain the
following rule for the most appropriate construction of the
instrument:-Of the metals which appear to be suitable,
choose that which expands most on heating: use as thin a
wire as can be procured, and choose its length so that the
internal resistance of the instrument is equal to the external
resistance for which the maximum sensitiveness is required.