1
ΧΙ
HOT-WIRE AMMETER OF SMALL RESISTANCE
AND NEGLIGIBLE INDUCTANCE
(Zeitschrift für Instrumentenkunde, 3, pp. 17-19, 1883.)
ALL the forms of the electro-dynamometer invented by
Wilhelm Weber which are intended for weak currents suffer
from two defects which are very inconvenient in many
investigations. In the first place, the resistance is high,
usually amounting to many hundred Siemens units; in the
second place, the self-inductance is large. In many respects
the second defect restricts the use of the instrument more than
the first; for it causes the instrument to offer an apparently
increased resistance to alternating currents, and in the case of
very rapidly alternating currents this increase can be very
considerable. If r is the resistance of the instrument, P its
self-inductance, and T the period of the alternating current,
the apparent resistance to this current is to the actual
resistance 7 as √1+P²²/T²7² : 1. For the instrument
described by Wilhelm Weber, and similar ones which are
actually in use, the self-inductance P can be estimated as
being of the order of one to two earth-quadrants. If we take
ras 200 Siemens units, or approximately 200 earth-quadrants
per second, it follows that for a current which alters its
direction 50 times per second the resistance is apparently
increased in the ratio of 2:1; and a current which altered
its direction 500,000 times per second would encounter in the
instrument an apparent resistance of 20,000 S.U. As to the
1 [Dynamometrische Vorrichtung.]