Field, Pulsatory.
A field produced by pulsatory currents. By induction such field can
produce an alternating current.
Field, Rotating.
In a dynamo the field magnets are sometimes rotated instead of the
armature, the latter being stationary. In Mordey's alternator the
armature, nearly cylindrical, surrounds the field, and the latter
rotates within it, the arrangement being nearly the exact reverse of the
ordinary one. This produces a rotating field.
Field, Rotatory.
A magnetic field whose virtual poles keep rotating around its centre of
figure. If two alternating currents differing one quarter period in
phase are carried around four magnetizing coils placed and connected in
sets of two on the same diameter and at right angles to each other, the
polarity of the system will be a resultant of the combination of their
polarity, and the resultant poles will travel round and round in a
circle. In such a field, owing to eddy currents, masses of metal,
journaled like an armature, will rotate, with the speed of rotation of
the field.
Field, Stray.
The portion of a field of force outside of the regular circuit;
especially applied to the magnetic field of force of dynamos expressing
the portion which contributes nothing to the current generation.
Synonym--Waste Field.
Field, Uniform.
A field of force of uniform density. (See Field Density.)
Figure of Merit.
In the case of a galvanometer, a coefficient expressing its delicacy. It
is the reciprocal of the current required to deflect the needle through
one degree. By using the reciprocal the smaller the current required the
larger is the figure of merit. The same term may be applied to other
instruments.
It is often defined as the resistance of a circuit through which one
Daniell's element will produce a deflection of one degree on the scale
of the instrument. The circuit includes a Daniell's cell of resistance
r, a rheostat R, galvanometer G and shunt S. Assume that with the shunt
in parallel a deflection of a divisions is obtained. The resistance of
the shunted galvanometer is (GS/G+S ; the multiplying power m of the
shunt is S+G/S; the formula or figure of merit is m d (r+R +G S/G+S).
The figure of merit is larger as the instrument is more sensitive.
Synonym--Formula of Merit.