N.E. and N.N.W. Telephone cables, containing any desired
number of insulated wires, each covered by a braiding of a distinctive
colour, can be obtained at a cost only slightly exceeding that of an equal
total amount of single insulated wire. The cable form is to be preferred,
on account of its greater convenience in fixing.
The amount of battery power required depends on the length of the circuit
and the delicacy of the dial. If an ordinary compass needle be used, as
indicated in Fig. 160, very little current is needed. In this case the
magnets, which can be made of a couple of dozen turns of fine insulated
wire round a 1/8-in soft iron bar, should be arranged spokewise round the
compass case, and care must be taken that all the cores are wound in the
same direction, so as to have the same polarity. Otherwise some will
attract the N. end of the needle and others repel it. The direction of the
current flow through the circuit will decide the polarity of the magnets,
so that, if one end of the needle be furnished with a little paper
arrow-head, the "correspondence" between vane and dial is easily
established.
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