In practice the primary current necessary is preferably
generated by a small high speed alternating dynamo on the locomotive,
the current being converted by means of an inductional transformer. To
avoid the necessity for electrically bridging the rail joints, a
modified arrangement may be employed, in which the electrical
connection is made directly with a fixed collar on the forward and
rear driving axles, the current dividing itself in parallel between
the two rails in such a manner that, if a defective joint exists in
the rail at one side, the circuit is still complete through the rail
on the other; and as the rails usually break joints on opposite sides,
this arrangement is found very effective. The insulation of the
driving wheels is very easily effected in either case.
As the amount of additional tractive adhesion produced depends upon
the _quantity_ of current flowing rather than upon its pressure, the
reason for transforming the current as described will be apparent, and
its advantages over a direct current of higher tension and less
quantity, both from an economical and practical standpoint, will for
this reason be clear. The amount of heat produced at the point of
contact between the wheels and rails is never large enough to injure
or otherwise affect them, although it may be quite possible to
increase the current sufficiently to produce a very considerable
heating effect.
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