The action is
analogous to the spark produced by a Ruhmkorff coil. This was
illustrated at Portrush; when the car was running perhaps fifteen
miles an hour, the current was suddenly reversed. The car came to a
standstill in little more than its own length, but at the expense of
breaking down the insulation of one of the wires of the magnet coils.
The way out of the difficulty is evidently at the moment of reversal
to insert a high resistance to diminish the momentum of the current.
In determining the proper dimensions of a conductor for railway
purposes, Sir William Thomson's law should properly apply. But on a
line where the gradients and traffic are very irregular, it is
difficult to estimate the average current, and the desirability of
having the rail mechanically strong, and of such low resistance that
the potential shall not vary very materially throughout its length,
becomes more important than the economic considerations involved in
Sir William Thomson's law. At Portrush the resistance of a mile,
including the return by earth and the ground rails, is actually about
0.23 ohm. If calculated from the section of the iron, it would be 0.15
ohm, the difference being accounted for by the resistance of the
copper loops, and occasional imperfect contacts. The E.M.F. at which
the conductor is maintained is about 225 volts, which is well within
the limit of perfect safety assigned by Sir William Thomson and Dr.
Siemens. At the same time the shock received by touching the iron is
sufficient to be unpleasant, and hence is some protection against the
conductor being tampered with.
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