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.

Consider a car requiring a given constant current; evidently the
maximum loss due to resistance will occur when the car is at the
middle point of the line, and will then be one-fourth of the total
resistance of the line, provided the two extremities are maintained by
the generators at the same potential. Again, by integration, the mean
resistance can be shown to be one-sixth of the resistance of the line.
Applying these figures, and assuming four cars are running, requiring
4 horse power each, the loss due to resistance does not exceed 4 per
cent. of the power developed on the cars; or if one car only be
running, the loss is less than 1 per cent. But in actual practice at
Portrush even these estimates are too high, as the generators are
placed at the bottom of the hills, and the middle portion of the line
is more or less level, hence the minimum current is required when the
resistance is at its maximum value.

The insulation of the conductor has been a matter of considerable
difficulty, chiefly on account of the moistness of the climate. An
insulation has now, however, been obtained of from 500 to 1,000 ohms
per mile, according to the state of the weather, by placing a cap of
insulite between the wooden posts and T-iron. Hence the total leakage