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
cannot exceed 2.5 amperes, representing a loss of three-fourths of a
horse power, or under 5 per cent, when four cars are running. But
apart from these figures, we have materials for an actual comparison
of the cost of working the line by electricity and steam.
Pages:
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62