FUNDAMENTAT, PRINCIPLES. 25 
ductors will be laid out for the average load based on the 
average consumption of energy per car. With a normal 
drop so computed and with care taken to allow a reason- 
able margin for maximum loads, these variations of the 
first class need not constitute a serious embarrassment. 
The diurnal changes of load based on average readings 
in which the minor oscillations are suppressed, are great in 
amount and of much interest. They are due to the habits 
and occupations of the community served, and often exhibit 
very curious peculiarities. Further, they are almost as 
strongly marked in very large systems as in quite small 
ones and serve to determine the relation of average to max- 
imum load, which in turn determines the allowance which 
must be made for drop at extreme loads. Even under very 
favorable circumstances the difference between average and 
maximum load is great. ‘This is very forcibly shown in 
Fig. 16, which gives the load line on one of the largest 
electric railway systems for a December day, just before 
the holidays. 
The minimum load is quite uniform from 2 A. M. until 
5 A. M. and is only about six per cent of the maximum. At 
about 5 A. M. the load comes on quite suddenly and con- 
tinues to rise until about 9 A. M., when it begins to fall, 
and keeps diminishing until about 2 . M.  Then it rises, 
slowly at first and then more rapidly until it reaches a 
second maximum, about equal to the first, at 6 p. M. Then 
it falls somewhat irregularly until only the night cars are 
left. 
The average load for the twenty-four hours is about 
six-tenths of that at the two maxima. ‘This difference is 
what must be kept in mind in providing a due factor of 
safety in the conductors. ‘The load line is not, of course, 
invariable, being subject both to accidental and yearly varia- 
tions, but, in spite of these, it preserves its characteristics 
and the value of its ‘‘load factor’’ with remarkable uni- 
formity. In small systems there are practically no night 
cars, the service being generally about eighteen hours. 
Were such the case in Fig. 16, the ‘‘load factor >’ would be