,- 
D94 | +. ‘TRANSACTIONS OF SECTION G. : a 
a 
south of London. On this line no locomotives are used, but only motor 
coaches. It is therefore not possible to make a direct comparison as to weight 
efficiency with a locomotive. The latter has only to carry the propelling 
machinery, whilst the former has to provide accommodation for passengerg as 
well. ‘The 600 horse-power motor coaches on the Brighton line weigh 50 tons, 
or at the rate of 83 kilogrammes per horse-power. A 1,000 horse-power C.C. 
electromotive taking current at 1,200 volts weighs 74 tons.’ By making a suitable 
reduction for the extra weight of the passenger accommodation in the A.C. 
coach, its weight per horse-power comes out at something like 60 kilogrammes, 
against 62 kilogrammes in the C.C. engine. 
Series motors are employed on the electrified lines of the Midland Company 
between Heysham, Morecambe, and Lancaster. Also in this case motor coaches, 
and not electromotives, are used. At the hour-rating a motor coach develops 
420 horse-power, and as its total weight is about 35 tons, we have here the same 
weight-efficiency as on the Brighton lnes—namely, 83 kilogrammes per horse- 
power for the whole coach. . 
Of high-pressure continuous-current lines there are many examples, both in 
Europe and America. The term high-pressure does, of course, not imply the 
same order of magnitude as in single-phase A.C. lines. There high-pressure may 
mean anything up to 15,000 volts, the pressure which is likely to become a 
standard in future electrifications; but in C.C. work one must class anything 
over 1,000 volts or 1,500 volts as high-pressure. The general rule is to employ 
motor coaches, and not electromotives; but there is a private line belonging to 
a steel-works in Lorraine, where two electromotives, each of 600 horse-power 
(four C.C. motors of 150 horse-power) are working the mineral trains under a 
pressure of 2,000 volts. The Southern Pacific Railway also employs C.C. electro- 
motives of 1,000 horse-power each. Each engine weighs 74 tons, and hauls a 
train of 270 tons on grades of 40 per mille. This is a remarkable performance, 
rendered possible by the fact that with the even torque exerted by the electric 
motor a much larger co-efficient of friction than is possible in steam traction may 
safely be permitted. Hlectrical engineers generally base their calculation of the 
possible tractive effort on a co-efficient of 0°17, without sand, and as high as 
0°25, or even 0°28 if sand is used. The voltage in the case of the Southern Pacific 
engines is only 1,200 volts, taken by two motors in series, and there is provision 
made to change over from the overhead wire to third rail, with 600 volts, when 
the motors are all in parallel. On European C.C. lines the voltage is higher— 
generally 2,000 volts, as on the Chur-Arosa and some other Swiss lines—and the 
tendency is still in the direction of higher pressures. Continental makers are now 
prepared to go as far as 1,200 volts per motor, so that with the usual system of 
series-parallel control a line-pressure of 2,400 volts becomes possible. The 
greatest step in advance in this direction has, however, been made in England, 
where Messrs. Dick Kerr, Ltd., have adopted a line-pressure of 3,500 volts as 
their standard, involving the use of motors constructed for 1,750 volts. After 
laying experimented with this high-pressure system for two years, they have 
undertaken the electrification of a short section of the Lancashire and Yorkshire 
Railway with continuous current at 3,500 volts. I am indebted to the firm for 
the following particulars : The current is collected by pantograph from an over- 
head wire with catenary suspension. The train consists of a motor coach and 
two trailers. The motor coach is equipped with four 300 horse-power motors, 
and weighs 62 tons; the trailers weigh each 26 tons. From these figures it 
will be seen that the weight of the motor coach per horse-power is only 52 kilo- 
grammes, and thus considerably below what the weight of an equivalent single- 
phase motor coach would be. It is especially the saving in weight and the avoid- 
ance of any telephonic disturbances which renders the C.C. system so attractive 
that, in spite of a natural reluctance against the use of high-pressure on a com- 
mutator, designers are giving increased attention to the use of continuous current 
for electric traction. The difficulties which some engineers anticipate with 
commutator and brushes seem, however, rather imaginary than real, if we may 
judge from the experience with the 3,500-volt motor coach. The makers inform 
me that they estimate the mileage for a set of carbon brushes at 50,000 miles. 
* See Gratzemueller’s paper read at the Paris meeting of the I.E.E. and 
S. Intern. des Electr. (Paris, May 1913). 
