596 TRANSACTIONS OF SECTION G. 
and a current density of 15 A. per sq. cm., is permissible. If, then, we use 
narrow brushes, covering at any time not more than three segments, use coils of 
only one turn to each segment, and work at a reasonably low frequency, and not 
too high a total flux, it is possible to keep the transformer voltage and current 
density well within the above limits. This is not a severe limitation, for it enables 
the designer to use a flux out of one pole of 2'4 megalines if the frequency is 25, 
and 3°6 megalines if it is 15. The number of poles has then to be selected in 
accordance with the power desired. Obviously the lower periodicity is to be pre- 
ferred, because the motor may be built with a lesser number of poles, and will then 
occupy less room—a matter of considerable importance, considering the limited 
space which is available in an electromotive. The frequency of 15 has also some 
other advantages over that of 25. The e.m.f. of self-induction is proportionately 
less, and, in consequence, the power-factor is about five per cent. better. The 
skin effect in the rails is much reduced, and also disturbances on neighbouring 
circuits which may be due to inductive or capacity effects. On the other hand, 
the generators become a little more expensive and the transformers on the 
electromotives a little heavier. But, notwithstanding these drawbacks, the 
balance of advantage is with the lower frequency, and that is the reason why the 
Commission of Experts called together in 1904 by the Swiss Government to 
establish standards for the electrification of the Swiss railways has decided that 
15 shall be the standard frequency, with a tolerance down to 14, and up to 162. 
Since then other States have fallen into line, so that 15 is now the standard 
frequency nearly all over the continent of Europe. The standard pressure is 
likely to be 15,000 volts. For three-phase traction the standard pressure is 3,000 
to 3,300 volts. 
The subject of electric main-line traction is so vast that in the limited time 
at my disposal [ have only been able to mention a few of the important features 
of this interesting problem. A detailed account of all that has been done in 
electrification would take far more time than we can spare; but, by way of 
example, I give below two tables referring to the Italian State Railways. I 
am indebted for the information to Mr. v. Kando, who may justly be described 
as the father of three-phase traction, 
In Service | In Construction 
a vot. . : ob 
ee Sl iy Ahs oy lesb toe fin | 
Loeation of Line ES oe 2S \ me | 3 
of] 28a | SQe |) 8 ; a 
e855) 28a | ess | Be) 28 | Sas 
SSes| 883 | 288 | gos | 88 | 688 
IHOnO |] OFM | BHe | nwo Ha Snm | 
| Length, in kilometres 107 19). 1,58 - ol, 446 38 28.5 tl 
| Heaviest Grade per mille 22 35. | 30 || 25 12 ivierare! 
| Number of Transforming | 10 4 7 | 4 4 2H 
Stations | | 
| Transmission Voltage . 20,000 | 13,000 | 59,000 | 62,000 | 25,000 | 57,000 | 
| Trolley Voltage . . 3,000 3,000 3,300 | 3,300 3,300 3,000 
, Frequency (cycles per — UD ln $Date Ws 162 | 16% | 15 
| second) | 
Source of Power . . Water | Steam | Water | Water | Water | Water 
| | | (Steam (Steam | (Steam 
| Reserve) Reserve) Reserve) 
| Number of  Electro- 14 200 et © 15. | 61 for the three lines 
| motives | 
| Number of Motor yy = _— > 
| Coaches | | | 
Weight of } minimum . 150 190 — | 5 
| Trains J maximum . 370 380 220 it nae 
