148 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
wise may be made less distinctive if they have truss members for a 
background. 
Relative direct-current potentials in overhead trolley and third 
rail—Now that the improvements in direct-current motor construc- 
tion, not only those promised but those actually accomplished, have 
made it possible, quite irrespective of what may be done with alter- 
nating-current motors, to use much higher potentials than ordinary 
not, of course, as high as those available in single-phase alternating- 
current systems—the question sometimes arises, Will not the permis- 
sible potential be high enough, taking into account certain other 
facts, to meet in large measure the demands of railroad operation, 
whether by overhead or third rail? 
Engineers have generally proceeded on the assumption that the use 
of a sufficiently high potential for practical purposes is possible only 
with overhead conductors. In the Siemens-Schuckert installation at 
Maziéres, where 2,000 volts direct current are used, the current is 
taken from two trolley wires of lke potential supported by cross- 
wire catenaries from side poles of the same construction as is com- 
monly used to carry the warning tickler. 
Again, the third-wire system has been proposed, as on the Krizik 
road, and, on a recent installation with many grades and heavy tun- 
nels in the Iselle mining district in France, where two overhead 
trolley wires are used at 2,400 volts, with the track as a neutral, and 
with the motors grouped in series of two, current being supplied by 
two Thury generators in series and grounded in the middle. 
A comparison of potential relations giving the same losses on three 
systems is interesting. The systems are: 
1. Three-wire, with two No. 0000 trolleys and 75-pound bonded 
single track. 
2. Two-wire, with same trolley wires and track return. 
3. Third rail, 70-pound special, and with same track return. 
The following table gives the comparisons: 
Resistance : 3 
7 Ratio of /Ratjo. |Comparative 
System. Dee | resistance.| VR@tio. |~ Voitages. 
Ohms. 
IN Olea ae a a ee ee ee ee ee ee 0.52 6.6667 oO 2,400 | 3,100 
ENON potent ete cit? peed Beh aie ba ae ee Rd ye .165 2.1154 45 | 1,350 | 1,740 
IN Olesen se ee eee meee nan eeeeeae Sae eee ee .078 1.0000 1.00 930 | 1,200 
On such a showing there is little excuse for departing from the 
lower potentials and the simpler systems, and being handicapped with 
the higher voltage problems and complication of switches in the three- 
wire system. If any smaller trolley wire be used, then the disparity 
between Nos. 1 or 2 and No. 3 would be emphasized. 
