34 POWER DISTRIBUTION F¥FOR ELECTRIC RAILROADS. 
ing half a dozen values of contact resistance from various 
experiments we get the following table: 
Single bond contact. 
R == .0008 ohm. 
= 00045 
== 00OBHoS 
= .00023 
= L0001 
— - .00000 
As nearly as may be judged, the resistance of a single 
contact, carefully made, can be counted on to be consider- 
ably less than .oor ohm. With bond plugs of large surface 
well set, it would seem safe to count upon a resistance not 
exceeding .0002 ohm. per contact. 
The bonding wires should be as short as can be con- 
veniently handled. If the work is properly prepared for, 
there is no good reason for having more than a foot of wirein 
the bonding connection, although more is frequently used. 
As to size, there is little reason for using anything smaller 
than No. 0oo or No. oooo. With abouta foot of No. oooo 
at each joint, and thorough contacts carefully made, the 
resistance of bonds ought to foot up about as follows per 
thousand feet. 
66 bond contacts = .0132 
33 ft. cooo wire = .00165 
Total 0.0148 ohm. 
'This is about one and a half times the resistance of a thou- 
sand feet of sixty pound rail and corresponds well with 
actual tests of well bonded track. It is quite near the truth 
to assume that under average circumstances of good con- 
struction the bond wire and contact resistance will aggregate 
about twice the resistance of the rails themselves. 
As regards the earth there is great misconception both 
as to its conducting power and the part it takes in modify- 
ing the rail and bond resistance which we have just been 
considering. Outside of the metals there are no sub- 
stances that have even fair conducting properties. That