Another method of expanding bond terminals into the holes made to receive them, is that employed in the General Electric Company’s bond. In it a soft pin in the center of the terminal is expanded by compression of the terminal so that it forces the copper surrounding it outward. The copper terminal, in expanding to fill the hole, is therefore backed by the steel center pin.
All types of bonds must be installed with great care if they are to be efficient. Unless the bond terminal thoroughly fills the hole and is tightly expanded into it, moisture will creep into the space between the copper and the iron, and the copper will become coated with a non-conducting scale which destroys the conductivity of the contact. The plastic rail bond, so called because it depends for the contact between the rail and the bond upon a plastic, putty-like alloy of mercury and some other metal, is applied in a number of different ways. One form consists of a strip of copper held by a spring against the rail ends under the fish-plate. The rail ends at the point of contact with this strip of copper are amalgamated and made bright by the use of a mercury compound similar to the plastic alloy. These points of contact are then daubed with plastic alloy, and the copper bond plate applied. It is not necessary, with any form of plastic bond, that the mechanical contact be unyielding, as the amalgamated surfaces with the aid of the plastic alloy between them, maintain a good conductivity in spite of any slight motion. The plastic alloy can be applied in a number of other ways, one of which is to drill a hole forming a small cup in the rail base in adjacent rail ends, fill these cups with plastic alloy, and bridge the space between them with a short copper bond having its ends projecting down into the cups.
Resistance of the Track. The resistance of the return circuit is usually much higher than it should be owing to the bad contact of the bonds. The resistance of rails varies greatly with the proportions of carbon, manganese and phosphorus. The following figures, however, may be regarded as the average.
| Weight per Yard. | Resistance Single Rail per Mile. |
|---|---|
| 50 | .0253 ohms |
| 60 | .0211” |
| 70 | .0180” |
| 80 | .0159” |
| 90 | .014” |
A track laid with continuous rails as in the case of welded joints, would have one-half the resistance given since there are two rails to be considered.
Tests of new unbonded track constructed with rails 60 feet long show that the joints cause an increase of .25 ohms or more per mile.
Several roads in testing bonds consider a bond good when the bond and one foot of the rail over it have a resistance equal to five feet of the solid rail.
Supplementary Return Feeders. On some large roads it is necessary to run additional return feeders from the power house to various points on the system, to supplement the conductivity of the rails. Otherwise the track rails near the power house would have to carry all the current, and in some cases there are not enough such lines of track passing the power house to do this properly. Sometimes these feeders are laid underground in troughs; sometimes they are laid bare in the ground, and sometimes on overhead pole lines. When laid in the ground, frequently old rails are used instead of copper or aluminum cables. The old rails are, of course, thoroughly bonded together with bonds giving a conductivity nearly equal to that of the unbroken rail.
FEEDER SYSTEMS.
