fast traffic is maintained on the tracks, as they clamp over the head of the rail, making a dangerous condition due to the possibility of causing derailment. The other is a single or multiple stud terminal bond applied to the head of the rail, the terminal studs being set in holes and expanded into contact by hammer blows. This type of bond has been largely superseded by the modern types of brazed and welded head bonds.

(d) Welded Rail Joints. The difficulties and uncertainties attending the proper maintenance of rail joints and bonds have been eliminated to a large degree by the successful use of several modern types of welded joints, such as electric resistance and arc welding, cast welding, and thermit welding. The welded joint in one form or another has been adopted as a standard of construction in nearly every large city in the United States. Most types of welded joints have a conductivity equal to or greater than the continuous rail and are less subject to failure than any form of rail bond. They must be considered, therefore, as a most important factor in the reduction of stray current.

Electric Rail Welding is performed by clamping heavy iron bars to the web of the rail and bringing the bars and the adjacent rail to a white heat by means of an electric current. The process requires a heavy and expensive plant, and is usually carried out by contract on a comparatively large scale. For this reason it is not well suited to installations on small systems. It is well adapted to the reclaiming of old track as well as for new work and has been applied on open T-rail construction where expansion joints are installed at intervals to provide for expansion and contraction. Arc Welding. There are several forms of arc welding where the splice bars are welded to the rail at a number of points by the use of an electric arc. Electric arc welding may be done under traffic conditions, and is more extensively used in maintenance work than other methods.

Cast Welding is accomplished by setting a mold around the rail joint and pouring molten iron from a crucible around the joint. This process requires transporting a portable cupola along the street adjacent to the work. On account of the improvement in similar types of joints with more portable equipment, this method is not now used as much as formerly.

The Thermit process is a modification of the cast weld, the iron being liberated at white heat from a mixture of iron oxide and aluminum, which is ignited in a crucible. Cast welding is used chiefly on new construction and cannot be done under traffic. The renewal of a cast weld joint requires cutting in a short length of new rail which adds another joint to the track.

(e) Cross-bonding. The important objects of cross-bonding are to equalize the current flow between the rails, thus reducing the voltage drop and also to insure continuity of the return circuit in case of a broken length of rail or a broken bond in any rail. It is good practice to place cross-bonds at intervals of 1,000 to 2,000 feet on suburban railways and not to exceed 500 feet on urban railways. Cross-bonding between parallel tracks is in some cases installed with the same frequency

as between the rails of the single track; in other cases at less frequent intervals. Some companies make a practice of installing cross-bonds under each feeder tap to the trolley wire or at every fourth or fifth span wire, thus enabling them to conveniently preserve a record of their locations. In cases where the track has been carefully insulated crossbonds should preferably be rubber insulated so as to increase their electrical resistance to earth, and where subject to damage from track tools and to other mechanical injury the insulation should be protected by circular loom or conduit.

The common practice of electrified steam railroads is to use crossbonds with a conductance equal to one track rail, or of about 1,000,000 circular mils cross-section. Street and interurban railways employ bonds having a cross-section of from 200,000 to 500,000 circular mils.

(f) Special Track Work Bonding. It is good practice to provide jumpers at switches, frogs and at other special track work to insure that the electrical continuity of the bonded rail will be maintained. This is usually accomplished by jumpers extending around the special work, and in such cases the frogs are bonded into the track system, or where practicable the special work is bonded as other track rails. The size of the jumper cables to be used will depend upon the nature of the traffic. On tracks bearing heavy traffic a separate cable is usually provided for each rail, while for light traffic a single jumper connecting to all rails on both sides of the special work is sometimes used. In all cases the jumpers should be proportioned to the current carried in the track and in no case less than a No. 0000 for one track.

In cases where the track has been carefully insulated the best practice provides for the use of insulated cables for jumpers, except in dry locations, as for instance, on bridges or on other elevated structures where the ties are not in contact with earth or ballast. The electrical leakage from one bare track jumper to damp earth has been known to offset the effect of many miles of most careful track insulation. Under such conditions, if positive to the earth, the bond is gradually destroyed by electrolysis.

(g) Bonding Tracks with Signal Systems. In determining the location of cross-bonds and jumpers in connection with alternating current track signal circuits, a departure from ideal spacing becomes necessary, owing to the fact that cross-bonds are permissible only at the reactance bonds. The signal reactance bonds are located between the signal block sections, and these sections are more or less fixed for train operating conditions. The method used where tracks carry heavy currents is to cross-bond at all signal reactance bonds and install additional cross-bonds with reactance bonds at intermediate locations to obtain the most satisfactory resistance conditions in the sections fixed by the signal system.

(h) Conductivity and Composition of Rails. The conductivity of the track rails used by several interurban and electrified steam railroads has been found to be equivalent to about 1/11 that of copper, and this figure generally holds approximately true for girder types of rails, ex

cept when alloy steel is used, in which case higher resistivities are found. The track rails are specified for their mechanical qualities, and where these interfere with the electrical requirements, it is customary to give the mechanical qualities preference. The composition of rails for heavy service used by one of the large electrified steam railroads, in percentage, is as follows:

Carbon

Manganese
Silicon

Phosphorous

0.62 to 0.75

0.70 to 1.00

0.10 to 0.20

.Not to exceed 0.04

The American Railway Engineering Association has adopted the following composition for heavy rails:

(a) Degrees of Insulation. Under this sub-heading have been considered, (1) Substantial Insulation, in which the type of construction largely prevents the escape of stray current, and (2) Partial Insulation, which comprises using such means as are available to insulate from the earth the running rails of ordinary street railways in so far as practicable.

Substantial Insulation. Interurban and electrified steam roads generally require the rail to be supported on wooden ties set in well drained broken stone or gravel ballast. Such construction affords a very high resistance between the tracks and earth and reduces the danger of electrolysis to a minimum.

With 10 volts between rail and ground the leakage in some instances is found to be as low as 0.00016 amperes per rail per tie under dry weather conditions, increasing to 0.0055 amperes when wet. On double track with ties spaced 2 feet apart these values represent 0.32 and 11.0 amperes, respectively, per 1,000 feet, or 31 and 0.91 ohms respectively for 1,000 feet. On steel structures where the ties are only partially in contact with the ground and cannot become waterlogged, this leakage is even less. The substantial insulation of a ballasted roadbed has, in some installations, been rendered ineffective by bare negative cables in damp earth or by metallic connections between the tracks and steel supporting structures. Conditions are found to be very favorable for rail insulation where the tracks are in subways or under cover protected from the weather, permitting the ballast and ties to become permanently dry.

Partial Insulation. Tracks placed in city streets where rails are depressed to the surface of the ground and have only their upper surface exposed can be but partially insulated. The character of the material in immediate contact with the rails has a large influence on the resistance to ground, but it has been repeatedly demonstrated that coating the rails with an insulating material is not advisable, and the best plan is to

provide a roadbed, which, taken as a whole, is of an insulating character. The use of well drained broken stone or gravel ballast results not only in a good roadbed, but also affords a much higher resistance to the escape of stray current than does a roadbed of concrete. It is desirable to keep vegetation down and otherwise keep the ballast dry and prevent foreign material from washing into it. Salt, which is frequently used to prevent freezing at switches and frogs, greatly increases the conductivity of the roadbed and thereby facilitates the escape of stray current.

Electric railways have experienced some damage due to the corrosion of the base of the rail or of elevated structures connected to the rails in districts where the stray current leaves the structure for the earth. Cases are on record where this corrosion is serious and where steps have been taken to reduce the damage to elevated structures by insulating the rail from the steel structure. Any measure which tends to insulate the track from the soil or any mitigative system which tends to reduce stray current will tend to retard the electrolytic corrosion of the base of the rails and other grounded steel structures.

(b) Leakage to be Expected. Under conditions of substantial insulation and where the roadbed is of open construction the leakage varies widely, depending upon the character of the ballast and whether it is wet or dry. In dry weather the resistance may be from 10 to 15 ohms or even more per 1,000 feet of single track. In wet weather this may drop to 3 to 5 ohms. If ties are treated with a 3 to 1 mixture of gas oil and creosote, the resistance may be double the above values whereas with ties treated with zinc chloride or other chemical salts the resistance may be one-half of these values.

The leakage where tracks are only partially insulated will not only be much greater than where they are substantially insulated but will vary over a much wider range. This is because the type of roadbed, character of soil, and drainage conditions vary greatly. It is known that well drained crushed stone ballast with a Tarvia finish will have a resistance from 2 ohms to 5 ohms per 1,000 feet of single track. On the other hand the resistance of roadbeds with solid concrete ballast in contact with the rails and also earth roadbeds, in which the ties are embedded and therefore in a more or less moist condition, are much lower and may be only from 0.5 to 1.5 ohms for 1,000 feet of single track.

3. Reinforcement of Rail Conductivity.

Early track construction practice in this country often included bare wire laid between the rails and connected to each bond. Sometimes one such wire was used for each rail, sometimes one for each track, and sometimes one served for a double track. The wires varied from No. 4 to No. 1, and were either of copper or galvanized iron. Their conductivity was small and they were subject to electrolytic corrosion and mechanical injury. This construction has practically gone out of use. It

is, however, common to find the rails in the vicinity of supply stations supplemented by large conductors connected in parallel with the rails. This is not infrequently accomplished by the use of bare copper wire or cable buried between rails, and hence in full contact with the earth. Old rails, bolted and bonded together and buried beneath or beside the track, have also been used in some cases. Such buried conductors increase the leakage from the tracks and should be avoided.

Supplementary conductors in parallel with the track and connected to it at frequent intervals tend greatly to insure the continuity of the return circuit, where the track bonds cannot be well maintained. Where copper cables are so used the occasional failure of bonds does not materially affect the track drop and their use may be justified where tracks are laid on filled or spongy ground or where the proper maintenance is unusually difficult.

Buried bare conductors, however, increase the contact area between the return circuit and the earth, and the tendency to augment stray currents thus caused offsets to a greater or less extent the benefits attained by the reduction of drop.

Copper installed in this manner is in parallel with the rails, and therefore has the same drop as exists in the rails. As track gradients rarely exceed two or three volts per thousand feet, this would mean that the drop on such cables would not exceed two or three volts per thousand feet, which corresponds to a current density of about 190 or 280 amperes respectively, per 1,000,000 circular mils. It will be seen that these densities are so low that such use of the copper is very uneconomical, and for this reason this method of reinforcement of the rail conductivity should not ordinarily be used.

Conductors are regarded as being in parallel with the rails when both ends are connected to the tracks or when one end is connected to the track and the other to a station busbar which is connected directly to the rail by a conductor of negligible resistance. The use of such conductors should not be confused with the insulated negative feeder system.

4. Power Supply.

Among the various features of railway construction which tend to reduce stray current none has made more rapid advancement during recent years than the development of multiple feeding points, principally from use of additional substations supplying the railway systems. Increasing the number of substations will reduce the feeding distances and effect a saving in distribution copper and in line and return losses, and will also reduce the amount of current to be returned to any one point. The general effect is to reduce the track voltage drops, thereby reducing the amount of current which will stray from the rails to subsurface metallic structures.

The ordinary street railway system employs direct current at from 550 to 750 volts. Some interurban lines operate at 1.200 volts direct cur