Even in very wet or snowy weather a single jar of gravity battery is generally found to furnish sufficient current to properly work the relay at the other end of any section less than three-quarters of a mile in length; although it frequently happens on longer sections and occasionally on those of ordinary length that two jars are necessary. A greater number of jars is never advisable since by increasing the intensity of the current, the liability of its leaking from one rail to the other during wet weather is correspondingly increased, and as this is attended with some uncertainty in the working of the relay of the section—due to the varying intensity of the current—it should be carefully guarded against. As two jars of gravity battery are not sufficient to operate a signal, lock, bell or any similar instrument with any degree of certainty, it becomes necessary to have a second set of batteries of a greater number of jars for that purpose. The armature of the magnet controlled by the track section is therefore made to control a second circuit using a battery of this sort (Figs. 3, 4 and 5) and which includes the magnet of the signal mechanism. The use of a relay on a track section is therefore necessary; and when it becomes necessary to control two or more devices, each requiring independent circuits, by one track, the use of a relay is indispensable.
While the fundamental principles of the track circuit are the same today as they were when it was originally invented by Dr. Robinson in 1872, it has been found that it is not as simple a device as was formerly supposed to be the case and many problems have arisen which have required and is requiring the careful study of the signal engineers. Accordingly, it is well to present briefly some of the track circuit characteristics as they are known today. In the following presentation, information has been collected from many different sources, including abstracts from papers presented on the track circuit by Mr. A. R. Fugina, signal engineer, and Mr. J. B. Weigle, signal inspector on the Louisville & Nashville.
There are two general classes of track circuits, direct current and alternating current, which may be further subdivided between single or double rail circuits. The essential feature of the track circuit is the insulation of each section of track from the adjoining sections. Each rail in the section is connected to the one adjoining by bond wires, for the purpose of making a continuous conductor from one end of the section to the other.
Rail Bonding
Under the present methods of bonding, the angle bar carries the greater part of the current, and bond wires frequently carry as little as 20 per cent. of it and sometimes even less. The rail resistance is lowest with new rails, but it gradually gets higher, due to rust and dirt formations between the angle bar and the rail. But even with new rail, the rail resistance varies greatly at different periods and even at different times during any twenty-four hours. This variation is entirely due to the fact that the angle bars carry more of the current than the bond wires, and that the bond wires under any condition are only large enough to carry the smaller part of the current from the battery. The lower the resistance of the bonds the less variable will be the rail resistance.
The resistance of the angle bars increases greatly as rail resistance increases, as a result of which the angle bars rapidly carry less of the current.
It is not infrequent to find the rail resistance to be as high as 0.20 ohms per 1,000 ft. of track, and we have known it to run as high as 0.264 in new rail, where especial attention had been given to obtaining as good bonding as possible. Under such conditions the angle bar carries very little of the current, the capacity of the bond is not sufficient to carry the current, and the net result is a failing track circuit, which is probably attributed to bad ballast, zinc treated ties or other causes.
The principal defect in the track circuit is that of improper bonding. The only explanation as to why No. 8 iron wires became standard for bonding appears to be that the bond wires were cut from this size iron telegraph wire which was in general use at the time rails began to be bonded. It is important to obtain better bonding to obtain a minimum constant rail resistance.
It has been recommended that: