Single rail track circuits, so called from the fact that but one rail is insulated, are also used. Installations of this kind are made to avoid the expense of two insulated joints or where one rail is needed for another circuit. Such track circuits are more liable to failure than those having both rails insulated for the reason that the break-down of one insulated joint will extend the circuit beyond the proper limit and cause interference of neighboring circuits or extended shunting of the relay, due to the presence of a train beyond the insulated joint. A track circuit may be made to perform two separate functions in which the direction or polarity as well as the presence of current is made use of in the relay, provided the first or principal function actuated by the presence or absence of current does not interfere with the secondary function, actuated both by the presence of current and its polarity. Where switches occur in a track circuit, special means must be employed to prevent short-circuiting through the switch rods and leakage of current to the turn-out rail. The usual method is the use of insulated switch rods with insulated joints in the leads of the turnout and at the fouling point of the turnout. The switch points are bonded to the stock rails to insure shunting by a pair of wheels on any part of the track. None of the methods employed in running track circuits through switches show any protection against an open switch. In order to obtain this protection a switch instrument or switch box is used. This consists of a device with electrical contacts, the whole mounted on a switch timber and connected to the switch point by means of a rod so arranged that when a switch slips open or is thrown open the movement of the rod actuates contacts which, on being closed, form a closed path from one rail to the other through wires connecting the rails to the contacts, thus when the contacts are closed by a switch being opened, the same effect is produced as if a train was on the circuit, shunting it out. On electrically operated roads where tracks are bonded for the return propulsion current with heavy copper bonds, no additional bond wires are necessary. The Track Battery The usual form of track circuit has a primary battery at one end of the insulated track section, with the positive terminal of the battery connected to one rail and the negative terminal to the other, while a relay at the other end of the section is connected to the rails in a similar manner. Current flows from the positive side of the battery through the one rail, the relay and the other rail back to the battery, thus keeping the relay energized.