Other Instruments.

After dabbling in relays, other instruments seem a trifle commonplace, but perhaps the “reactance” and “resistance” coils, being appendages to relays, may be interesting. Insulated tracks, as current-carrying circuits, may vary considerably, both according to the nature of the ballast and also with weather conditions. As the tracks are the medium of signal control by passing trains, their vagaries must be suffered, but these can be softened by judicious handling. To this end the current delivered by the track to the relays is passed through a “resistance,” which lessens the current in a fixed ratio, or through a “reactance,” which limits the current in a progressive ratio.

A relay requiring two currents for its operation (double element relay) needs those currents to be out of phase with other; that is, one operating current phase angle to lead that of the other, producing an effect on the driving mechanism of the relay somewhat similar to the lead given to one of the driving rods on a pair of locomotive wheels for the purpose of avoiding the dead centres.

This is where the “reactance” gets in its work. A “reactance” in a circuit has the effect of altering the phase angle in that circuit, and, being placed in the track feed to the relay, produces a stronger torque in that relay for closing its contacts; the effect being comparable to the conditions governing a yacht beating to windward; the greater the angle her direction makes with the wind the faster she sails. These adjustments put the relays in the best condition to be least affected by variations in the track circuit due to the weather or other cause. Transformers have been dealt with, and lightning guards, fuses, terminals, etc., are commonplaces of all electrical installations.