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The New Zealand Railways Magazine, Volume 2, Issue 2 (June 1, 1927)

Relays

Relays.

The key to the success of Automatic Signalling is the relay. Essentially, the relay is a device for switching in an electric current for performing work of some kind. The work may be that of selecting which light (green, yellow or red) is to show in a signal and lighting it to the correct colour; it may be that of lifting a lock from a signal lever or again it may be the withholding of current from another relay for a set time (say 2 seconds) in order to allow proper co-ordination in a series of electrically controlled movements of other relays and machines. In fact relays seem to have no limit to their possibilities for the automatic control of operating electric currents.

Do not think, however, that relays are of one form only. Though standardised for mass-production purposes, in appearance and in most of their fittings, their functions, operating currents, and the number of contacts (switches) embodied, vary widely;—so does their price! Adequately to describe their complicated structure and functions is impossible in this short article. A slight glimpse only can be attempted. I have already roughly described two types of “track” relays as regards their contacts or switching equipment: two-position, and three-position relays.

page 35

Two position relays may be operated (energised) by a single current of electricity passing through their coils (these are known as “single element”), or by two separate currents from different circuits acting together. In the latter case they are called “double element” relays.

Single element relays, which are always of the two-position type, are operated (energised) by low voltage (say two volts) when controlled by a track, and higher voltage (110 volts) if line controlled.

Double element relays may be of either the two or three-position types, and the two currents operating them may be both of 110 volts, or one of 110 volts and the other of low voltage (say two volts), the low voltage side, in the latter case, coming from the track. Double element relays thus have dual control, which means that two separate factors must co-ordinate in their operation.

Again relays are used which wait for a second or two before closing their contacts after their operating current is applied. Others will wait for as long as twenty-five seconds before deigning to open; of course all being timed “according to plan.”

Some relays have small motors embodied, to open and close their contacts (a very positive and reliable type). Others are of the magnet and coil variety, but the most fascinating are the Vane type relays, with their eddy currents principle and other complications.

With such an array of mechanism to draw upon it becomes a simple matter to control the operations of the nimble spark, which moves at the rate of a mere 186,000 miles per second, or thereabouts.