II. Use in telephone switching

ATYPICAL RELAY, used by the millions in telephone offices, in shown in Figure 1 — the U-type relay. This particular type is not a slow relay; its action time is about 1/20th second or less. It can be slowed, however, to give times in the range from 50 to 500 milliseconds, which is the ordinary range of delay in telephone relays. These delays can be obtained — at reasonable cost, with reasonable accuracy, and without too great a sacrifice in number of contacts — through application of the principles to be outlined. Primarily, these delays are obtained by the use of short-circuited turns to oppose a change in magnetization, and by using the properties of the relay upon disconnection of the circuit, not on closure, because of the greater stability of a relay upon its release. The subject of this article then narrows down to the principles underlying slow-release relays.