With the adoption of relays the signalling between the subscribers and the exchange became automatic, and, with the introduction of the principle of double and automatic supervision on the cord circuits, it became possible for the operators to tell at any instant the state of a connexion.
The large modern trunk exchanges are equipped with relays and lamps for signalling purposes.
This arrangement can be duplexed in the way already explained, by providing differential relays and arranging for the outgoing currents to divide differentially through the two relays at each end.
Davila was murdered, while on his way to take possession of the government of Cremona for Venice in July 1631, by a ruffian, with whom some dispute seems to have arisen concerning the furnishing of the relays of horses ordered for his use by the Venetian government.
Each transmitter is therefore able to transmit a separate series of positive and negative currents in different combinations; these are distributed, by suitably arranged distributors and relays at the receiving end of the line, into their respective receivers.
The movements of the shaft are controlled by relays and electro-magnets which operate in response to the action of the subscriber whose telephone is fitted with a 'calling mechanism which, when the subscriber calls, earths the line a certain number of times for each figure in the number of the wanted subscriber.
Two years of incessant labor with relays of artisans working steadily throughout the twenty-four hours were required to finish this piece.
Such a leisurely fashion of transacting business soon grew intolerable, and in 1635 a system of relays was instituted which enabled the journey between the two cities to be accomplished in three days, the charge for a letter being 8d.
Following on a period of good rule and prosperity under Rhampsinitus, Cheops closed the temples, abolished the sacrifices and made all the Egyptians labour for his monument, working in relays of ioo,000 men every three months (see Pyramid).
At regular intervals a rotating arm on the distributor connects the five keys of each keyboard to line, thus passing the signals to the distant station, where they pass through the distributor and certain relays which repeat the currents corresponding to the depressed keys and actuate electromagnets in the receivers.