Many attempts have been made to overcome these disadvantages of the party line in sparsely settled communities, by producing what are commonly called lock-out systems.  These, as their name implies, employ such an arrangement of parts that when the line is in use by any two parties, all other parties are locked out from the circuit and cannot gain access to it until the parties who are using it are through.  System after system for accomplishing this purpose has been announced but for the most part these have involved such a degree of complexity and have introduced so many undesirable features as to seriously affect the smooth operation of the system and the reliability of the service.

We believe, however, in spite of numerous failures, that the lock-out selective-signaling party line has a real field of usefulness and that operating companies as well as manufacturing companies are beginning to appreciate this need, and as a result that the relief of the rural subscriber from the almost intolerable service he has often had to endure is at hand.  A few of the most promising lock-out party-line systems now before the public will, therefore, be described in some detail.

Poole System.  The Poole system is a lock-out system pure and simple, its devices being in the nature of a lock-out attachment for selective-signaling lines, either of the polarity or of the harmonic type wherein common-battery transmission is employed.  It will be here described as employed in connection with an ordinary harmonic-ringing system.

In Fig. 188 there is shown a four-station party line equipped with Poole lock-out devices, it being assumed that the ringers at each station are harmonic and that the keys at the central office are the ordinary keys adapted to impress the proper frequency on the line for ringing any one of the stations.  In addition to the ordinary talking and ringing apparatus at each subscriber’s station, there is a relay of special form and also a push-button key.

[Illustration:  Fig. 188.  Poole Lock-Out System]

Each of the relays has two windings, one of high resistance and the other of low resistance.  Remembering that the system to which this device is applied is always a common-battery system, and that, therefore, the normal condition of the line will be one in which there is a difference of potential between the two limbs, it will be evident that whenever any subscriber on a line that is not in use raises his receiver from its hook, a circuit will be established from the upper contact of the hook through the lever of the hook to the high-resistance winding 1 of the relay and thence to the other side of the line by way of wire 6.  This will result in current passing through the high-resistance winding of the relay and the relay will pull up its armature.  As soon as it does so it establishes two other circuits by the closure of the relay armature against the contacts 4 and 5.