Page 18 4001-A Selector Key, Coded 3-11-3 Fig. 7.
The keys are easily removed from the key case by simply turning the mounting screw U counter-clockwise for a quarter turn. This causes the spring latch B to disengage from the framework bars. The spring latch B, in addition to securing the key, completes the K3 connection through the key case framework to the external K3 terminal. The keys, when mounted, make contact with springs in the back of the case, and these springs are connected with the external terminals K1 and K2. Connections are therefore automatically accomplished when a key is mounted in its key case making the changing of a key a very simple matter. General Requirements and Description The speed of the key is regulated by a governor Q which can be adjusted by bending the springs carrying the weights, inwards or outwards, to increase or decrease respectively the speed of the key. The springs should be so adjusted that the impulse wheel makes one complete revolution in 7½ to 8 seconds. Fig. 7 illustrates how a key is coded, and will be referred to in the following description. When the key is normal, the end of the inner spring J1 should clear the impulse wheel K3, as shown at A in Fig. 7, and when the wheel is revolving, should make contact with it for the duration of the revolution. It is this contact between K1 and K3 which operates the battery relay of the system, thus applying battery to the line. K1 and K2 should be closed only when K1 passes over a tooth of the impulse wheel, or when passing over a flat coding segment, and should be open when K1 drops into a hollow between teeth, or is passing over a bent-up segment. These makes and breaks of K1 and K2 are reproduced by the battery reversing relay of the system, thus sending reversed impulses to line. Impulse Ratio The springs K1 and K2 should be so adjusted that the time the contacts are closed when passing over the teeth of the impulsing wheel equals the time they are open. The impulse ratio, or comparison of the closed and open periods, of K1 and K2 can be determined with the use of a damped voltmeter and battery or a milli-ammeter, battery and non-inductive resistor in series with KI and K2. The coding segments should be adjusted so that an uninterrupted train of impulses, such as General Call (see General Call, page 22) are transmitted, then the average reading of the meter, during the impulsing period, should be one half of the steady reading of the meter when the contacts K1, K2 are closed or shorted. (See also Mechanical Requirements page 23 paragraph 6.) Method of Coding the Key When the impulse wheel is rotating each closure of KI, K2 is one impulse, and each opening of the contacts is another impulse. The code consists of three digits, the sum of which, for any code is always 17 impulses. The inter-digital pauses in impulsing are obtained by the use of two segments, of which there are two types. Page 19