Fig. 46
A form of bell and relay combined is shown in [Fig. 46]. Here the armature A is held against the magnets while the circuit through the spring F and battery G is closed. But on opening this circuit the armature flies back and makes contact with an adjustable contact screw S putting in circuit a local battery C. The bell is now practically a vibrating bell; on a closed circuit it rings until the circuit is again closed or the battery runs down.
Fig. 47
A different connection of the same scheme is [Fig. 47], where only one battery is used. This must be a gravity battery or some other closed-circuit battery. The circuit can be easily traced in the figure and needs no special description.
Both of the latter schemes are inferior to one using a separate relay. If the circuit at the spring were quickly closed again the bell would either stop ringing, or be so hampered as to ring very weakly.
Fig. 48
A relay made as in [Fig. 48] has no spring support to the armature A, which falls down by gravity. The adjustable contact C is screwed far back, so that the armature must fall a considerable distance away from the electromagnets before it makes contact. This ensures that the armature will not be attracted and the bell stopped from ringing by a re-closing of the circuit at the door or window spring.