Faults in Bells.
On examining many electric bells it will be noted that only one binding post is insulated from the frame when the latter is of iron ([Fig. 34]). As the armature spring S is in electrical connection with the frame F by reason of its metal screws and support, the circuit may run from the insulated post U to the magnet coils, thence through the insulated contact screw C through the armature spring (when it is making contact) and through the frame to the uninsulated post I.
Fig. 34
This saves labor, wire and complication, but if the insulation of the post U, the wires W V, or the contact screw C be injured, the current may take a short path back to the frame.
If C were thus grounded, the bell would act as a single-stroke bell.
If U were grounded, the bell would not ring at all, as that would be a short circuit on the battery between I and U and the latter would also result if the bare wire were touching the frame at V.
If the bare wire touched the frame beyond M M, that is, along W, it would be a single-stroke bell, as if C were grounded.
As any one of these faults is likely to occur, they should be looked for when the bell acts imperfectly, or not at all.
A very common fault in a bell is when its armature sticks to the cores and thus does not make contact with the contact screw. This may be from a weak spring or because of the loss of the pieces of brass inserted in the ends of the cores to keep the armature away from actual contact. A piece of a postage stamp stuck over the core end will often help out in the latter case.
A high screeching noise from the armature vibrating too rapidly but with too little play, may be from excessive battery power or the contact screw being too far forward. The former will generally be detected by the violent sparking as well as the rapid vibration.
In very cheap bells the platinum contacts may be replaced by German silver or some other metal.
Platinum is necessary because the sparking would soon corrode other metals, but it is very expensive. To test for platinum put a tiny drop of nitric acid on the suspected metal. If bubbles or smoke appear it is not platinum. After applying this test in any case however, carefully wash off and remove all traces of the acid, as it will corrode the metal into which the platinum is riveted.
Dirty contacts will decrease the current in the bell coils and it will not work well, if at all.
Loose contact screws and wires also give trouble. The adjusting of the contact screw is of the utmost importance, and should never be attempted unless it is clearly necessary.