Fig. 72.
We close this chapter with an engraving of a very compact and neat form of drop indicator devised by Messrs. Gent, and called by them a "Tripolar Indicator." It consists, as the name implies, of a single magnet, having one end of the iron core as one pole, the other end extending on each side like a V, forming, as it were, three poles. Though but one bobbin is used, the effect is very powerful. There are no springs or other complications, so that the arrangement is adapted for ship use, as are also those represented at Figs. [67] and [68]. Pendulum and fall-back indicators, as well as polarised indicators, owing to the delicacy of the adjustments, are unfitted for use on board ship, or in the cabs of lifts, where the sudden jolts and jerks are sure to move the indicators, and falsify the indications. The tripolar indicator is illustrated at [Fig. 72].
CHAPTER V.
ON WIRING, CONNECTING UP, AND LOCALISING FAULTS.
§ 66. However good may be the bells, indicators, batteries, etc., used in an electric bell installation, if the wiring be in any wise faulty, the system will surely be continually breaking down, and giving rise to dissatisfaction. It is therefore of the highest importance that the workman, if he value his good name, should pay the greatest attention to ensure that this part of his work be well and thoroughly done. This is all the more necessary, since while the bells, batteries, relays, pushes, etc., are easily got at for examination and repair, the wires, when once laid, are not so easily examined, and it entails a great deal of trouble to pull up floor boards, to remove skirtings etc., in order to be able to overhaul and replace defective wires or joints. The first consideration of course, is the kind and size of wire fitted to carry the current for indoor and outdoor work. Now this must evidently depend on three points. 1st, The amount of current (in ampères) required to ring the bell. 2nd, The battery power it is intended to employ. 3rd, The distance to which the lines are to be carried. From
practical experience I have found that it is just possible to ring a 2½" bell with ½ an ampère of current. Let us consider what this would allow us to use, in the way of batteries and wire, to ring such a bell. The electro-motive force of a single Leclanchè cell is, as we have seen at [§ 38], about 1·6 volt, and the internal resistance of the quart size, about 1·1 ohm. No. 20 gauge copper wire has a resistance of about 1·2 ohm to the pound, and in a pound (of the cotton covered wire) there are about 60 yards. Supposing we were to use 60 yards of this wire, we should have a wire resistance of 1·2 ohm, an internal resistance of 1·1 ohm, and a bell resistance of about 0·1 of an ohm, altogether about 2·4 ohms. Since the E.M.F. of the cell is 1·6 volt, we must divide this by the total resistance to get the amount of current passing. That is to say:—
| Ohms. | Volts. | Ampères. |
| 2·4) | 1·60 | (0·66, |
or about 2/3 of an ampère; just a little over what is absolutely necessary to ring the bell. Now this would allow nothing for the deterioration in the battery, and the increased resistance in the pushes, joints, etc. We may safely say, therefore, that no copper wire, of less diameter than No. 18 gauge (48/1000 of an inch diameter) should be used in wiring up house bells, except only in very short circuits of two or three yards, with one single bell in circuit; and as the difference in price between No. 18 and No. 20 is very
trifling, I should strongly recommend the bell-fitter to adhere to No. 18, as his smallest standard size. It would also be well to so proportion the size and arrangement of the batteries and wires, that, at the time of setting up, a current of at least one ampère should flow through the entire circuit. This will allow margin for the weakening of the battery, which takes place after it has been for some months in use. As a guide as to what resistance a given length of copper wire introduces into any circuit in which it may be employed, I subjoin the following table of the Birmingham wire gauge, diameter in 1,000ths of an inch, yards per lb., and resistance in ohms per lb. or 100 yards, of the wires which the fitter is likely to be called upon to employ:—