Electric Bells and All About Them.
by S. R. Bottone.
PREFACE.
So rapidly has the use of electric bells and similiar signalling appliances extended, in modern houses, offices, hotels, lifts, and ships, that every bell-fitter must have felt the need of accurate knowledge of the manner in which these instruments act and are made.
In the following pages the author has attempted to supply this need, by giving full details as to the construction of batteries, bells, pushes, detectors, etc., the mode of wiring, testing, connecting up, localizing faults, and, in point of fact, by directing careful attention to every case that can present itself to the electric-bell fitter.
CARSHALTON, SURREY, _November, 1888_.
ELECTRIC BELLS.
CHAPTER I.
PRELIMINARY CONSIDERATIONS.
-- 1. ELECTRICITY.--The primary cause of all the effects which we are about to consider resides in a force known as _electricity_, from the Greek name of amber (electron), this being the body in which the manifestations were first observed. The ancients were acquainted with a few detached facts, such as the attractive power acquired by amber after friction; the benumbing shocks given by the torpedo; the aurora borealis; the lightning flash; and the sparks or streams of light which, under certain conditions, are seen to issue from the human body. Thales, a Grecian philosopher, who flourished about 600 years B.C., observed the former of these facts, but nearly twenty centuries elapsed before it was suspected that any connection existed between these phenomena.
-- 2. According to the present state of our knowledge, it would appear that electricity is a mode of motion in the const.i.tuent particles (or atoms) of bodies very similar to, if not identical with, _heat_ and _light_. These, like _sound_, are known to be dependent on undulatory motion; but, whilst _sound_ is elicited by the vibration of a body _as a whole_, electricity appears to depend, in its manifestations, upon some motion (whether rotary, oscillatory, or undulatory, it is not known) of the atoms themselves.
However this be, it is certain that whatever tends to set up molecular motion, tends also to call forth a display of electricity. Hence we have several practical means at our disposal for evoking electrical effects.
These may be conveniently divided into three cla.s.ses, viz.:--1st, mechanical; 2nd, chemical; 3rd, changes of temperature. Among the _mechanical_ may be ranged friction, percussion, vibration, trituration, cleavage, etc. Among the _chemical_ we note the action of acids and alkalies upon metals. Every chemical action is accompanied by electrical effects; but not all such actions are convenient sources of electricity.
_Changes of temperature_, whether sudden or gradual, call forth electricity, but the displays are generally more striking in the former than in the latter case, owing to the acc.u.mulated effect being presented in a shorter time.
-- 3. We may now proceed to study a few of these methods of evoking electricity, so as to familiarise ourselves with the leading properties.
If we rub any resinous substance (such as amber, copal, resin, sealing-wax, ebonite, etc.) with a piece of warm, dry flannel, we shall find that it acquires the power of attracting light bodies, such as small pieces of paper, straw, pith, etc. After remaining in contact with the rubbed (or electrified) substance for a short time, the paper, etc., will fly off as if repelled; and this apparent repulsion will be more evident and more quickly produced if the experiment be performed over a metal tray. If a small pith-ball, the size of a pea, be suspended from the ceiling by a piece of fine cotton, previously damped and then approached by an ebonite comb which has been briskly rubbed, it will be vigorously attracted, and never repelled; but if for the cotton there be subst.i.tuted a thread or fibre of very fine dry silk, the pith-ball will be first _attracted_ and then _repelled_. This is owing to the fact that the damp cotton allows the electricity to escape along it: _id est_, damp cotton is a CONDUCTOR of electricity, while silk does not permit its dissipation; or, in other words, silk is a NON-CONDUCTOR. All bodies with which we are acquainted are found, on trial, to fall under one or other of the two heads--viz., conductors and non-conductors. Nature knows no hard lines, so that we find that even the worst conductors will permit the escape of some electricity, while the very best conductors oppose a measurable resistance to its pa.s.sage. Between the limits of good conductors, on the one hand, and non-conductors (or insulators) on the other, we have bodies possessing varying degrees of conductivity.
-- 4. As a knowledge of which bodies are, and which are not, conductors of electricity is absolutely essential to every one aspiring to apply electricity to any practical purpose, the following table is subjoined, giving the names of the commoner bodies, beginning with those which most readily transmit electricity, or are _good_ conductors, and ending with those which oppose the highest resistance to its pa.s.sage, or are insulators, or non-conductors:--
-- 5. TABLE OF CONDUCTORS AND INSULATORS.
-----------------+------------------------------+--------------------- Quality.Name of Substance.Relative Resistance.
-----------------+------------------------------+--------------------- Good {Silver, annealed1.
Conductors {Copper, annealed1.063 {Silver, hard drawn1.086 {Copper, hard drawn1.086 {Gold, annealed1.369 {Gold, hard drawn1.393 {Aluminium, annealed1.935 {Zinc, pressed3.741 {Bra.s.s (variable)5.000 {Platinum, annealed6.022 {Iron6.450 {Steel, soft6.500 {Gold and silver alloy, 2 to 17.228 {Nickel, annealed8.285 {Tin, pressed8.784 {Lead, pressed13.050 {German silver (variable)13.920 {Platinum-silver alloy, 1 to 216.210 {Steel, hard25.000 {Antimony, pressed23.600 {Mercury62.730 {Bis.m.u.th87.230 {Graphite145.000 {Nitric Acid976000.000Imperfect {Hydrochloric acid[1]
Conductors {Sulphuric acid1032020.000 {Solutions of metallic saltsvaries with strength {Metallic sulphides[1]
{Distilled water[1] 6754208.000Inferior {Metallic salts, solid[1]
Conductors. {Linen }{Cotton } and other forms of[1]
{Hemp } cellulose{Paper }{Alcohol[1]
{Ether[1]
{Dry Wood[1]
{Dry Ice[1]
{Metallic Oxides[1]
Non-conductors, {Ice, at 25 c.[1]
or {Fats and oils[1]
Insulators. {Caoutchouc1000000000000.
{Guttapercha1000000000000.
{Dry air, gases, and vapours[1]
{Wool[1]
{Ebonite1300000000000.
{Diamond[1]
{Silk[1]
{Gla.s.s[1]
{Wax[1]
{Sulphur[1]
{Resin[1]
{Amber[1]
{Sh.e.l.lac[1]
{Paraffin1500000000000.
[Footnote 1: These have not been accurately measured.]
The figures given as indicating the relative resistance of the above bodies to the pa.s.sage of electricity must be taken as approximate only, since the conductivity of all these bodies varies very largely with their purity, and with the temperature. Metals become worse conductors when heated; liquids and non-metals, on the contrary, become better conductors.
It must be borne in mind that _dry air_ is one of the _best insulators_, or worst _conductors_, with which we are acquainted; while damp air, on the contrary, owing to the facility with which it deposits _water_ on the surface of bodies, is highly conducive to the escape of electricity.
-- 6. If the experiment described at -- 3 be repeated, subst.i.tuting a gla.s.s rod for the ebonite comb, it will be found that the pith-ball will be first attracted and then repelled, as in the case with the ebonite; and if of two similar pith-b.a.l.l.s, each suspended by a fibre of silk, one be treated with the excited ebonite and the other with the gla.s.s rod, until repulsion occurs, and then approached to each other, the two b.a.l.l.s will be found to attract each other. This proves that the electrical condition of the excited ebonite and of the excited gla.s.s must be different; for had it been the same, the two b.a.l.l.s would have repelled one another. Farther, it will be found that the _rubber_ with which the ebonite or the gla.s.s rod have been excited has also acquired electrical properties, attracting the pith-ball, previously repelled by the rod.
From this we may gather that when one body acting on another, either mechanically or chemically, sets up an electrical condition in one of the two bodies, a similar electrical condition, but in the opposite sense, is produced in the other: in point of fact, that it is impossible to excite any one body without exciting a corresponding but opposite state in the other. (We may take, as a rough mechanical ill.u.s.tration of this, the effect which is produced on the pile of two pieces of plush or fur, on being drawn across one another in opposite directions. On examination we shall find that both the piles have been laid down, the upper in the one direction, the lower in the other.) For a long time these two electrical states were held to depend upon two distinct electricities, which were called respectively _vitreous_ and _resinous_, to indicate the nature of the bodies from which they were derived. Later on (when it was found that the theory of a single electricity could be made to account for all the phenomena, provided it was granted that some electrified bodies acquired more, while others acquired less than their natural share of electricity), the two states were known as _positive_ and _negative_; and these names are still retained, although it is pretty generally conceded that electricity is not an ent.i.ty in itself, but simply a mode of motion.
-- 7. It is usual, in treatises on electricity, to give a long list of the substances which acquire a positive or a negative condition when rubbed against one another. Such a table is of very little use, since the slightest modification in physical condition will influence very considerably the result. For example: if two similar sheets of gla.s.s be rubbed over one another, no change in electrical condition is produced; but if one be roughed while the other is left polished, this latter becomes positively, while the former becomes negatively, electrified.
So, also, if one sheet of gla.s.s be warmed, while the other be left cold, the colder becomes positively, and the latter negatively, excited. As a general law, _that body, the particles of which are more easily displaced, becomes negatively electrified_.
-- 8. As, however, the electricity set up by friction has not hitherto found any practical application in electric bell-ringing or signalling, we need not to go more deeply into this portion of the subject, but pa.s.s at once to the electricity elicited by the action of acids, or their salts, on metals.
Here, as might be expected from the law enunciated above, the metal more acted on by the acid becomes negatively electrified, while the one less acted on becomes positive.[2] The following table, copied from Ganot, gives an idea of the electrical condition which the commoner metals and graphite a.s.sume when two of them are immersed at the same time in dilute acid:--
{ v Zinc. ^ } { v Cadmium.} {Tin.} {Lead.} {Iron.} The portion {Nickel.} The portion out immersed in the {Bis.m.u.th.} of the acid fluid acid fluid. {Antimony.} {Copper.} {Silver.} {Gold.} {Platinum. ^ } { v Graphite. ^ }
[Footnote 2: This refers, of course, to those portions of the metals which are out of the acid. For reasons which will be explained farther on, the condition of the metals in the acid is just the opposite to this.]