Electric Bells and All About Them

§ 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 constituent 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 classes, 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 accumulated 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 substituted 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 passage. 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 passage, or are insulators, or non-conductors:-

§ 5. TABLE OF CONDUCTORS AND INSULATORS.

Quality. Name of Substance. Relative Resistance.

Good Conductors Silver, annealed 1.

Copper, annealed 1.063

Silver, hard drawn 1.086

Copper, hard drawn 1.086

Gold, annealed 1.369

Gold, hard drawn 1.393

Aluminium, annealed 1.935

Zinc, pressed 3.741

Brass (variable) 5.000

Platinum, annealed 6.022

Iron 6.450

Steel, soft 6.500

Gold and silver alloy, 2 to 1 7.228

Nickel, annealed 8.285

Tin, pressed 8.784

Lead, pressed 13.050

German silver (variable) 13.920

Platinum-silver alloy, 1 to 2 16.210

Steel, hard 25.000

Antimony, pressed 23.600

Mercury 62.730

Bismuth 87.230

Graphite 145.000

Nitric Acid 976000.000

Imperfect Conductors Hydrochloric acid [1]

Sulphuriacid 1032020.000

Solutions of metallic salts varies with strength

Metallic sulphides [1]

Distilled water [1] 6754208.000

Inferior Conductors. Metallic salts, solid [1]

Linen } and other forms of cellulose [1]

Cotton

Hemp

Paper

Alcohol [1]

Ether [1]

Dry Wood [1]

Dry Ice [1]

Metallic Oxides [1]

Non-conductors,

or Insulators. Ice, at 25 c. [1]

Fats and oils [1]

Caoutchouc 1000000000000.

Guttapercha 1000000000000.

Dry air, gases, and vapours [1]

Wool [1]

Ebonite 1300000000000.

Diamond [1]

Silk [1]

Glass [1]

Wax [1]

Sulphur [1]

Resin [1]

Amber [1]

Shellac [1]

Paraffin 1500000000000.

[1] These have not been accurately measured.

The figures given as indicating the relative resistance of the above bodies to the passage 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, substituting a glass 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-balls, each suspended by a fibre of silk, one be treated with the excited ebonite and the other with the glass rod, until repulsion occurs, and then approached to each other, the two balls will be found to attract each other. This proves that the electrical condition of the excited ebonite and of the excited glass must be different; for had it been the same, the two balls would have repelled one another. Farther, it will be found that the rubber with which the ebonite or the glass 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 illustration 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 entity 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 glass 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 glass 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 pass 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 assume when two of them are immersed at the same time in dilute acid:-

The portion immersed in the acid fluid. ? ↓ Zinc. ↑ ? The portion out of the acid fluid.

? ↓ Cadmium. ↑ ?

? ↓ Tin. ↑ ?

? ↓ Lead. ↑ ?

? ↓ Iron. ↑ ?

? ↓ Nickel. ↑ ?

? ↓ Bismuth. ↑ ?

? ↓ Antimony. ↑ ?

? ↓ Copper. ↑ ?

? ↓ Silver. ↑ ?

? ↓ Gold. ↑ ?

? ↓ Platinum. ↑ ?

? ↓ Graphite. ↑ ?

The meaning of the above table is, that if we test the electrical condition of any two of its members when immersed in an acid fluid, we shall find that the ones at the head of the list are positive to those below them, but negative to those above them, if the test have reference to the condition of the parts within the fluid. On the contrary, we shall find that any member of the list will be found to be negative to any one below it, or positive to any above it, if tested from the portion NOT immersed in the acid fluid.

Fig. 1.

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Fig. 2.

§ 9. A very simple experiment will make this quite clear. Two strips, one of copper and the other of zinc, 1" wide by 4" long, have a 12" length of copper wire soldered to one extremity of each. A small flat piece of cork, about 1" long by 1" square section, is placed between the two plates, at the end where the wires have been soldered, this portion being then lashed together by a few turns of waxed string. (The plates should not touch each other at any point.) If this combination (which constitutes a very primitive galvanic couple) be immersed in a tumbler three-parts filled with water, rendered just sour by the addition of a few drops of sulphuric or hydrochloric acid, we shall get a manifestation of electrical effects. If a delicately poised magnetic needle be allowed to take up its natural position of north and south, and then the wires proceeding from the two metal strips twisted in contact, so as to be parallel to and over the needle, as shown in Fig. 1, the needle will be impelled out of its normal position, and be deflected more or less out of the line of the wire. If the needle be again allowed to come to rest N. and S. (the battery or couple having been removed), and then the tumbler be held close over the needle, as in Fig. 2, so that the needle points from the copper to the zinc strip, the needle will be again impelled or deflected out of its natural position, but in this case in the opposite direction.

§ 10. It is a well-known fact that if a wire, or any other conductor, along which the electric undulation (or, as is usually said, the electric current) is passing, be brought over and parallel to a suspended magnetic needle, pointing north and south, the needle is immediately deflected from this north and south position, and assumes a new direction, more or less east and west, according to the amplitude of the current and the nearness of the conductor to the needle. Moreover, the direction in which the north pole of the needle is impelled is found to be dependent upon the direction in which the electric waves (or current) enter the conducting body or wire. The law which regulates the direction of these deflections, and which is known, from the name of its originator, as Ampère's law, is briefly as follows:-

§ 11. "If a current be caused to flow over and parallel to a freely suspended magnetic needle, previously pointing north and south, the north pole will be impelled to the LEFT of the entering current. If, on the contrary, the wire, or conductor, be placed below the needle, the deflection will, under similar circumstances, be in the opposite direction, viz.: the north pole will be impelled to the RIGHT of the entering current." In both these cases the observer is supposed to be looking along the needle, with its N. seeking pole pointing at him.

§ 12. From a consideration of the above law, in connection with the experiments performed at § 9, it will be evident that inside the tumbler the zinc is positive to the copper strip; while, viewed from the outside conductor, the copper is positive to the zinc strip.[3]

§ 13. A property of current electricity, which is the fundamental basis of electric bell-ringing, is that of conferring upon iron and steel the power of attracting iron and similar bodies, or, as it is usually said, of rendering iron magnetic. If a soft iron rod, say about 4" long by ?" diameter, be wound evenly from end to end with three or four layers of cotton-covered copper wire, say No. 20 gauge, and placed in proximity to a few iron nails, etc., no attractive power will be evinced; but let the two free ends of the wire be placed in metallic contact with the wires leading from the simple battery described at § 9, and it will be found that the iron has become powerfully magnetic, capable of sustaining several ounces weight of iron and steel, so long as the wires from the battery are in contact with the wire encircling the iron; or, in other words, "the soft iron is a magnet, so long as an electric current flows round it." If contact between the battery wires and the coiled wires be broken, the iron loses all magnetic power, and the nails, etc., drop off immediately. A piece of soft iron thus coiled with covered or "insulated" wire, no matter what its shape may be, is termed an "electro-magnet." Their chief peculiarities, as compared with the ordinary permanent steel magnets or lodestones, are, first, their great attractive and sustaining power; secondly, the rapidity, nay, instantaneity, with which they lose all attractive force on the cessation of the electric flow around them. It is on these two properties that their usefulness in bell-ringing depends.

§ 14. If, instead of using a soft iron bar in the above experiment, we had substituted one of hard iron, or steel, we should have found two remarkable differences in the results. In the first place, the bar would have been found to retain its magnetism instead of losing it immediately on contact with the battery being broken; and, in the second place, the attractive power elicited would have been much less than in the case of soft iron. It is therefore of the highest importance, in all cases where rapid and powerful magnetisation is desired, that the cores of the electro-magnets should be of the very softest iron. Long annealing and gradual cooling conduce greatly to the softness of iron.

Fig. 3.

Magnets, showing Lines of Force.

§ 15. There is yet another source of electricity which must be noticed here, as it has already found application in some forms of electric bells and signalling, and which promises to enter into more extended use. If we sprinkle some iron filings over a bar magnet, or a horse-shoe magnet, we shall find that the filings arrange themselves in a definite position along the lines of greatest attractive force; or, as scientists usually say, the iron filings arrange themselves in the direction of the lines of force. The entire space acted on by the magnet is usually known as its "field." Fig. 3 gives an idea of the distribution of the iron filings, and also of the general direction of the lines of force. It is found that if a body be moved before the poles of a magnet in such a direction as to cut the lines of force, electricity is excited in that body, and also around the magnet. The ordinary magneto-electric machines of the shops are illustrations of the application of this property of magnets. They consist essentially in a horse-shoe magnet, in front of which is caused to rotate, by means of appropriate gearing, or wheel and band, an iron bobbin, or pair of bobbins, coiled with wire. The ends of the wire on the bobbins are brought out and fastened to insulated portions of the spindle, and revolve with it. Two springs press against the spindle, and pick up the current generated by the motion of the iron bobbins before the poles of the magnet. It is quite indifferent whether we use permanent steel magnets or electro-magnets to produce this effect. If we use the latter, and more especially if we cause a portion of the current set up to circulate round the electro-magnet to maintain its power, we designate the apparatus by the name of Dynamo.

Fig. 4.

Typical Dynamo, showing essential portions.

§ 16. Our space will not permit of a very extended description of the dynamo, but the following brief outline of its constructive details will be found useful to the student. A mass of soft iron (shape immaterial) is wound with many turns of insulated copper wire, in such a manner that, were an electrical current sent along the wire, the mass of iron would become strongly north at one extremity, and south at the other. As prolongations of the electro-magnet thus produced are affixed two masses of iron facing one another, and so fashioned or bored out as to allow a ring, or cylinder of soft iron, to rotate between them. This cylinder, or ring of iron, is also wound with insulated wire, two or more ends of which are brought out in a line with the spindle on which it rotates, and fastened down to as many insulated sections of brass cylinder placed around the circumference of the spindle. Two metallic springs, connected to binding screws which form the "terminals" of the machine, serve to collect the electrical wave set up by the rotation of the coiled cylinder (or "armature") before the poles of the electro-magnet. The annexed cut (Fig. 4) will assist the student in getting a clear idea of the essential portions in a dynamo:-E is the mass of wrought iron wound with insulated wire, and known as the field-magnet. N and S are cast-iron prolongations of the same, and are usually bolted to the field-magnet. When current is passing these become powerfully magnetic. A is the rotating iron ring, or cylinder, known as the armature, which is also wound with insulated wire, B, the ends of which are brought out and connected to the insulated brass segments known as the commutator, C. Upon this commutator press the two springs D and D', known as the brushes, which serve to collect the electricity set up by the rotation of the armature. These brushes are in electrical connection with the two terminals of the machine F F', whence the electric current is transmitted where required; the latter being also connected with the wire encircling the field-magnet, E.

When the iron mass stands in the direction of the earth's magnetic meridian, even if it have not previously acquired a little magnetism from the hammering, etc., to which it was subjected during fitting, it becomes weakly magnetic. On causing the armature to rotate by connecting up the pulley at the back of the shaft (not shown in cut) with any source of power, a very small current is set up in the wires of the armature, due to the weak magnetism of the iron mass of the field-magnet. As this current (or a portion of it) is caused to circulate around this iron mass, through the coils of wire surrounding the field-magnet, this latter becomes more powerfully magnetic (§ 13), and, being more magnetically active, sets up a more powerful electrical disturbance in the armature.

This increased electrical activity in the armature increases the magnetism of this field-magnet as before, and this again reacts on the armature; and these cumulative effects rapidly increase, until a limit is reached, dependent partly on the speed of rotation, partly on the magnetic saturation of the iron of which the dynamo is built up, and partly on the amount of resistance in the circuit.

[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.

[3] From some recent investigations, it would appear that what we usually term the negative is really the point at which the undulation takes its rise.

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