"That every substance and every atom of the world is pervaded by a peculiar, subtle, imponderable fluid which is termed Electricity, but which is not known to exist, or remains in a state of electrical equilibrium, until evoked by certain causes."

The effect of causing a disturbance of this equilibrium is to increase the normal, or natural, electricity in some particles, and to equally decrease it in other particles, i.e. what one loses the other gains. An excess of natural electricity is denoted by the term positive, or mathematical symbol (+) while a deficiency is denoted by the term negative, or symbol (-).

Like electricities repel each other.

That is to say, two bodies charged with an excess of, or positive, electricity, being brought together repel each other, neither wishing to increase the excess that has been evoked in them.

Similarly in the case of two bodies charged with a deficiency of, or negative, electricity, neither wish to add to the deficiency already there.

In both these cases there can be no tendency to electrical equilibrium, which is the principle at work. In the former case, there being already too much, more will but increase the disturbance.

In the latter case, further deficiency will but add to the irregularity.

Unlike electricities attract each other.

That is to say, if two bodies, one charged with positive, or having an excess of electricity, the other charged with negative, or having a deficiency of electricity, be brought together, they will attract each other; both being desirous of altering their existing state, the one by decreasing its excess, and the other by decreasing its deficiency of electricity.

In this case, there will be a tendency to equilibrium, caused by attraction. The earth is supposed to be a vast reservoir of electricity, from which a quantity can be drawn to fill up a deficiency, and which is always ready to receive an excess from other bodies. Every body in nature has its own natural quantity of electricity, and when an object is negatively electrified, or has a deficiency in its normal quantity, there is a tendency to receive a supply from any convenient source. Such an object would receive electricity from the earth if means were afforded; and a body positively electrified, would tend to part with its excess in the same manner. Where such facilities for establishing electrical equilibrium are afforded, the result is the passage of a current of electricity.

Conductors.—Sensible effects can be produced by electricity at great distances from the source, provided there be a medium of communication, that is, good conductors to transfer it. When a glass rod is rubbed with a piece of silk, it becomes charged with an excess of, or positive, electricity, and at the same time the silk becomes charged with negative electricity.

The glass rod will retain the positive electricity upon it for some time, unless touched with the wet hand, a wet cloth, a metal, &c., when it will instantly cease to be electrified. The electricity is then said to have been conducted away, and the bodies which allow it to run off the glass are called conductors of electricity. Metals, water, the human body, charcoal, damp wood, and many other bodies are conductors.

Those bodies which conduct electricity hardly at all, such as the air, silk, glass, sealing wax, gutta percha, india rubber, &c., are termed nonconductors or insulators.

Strictly speaking, all substances conduct electricity in some degree, and a nonconductor is merely a bad conductor.

In the following table the bodies are arranged in their order of conductivity, i.e. each substance conducts better than that which precedes it; the first-named body is the best insulator, and the last-named one is the best conductor.

• Dry air.
• Ebonite.
• Paraffin.
• Shellac.
• India rubber.
• Gutta percha.
• Resin.
• Sulphur.
• Sealing wax.
• Glass.
• Silk.
• Wool.
• Dry paper.
• Porcelain.
• Dry wood.
• Stone.
• Pure water.
• Rarefied air.
• Sea water.
• Saline solutions.
• Acids.
• Charcoal, or Coke.
• Mercury.
• Lead.
• Tin.
• Iron.
• Platinum.
• Zinc.
• Gold.
• Copper.
• Silver.

Though two substances are near one another in the above list, they do not necessarily approach one another in their power of conducting. For instance, taking the conducting power of pure silver as represented by the number 100, then

The metals being the best known conductors, are usually employed as the means of transferring the electric current from one place to another.

Electric Circuit.—The conditions attending this operation are different from those of any other known method of transmission.

A complete circuit must always be formed by the electric current, i.e. it cannot start from one place A, travel to another place B, and cease there, but the current must be completed before it can be said to have reached B. There cannot be a current of electricity without a means of recombination, which recombination must be at the source, or place of original disturbance.

This "place of disturbance" or source must be considered as having two sides, i.e. at some spot the normal or natural electrical equilibrium is disturbed, and electricity is separated into too much (positive) on one side, and too little (negative) on the other side. If then no means of recombination be afforded, the electricities remain separated, and no current exists; but if a conductor be made to connect the two sides, electricity is set in motion, and a current established. Originally to form a circuit between two stations A and B, a conducting wire and a return wire were necessary, but in 1837 Steinway discovered that the earth itself answered all the purposes of a return wire, in fact under favourable conditions much better. Thus, to form a circuit between A and B, a conducting wire is required, and a buried metal plate at A and B, the earth by these means taking the place of the return wire.

The aforesaid metal plates are technically termed earth plates. The greater the size of the earth plates (up to certain limits), the deeper they are buried, and the better the conducting power of the soil surrounding them, the better conductors the plates become, or the less resistance the earth portion of the circuit offers. If either plate be not in communication with the earth, or else be separated from the wire, the circuit is not complete, or, as it is termed, "it is broken," and no current will flow, the signal not made, torpedo not fired, &c.

"Short" Circuit.—Due to the fact that recombination, or a tendency to equilibrium, is always at work when electricity has been evoked, the conducting path along which the electric current flows must be covered with a nonconducting substance, or, as it termed, "insulated," or else the current would not perform its duty, but escape to earth, and so form what is termed a "short circuit."

A current of electricity always chooses the easiest path to effect recombination, or electrical equilibrium.

Insulators, &c.—On land, telegraph wires are as a rule laid above the ground, and therefore require supporting at every few yards; this is done by means of posts, and as these are formed of substances which are conductors of electricity, the wires require to be insulated from them. The insulators generally employed for such purposes are cup-shaped pieces of porcelain, or pottery, fixed to the head of the telegraph posts. By means of these insulators, the current of electricity is prevented from escaping to the earth by the post conductors.

A certain amount of leakage, or loss of electricity, must occur at each of these posts, as there is no such thing as a perfect insulator. When the wires are laid on the ground or under ground, or under water, they are insulated by covering them with gutta percha, india rubber, &c., and any loss of current is thus prevented.

Methods of generating Electricity.—For the purposes of torpedo warfare there are two methods of evoking electricity, viz.—

By Chemical Action.—Chemical action is the chief source of free electricity, the representative of which is the galvanic, or Voltaic, battery.

The electricity so generated is also termed dynamical electricity, due to there being a constant electric current, so long as the poles of the battery producing it are kept closed; the electricity being thus in a dynamic or moving state.

By chemical action is signified that which occurs when two or more substances so act upon one another as to produce a third substance differing altogether from the original ones in its properties, or when one substance is brought under such conditions that it forms two or more bodies differing from the original ones in their properties.

Definition and Properties of a Voltaic Cell.—The Voltaic cell consists of an insulating jar, containing a liquid, in which are placed two plates or pieces of dissimilar metals; the liquid must be composed of two or more chemical elements, one of which at least tends to combine with one or other of the metals, or with both in different degrees.

By a Voltaic battery is meant a number of cells above one; this term, however, is often applied to a single cell when working by itself.

A "simple Voltaic cell," "element," or "couple," consists of two metals placed in a conducting liquid. If two metals—for instance, zinc and copper—are placed in water slightly acidulated, without touching each other, no effect is apparent; but if they be made to touch, bubbles of hydrogen gas are formed over the copper plate, and continue forming these until the plates are separated. After being in contact for some time, the copper plate will be found unaltered in weight, but the zinc plate will have lost weight, and the portion so lost will be found in the liquid in the form of sulphate of zinc. The same effects are also produced by connecting the two plates by means of some conducting substance, instead of placing them in contact.

Zinc is invariably employed as one of the metal plates, on account of the ease with which it dissolves in dilute acids; and the greatest results are obtained when the second metal plate is not acted upon at all by the liquid, for then the whole effect due to the oxidation of the zinc plate is obtained; but when the second plate is also chemically acted upon, then only the effect due to the difference between the two chemical actions is obtained, for, as will be explained further on, they each act in directly opposite directions.

Voltaic Current.—The Voltaic current makes its appearance under the general laws of electrical action.

When a body charged with an excess of, or positive, electricity, is connected with the earth, electricity is transferred from the charged body to the earth; and similarly when a body is charged with a deficiency of, or negative, electricity, is connected with the earth, electricity is transferred from the earth to the body.

Generally whenever two conductors in different electrical conditions are put in contact, electricity will flow from one to the other. That which determines the direction of the transfer is the relative potential of the two conductors. Electricity always flows from a body at higher potential to one at lower potential, when the two are in contact, or connected by a conductor. When no transfer of electricity takes place under these conditions, the bodies are said to be at the same potential, which may be either high or low. The potential of the earth is assumed to be zero.

Definition of Potential.—"The potential of a body or point, is the difference between the potential of the body or point, and the potential of the earth."

Difference of potential for electricity is analogous to difference of level for water. Now, since, when a metal is placed in a vessel containing a liquid, electricity is produced, the liquid becomes of a different potential to the metal, each being electrified in an opposite way; and therefore, as above stated, there being a difference of potentials, electricity will tend to flow from one to the other.

This is evidence of a force being in action, for there can be no motion without some force to produce it.

Electro-motive Force.—Electro-motive force is the name given to a peculiar force to which is due the property of producing a difference of potentials. When it is said that zinc and water produce a definite electro-motive force, what is meant is, that by their contact a certain definite difference of potentials is produced.

The electro-motive force of a Voltaic element may be termed its working power, in the same way as the pressure of steam is the working power of a steam engine, though this is not to be considered as the real source of power, which, as will be seen, is uncertain. Due to the difference of potential of the metal and the liquid, a current of electricity will flow from one to the other, causing the chemical decomposition of the liquid, and the reaction may be taken as the origin of the power employed.

But while the expenditure of energy (which is necessary to produce a force) is accounted for by taking the chemical action as the source of power, the preceding cause of this chemical action, viz. the flowing of the current of electricity due to the difference of potential of the metal and the liquid, must also have first involved the expenditure of energy; thus the real source of power is very uncertain.

Electrolytes.—As before stated, a Voltaic cell consists of two plates of dissimilar metals, which must be immersed in a liquid composed of two or more chemical elements, one of which at least will combine with one or other of the metals, or both in a different degree. Those liquids which are thus decomposed by the passage of a current of electricity are termed electrolytes.

The elements, then, forming the electrolyte may have chemical affinity for both metals, though in a greater degree for one than the other.

"Oxygen" is the most important element of an electrolyte, and to the affinity for oxygen of the metals is the magnitude of the result and effect.

Terms Electro-positive and Electro-negative.—All metals have a definite relation to each other as to the potential which any one may have when brought into contact with another. Thus, when zinc is brought into contact with copper, the former has a potential positive to the latter, i.e. a current of electricity will tend to flow from the zinc to the copper. The metals may be so placed in a list that each one would be positive to any of those that follow it; it is then said to be electro-positive to them, and they are electro-negative to it. As those metals which are electro-positive to others have a greater affinity for oxygen, and those that are electro-negative to others a less affinity for this element, the terms electro-positive and electro-negative signify, in effect, greater or less affinity for this element. Conversely, oxygen will combine more readily with the former than with the latter.

The following list shows the commoner metals arranged in electro-chemical order.

Take the case of a Voltaic cell composed of zinc and copper plates immersed in water.

The passage of electricity through the water will decompose it into its elements hydrogen and oxygen, the latter having an affinity for both the plates, but considerably more so for the zinc plate.

Then, an electro-motive force will be generated at each metal, and these forces will act in opposition to each other, but the greater strength of the one will overcome the weaker, and the real power of the electric current will be the difference between the two.

Definition of "Elements."—The battery plates are termed the positive and negative elements. A Voltaic battery has two poles—a positive and a negative—which are the terminations of the plates.

Direction of Current.—The course of the current in a Voltaic cell is as follows:—Within it leaves the electro-positive plate (or element), and flows to the electro-negative plate, but outside the cell (or as it were on its return path) it flows from the positive pole to the negative pole. The current always leaves the battery by the positive pole, and thus the copper is the negative element, but the positive pole, because the current leaves the battery by it; and the zinc is the positive element because the current begins there, within the cell, and the negative pole because it ends there, outside.

The positive pole is the terminal of the negative plate, and vice versÂ. There is but one current from a battery, viz. a positive one; what is called a negative current is merely the positive current passing in the reverse direction from the same pole, that is, the positive pole.

Single and Double Fluid Batteries.—Galvanic batteries may be divided into single fluid and double fluid batteries. The simplest form of galvanic cell practically in use is a single fluid cell, consisting of a zinc and a copper element, immersed in water slightly acidulated by the addition of a little sulphuric acid. In a battery of several cells, the zinc and copper plates are generally soldered together in pairs, and placed in a long stoneware or glass trough, divided into separate cells by means of partitions. By filling the cells with sand, this battery is made more portable, the plates being thus supported, and the liquid prevented from splashing about during transit.

In this form it is called the common sand battery.

Action in a Single Fluid Cell.—The following process goes on in the single fluid cell when the circuit is closed—that is, when the battery is set to work.

The water (composed of hydrogen and oxygen) is decomposed by the passage of the electric current, and oxide of zinc is formed. The oxygen of the water having greater affinity for the zinc, leaves the hydrogen. The zinc during the process is being consumed, as coal is consumed when it burns, while combining with the oxygen of the air. This oxide of zinc combines with the sulphuric acid, and forms sulphate of zinc; this salt is found to accumulate in solution in the liquid of the cell. At the same time the hydrogen of the water goes to the negative or copper plate, and gathers over it in bubbles.

The process will be better seen by the accompanying plan of the chemical decomposition and recombinations.

No single fluid cell can give a constant electro-motive force because of the polarisation of the plates.

Definition of the term Polarisation.—The word polarisation means that the plates become coated with the products of the decomposition of the electrolyte, producing a diminution of current. In the above described battery, the hydrogen gathers on the surface of the copper plate, and an electro-motive force is set up which counteracts the electro-motive force producing the current—the copper plate is said to be polarised. By the bubbles of hydrogen collecting on the face of the negative plate, the surface in contact with the liquid is gradually decreased; thus the plate becomes practically smaller, and a single fluid cell which at starting gave a good current soon shows that it is really weakened. The consequence is that the zinc is consumed extravagantly as well as the acid, and the cell working with poor results. Also the resistance of the cell is increased, due to the sulphuric acid, which is added to the water to increase its conductivity, being gradually used up, by combining with the oxide (see plan) and forming sulphate of zinc. Liquids are very bad conductors of electricity; the greater part of the ordinary internal resistance of a battery arises from this cause. The common sand battery is the worst of all batteries as regards constancy of electro-motive force, the polarisation being greater in this battery than any other because the gas cannot readily escape. The common copper and zinc cell is the next in order of demerit. The Smee single fluid cell, in which the negative plate is a platinum instead of a copper one, is better than the copper zinc cell, because the free hydrogen does not stick to the rough surface of the platinum plate so much as to the copper.

Double Fluid Batteries.—All the defects of the single fluid battery, which are as follows—

The Chemical Action of a Daniell Cell.—The chemical action of this form of Daniell cell is as follows:—

The zinc electrode combines with oxygen; the oxide thus formed combines with sulphuric acid and forms sulphate of zinc. Oxide of copper is separate from the sulphate; and the copper in this oxide is separated from the oxygen. The oxygen of the water is separated at the zinc electrode from the hydrogen, and at the other electrode this hydrogen recombines with the oxygen from the oxide of copper. This alternate decomposition and recombination of the elements of water can neither increase nor decrease the E.M.F. of the cell, the actions being equal and opposite. The result of the series of actions above described is that the sulphuric acid and oxygen of the sulphate of zinc are transmitted to the zinc, combine with it, and form fresh sulphate of zinc; the sulphuric acid and oxygen of the sulphate of copper are transmitted to the zinc set free by the above process, and reconvert it into sulphate of zinc; the copper of the sulphate of copper is transmitted to the copper electrode, and remains adhering to it. The whole result is therefore the substitution of a certain quantity of sulphate of zinc for an equivalent quantity of sulphate of copper, together with a deposition of copper on the copper or negative electrode.[X] The following is a plan of the process:—

Description of the "Callaud" and "MariÉ-Davy" Batteries.—The Voltaic batteries in general use for the different purposes of torpedo warfare have been fully described in Chapter IV., and therefore it will be only necessary here to explain the construction of the "Callaud" and "MariÉ-Davy" batteries, these being much used abroad in connection with telegraphy.

The Callaud cell, named from the inventor, is a modification of the Daniell cell, and is also called a gravity battery, the liquids being simply prevented from mixing by the law of gravity forbidding the heavier of the two from rising through the lighter. It consists of a thin plate of copper, which is laid on the bottom of a good insulating jar having an insulated wire leading up the side, and on this plate are placed crystals of sulphate of copper. A solution of sulphate of zinc is then poured in, and on the top is fitted a zinc plate, which forms the positive element. The vessel must not be shaken, or the sulphate of copper when dissolving will mix with the solution above it.

The MariÉ-Davy cell consists of a carbon electrode in a paste of proto-sulphate of mercury and water contained in a porous pot, and a zinc electrode in dilute sulphuric acid, or in sulphate of zinc.

The Circuit.—In connection with the manipulation of batteries, there is one important item to consider, viz. the resistance in the circuit, which may be divided into external and internal.

Resistances.—The external resistance in practice is that which exists in the conducting line, and the various instruments connected with it.

The internal resistance is that which exists in the battery itself. All known conductors oppose a sensible resistance to the passage of an electric current, and the strength of the current, or in other words, the quantity of electricity passing per second from one point to another, when a constant difference of potentials is maintained between them, depends on the resistance of the wire on the conductor joining them. A bad conductor does not let the electricity pass so rapidly as a good conductor, that is, it offers more resistance.

Resistance in a wire of constant section and material is directly proportional to the length, and inversely proportional to the area of the cross section.

The electrical resistance of a conductor must not be considered as analogous to mechanical resistance, such as the friction which water experiences in passing through a pipe, for this frictional resistance is not constant when different quantities of water are being forced through the pipe, whereas electrical resistance is constant whatever quantity of electricity be forced through the conductor.

Application of Ohm's Law.—Ohm's law, which governs the strength of the current, is expressed by the equation

• Where C is the strength of the current;
• E is the E.M.F. or difference of potentials;
• and R is the resistance of the circuit.

In words, Ohm's law means that the strength of the current is directly proportional to the E.M.F., and inversely proportional to the resistance of the circuit.

As before stated, the resistance of the circuit consists of an external and an internal resistance, therefore when these resistances are separately considered, the equation C = E / R must be converted into C = E / (x + r), where x is the external, and r the internal, resistance.

The resistance of the battery or the internal resistance depends on the size of the plates and the distance between them, that is, it is directly proportional to the distance, and inversely proportional to the size.

The electro-motive force of a battery is dependent generally on the number of cells joined in series, and not on the size of the plates. The cells of a battery may be joined in two ways, as follows:—

If the conductor between the battery poles be such that the external resistance x may be practically left out, then C = E / r, and no change in the strength of the current will be effected by adding any number of cells in series, as r will increase equally with E, and therefore C will remain the same; but if under the same conditions the cells be joined in multiple arc, then r will decrease as E increases, and therefore C will be increased.

Thus with a short circuit of small external resistance, the strength of the current will be increased by increasing the size of the plates, or by joining the cells in multiple arc, but not in series.

If the conductor between the poles of the battery be such that the external resistance x becomes very great, then C = E / (x + r), where x is very great compared to r. By joining the cells in multiple arc r is decreased, but E and x remain the same, and therefore C is not materially altered, as x is very great compared to r. By connecting the cell in series, r is increased, and so is E, but as r is still very small compared to x, the strength of the current C is increased.

Thus with a long circuit of great external resistance, the strength of the current will be increased by joining the cells in series, but not in multiple arc.

When the external resistance x is neither very large nor very small in comparison with the battery or internal resistance r, then the strength of the current C will be increased by adding the cells in series, and also in multiple arc. By the former process the E.M.F. E is increased more than the resistance of the circuit R or (x + r), and by the latter process, the E.M.F. E is unaltered, whilst the circuit resistance (x + r) is decreased. All the above may be practically demonstrated by the employment of suitable galvanometers.

Frictional Electricity.—Frictional electricity is produced by the friction of two insulators. There is no difference whatever in kind between "Voltaic" and "frictional" electricity.

Comparison with Voltaic Electricity.—The electricity generated by friction possesses a great electro-motive force, producing on even a small conductor a large charge, whereas the electricity generated by the galvanic cell possesses a very small electro-motive force, and produces only a small charge on a small conductor. But when the conductor is large, the electricity produced by the galvanic cell will almost instantaneously charge the conductor to the maximum potential it can produce, the galvanic cell developing an immense quantity of electricity by the chemical reaction; whereas the quantity developed by friction between two insulators is so small, that if it be diffused over a large conductor the potential of the conductor will be very little increased.

The late Professor Faraday has proved that one cell of a Voltaic pile possesses the same quantity of electricity as an ordinary sized frictional machine after being wound round 800,000 times, thus showing the contrast between the qualities of frictional and Voltaic electricity.

The electricity of the frictional machine and that of the galvanic battery may be made to produce the same effect, there being no difference in kind between them. Frictional electricity can be made to pass in a current, but it is comparatively feeble. Again, Voltaic electricity can be made to produce a spark, but under ordinary circumstances it scarcely amounts to anything.

Description of a Frictional Electric Machine.—A frictional electrical machine consists of a vulcanite or glass disc or cylinder, which is made to revolve between cushions or rubbers of leather or silk. By the friction the (silk) rubbers become negatively, and the glass disc or cylinder positively, electrified. The revolving disc immediately after contact with the fixed rubbers passes close by a series of brass points, which are connected with a condenser. These points collect the positive electricity of the glass, the rubbers being put to earth. The positive electricity which the glass loses is supplied through the rubber; a stream of negative electricity flows from the rubbers to the earth during the charging of the conductor or condenser; in other words, the positive electricity flows from the earth to the rubber, whence it crosses to the glass disc and so to the condenser.

Definition of a "Condenser."—A condenser is an arrangement for accumulating a large quantity of electricity on a comparatively small surface.

The "Leyden Jar."—The Leyden jar, which is the original type of the condenser, or accumulator, consists of a glass jar coated inside and out, up to within a few inches of the mouth, with tinfoil pasted on, but having no connection with each other. The mouth is usually closed by means of a wooden stopper, through which a brass rod passes, to the head of which is affixed a brass knob, &c., the rod and knob being metallically connected with the inner coating by means of a chain.

The "Leyden jar" may be charged either by connecting the outer coating to earth (the rubbers of the machine being also to earth), and the inner coating to the conductor of the machine; or else by connecting the outer coating to the rubbers, and the inner coating to the conductor, a complete circuit being necessary to charge the jar as highly as the frictional electrical machine will admit of.

The conductor of the machine being charged, also forms a kind of Leyden jar, the conductor in this case being the inner coating, the air, the dielectric, and the nearest surrounding conductors, such as the walls of the room, &c., being the outer coating.

Meaning of "Dielectric."—By dielectric is meant a non-conducting medium, which in the case of the "Leyden jar" is the glass.

Frictional Electricity very little used for Torpedo Purposes.—Frictional electricity is now seldom used in connection with torpedo warfare, as on account of its very great power, or electro-motive force, a very perfectly insulated cable must be employed, which is somewhat difficult to obtain; it is also necessary to employ a condenser, which requires a certain time to charge. For these and other reasons, frictional electricity has been abandoned for the far more practical Voltaic electricity.

Magnetism.—A magnet is a piece of steel, which has the peculiar property, among others, of attracting iron to its ends.

Certain kinds of iron ore, termed the loadstone, have the same properties. The word "magnet" is taken from the country Magnesia, where the loadstone was first discovered.

Magnetism in a body is considered to be a peculiar condition caused by electrical action. Both electricity and magnetism have the power of communicating their properties to other bodies without being in contact with them, i.e. inducing the power, which on the bodies being placed far apart becomes insensible.

The "Poles" of a Magnet.—Every magnet has two poles, called the north and south poles. A magnetic steel needle if pivoted on an upright point, or suspended from its centre, will fix itself, pointing north and south; in England the end of the needle pointing to the north is termed the north pole, but in France it is termed the south pole. The reason of this difference is owing to the fact that the north pole of one magnet attracts the south pole of another, and therefore, as the earth is considered as one vast magnet, the end of the magnetic needle attracted to the north pole of earth magnet should be the south pole of the magnet; thus the French south pole in a magnet is the English north pole, and vice versÂ.

Permanent Magnets.—A piece of steel when magnetised is termed a permanent magnet, because it retains its magnetism for a considerable length of time; but soft iron cannot be permanently magnetised.

A piece of soft iron rendered magnetic by induction retains a portion of its magnetism for some time after it has been removed from the magnetic field, by reason of what is called its coercive force. This remnant of magnetisation is called residual magnetism.

Effect of an Electrical Current on a Magnetic Needle.—A magnetic bar or needle pivoted on its centre will point north and south, but if an electric current is caused to flow along a wire parallel to and either over or under the magnetic needle, the latter will be turned from its position, and remain so as long as the current continues; on the current ceasing the needle will resume its original position.

The magnetic needle can be turned either to the east or the west, according to the direction and course of the electrical current.

Thus:—

• Current from S. to N. over deflects to W.
• Current from N. to S. under deflects to W.
• Current from N. to S. over deflects to E.
• Current from S. to N. under deflects to E.

The Galvanometer, the "Mirror," and "Thomson's reflector" all depend on this principle for their usefulness. These instruments have been fully described in Chapter IV.

The Electro-Magnet.—If a piece of insulated wire be coiled round a rod of soft iron, and a current of electricity be made to pass through the coil, the iron core becomes magnetic as long as the current passes; when the current ceases the magnetism disappears.

During the passage of the electric current, the iron core possesses all the properties of a magnet. Therefore if a piece of iron were placed near its poles it would be attracted and released from attraction as often as the current passed or ceased; and supposing such a piece of iron to be retained by a spring, &c., a series of movements, attraction, and drawing back would be effected.

A piece of iron so arranged is termed an armature, and the instrument is called an electro-magnet.

The coil of wire must be carefully insulated, or else the electric current will pass through the iron core to earth instead of performing its proper work.

An electro-magnet is much more powerful than a steel magnet of equal dimensions, and depends on the strength of the current by which the magnetism is induced, and the number of turns of wire round the core. The north and south poles of an electro-magnet are determined by the direction in which the current flows through the wire.

At the south pole the current passes with the hands of a watch, and at the north pole against the hands of a watch.

Definition of the "Ohm."—The "ohm" is the standard used for electrical resistance; it is obtained by observing what effect is produced by a current of electricity on a certain conductor in a certain time.

The ohm is a small coil of German silver wire representing the resistance overcome by a current in a certain time.

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