(846.) Voltame

ter.

(847.)

(848.) Rheostat of MM. Wheat

Jacobi.

The other method is by Dr Faraday's Voltameter. The amount of water decomposed is directly as the quantity of the current. The unit in this case is one cubic centimètre of gas produced from water in a minute.

These two measures agree. After being once compared, we may in all cases deduce the decomposing force of a current from its effect upon a tangent compass.

Mr Wheatstone has facilitated the measurement of voltaic effects by the invention of the rheostat, a simple contrivance for introducing any desired length stone and of wire into a circuit, and thus estimating resistances both of conductors and of batteries, and also the electro-motive forces of different batteries. The results appear to be extremely satisfactory. (Phil. Trans., 1843.) The conducting power of different metals drawn into wire will be inversely as the lengths required to be introduced into the circuit to reduce the strength of the current in a given proportion. The principle of the rheostat was independently applied to similar inquiries by M. Jacobi of St Petersburg, in 1840.

(849.) Farther deductions

theory.

The laws of Ohm farther proceed to expound the effect of the size and number of the elementary cells from Ohm's combined in a voltaic battery. The size of the plates increases the quantity of electricity which escapes through a short conductor, but has little effect upon a long current. On the other hand, the multiplication of elements produces no increase in the voltaic stream when the connecting wire is short and when it is also a good conductor, for the chief resistance in the circuit is in that case the battery itself, which resistance increases with the number of elements, just as the force which overcomes it increases. If, on the other hand, the chief resistance be extraneous to the battery, the addition of more elements increases the power without much increasing the resistance. All this scarcely requires mathematical proof. It is very evident, and very just, and it is borne out by experiment.

(850.)

Ohm's theory farther gives the partial effects of a current branching into various unequally good conductors, and into other details, particularly as to the electric tension in different parts of a circuit. It is, however, to be observed, that the whole is based on the assumption that the dissipation of electricity from the surface of the conductor is insensible.

(851.) It is only justice here to add, that the theory of Fechner's Ohm owes much, if not most, of its value to the exexperiperiments of Fechner in Germany; and that its rements.

ingenious and (in many cases) independent experiments of M. Pouillet and Mr Wheatstone.

John Fre

DANIELL'S Constant Battery.—This seems the pro- (852.) per place to mention an invention which has exer- deric Dacised a remarkable influence on the progress of prac- niell-the tical electricity—I mean the Constant Battery. I constant believe that the merit of this application is entirely battery. due to the late Professor Daniell, although the German writers (who manifest throughout a singular sensibility with regard to their national claims to electrical improvements) seem to claim it for their countrymen. Every battery previously constructed diminished rapidly in energy from the instant of being charged. This was chiefly due to two causes; first, to the acid becoming gradually charged with oxide of zinc; and, secondly, to the appearance of " nascent" hydrogen arising from the decomposition of water at the copper surface where it prevented effectual conduction of the electricity.. These sources of diminished effect were prevented in the following way:-Instead of a single cell containing one fluid moistening both the copper and the zinc, a double cell was formed by means of a partition of bladder or porous earthenware. The partition next the zinc was filled with dilute sulphuric acid; the partition next the copper with a solution of sulphate of copper, also acidulated. When galvanic action proceeds, both fluids are decomposed; but whilst that in the zinc cell becomes charged with oxide of zinc, it is at the same time continually acidulated by the electro-chemical transfer of acid from the decomposition of sulphate of copper in the copper cell; and the copper set free from the same combination in the form of oxide is metallically reduced by combining with the "nascent" hydrogen (the oxygen derived from the water decomposed having combined with the zinc), and the metallic copper is deposited in an ever fresh film on the surface of the copper plate. This beautiful invention was described in 1836. Many other batteries on the same principle have been since contrived and described; several are more powerful, but none perhaps are so constant in their action.

(854.)

stone on

the velo

tric con

duction.

adverted (32, &c.) The requirements of practice are magnificent experiments, such as no individual and no scientific Society would think of executing for the illustration of theory. It is not in the least my purpose to transfer to these two gentlemen an exclusive merit which they need not be unwilling to share with other energetic and able competitors in the hard-run race of scientific applications. They occupy, however, perhaps the most marked and distinguished place, and the field is so wide and includes so many minute details, that it requires all our resolution to fix our eyes steadily on the most considerable acquisitions—the nobler sheaves of so prolific a harvest.

I shall connect, then, with the name of Mr Wheatstone, (1), the apparatus for determining the velocity of electrical conduction, (2), the electric telegraph and clock,-with that of M. Jacobi, (3), electrodynamic machines, and (4), the electrotype. (855.) I. The apparatus used by Mr Wheatstone in 1834 Mr Wheat for measuring the velocity of the passage of the electrical impulse through a good insulated conductor city of elec- such as a copper wire, deserves particular notice from its great ingenuity, and from its general application to the measurement of short intervals of time. Let a copper conducting wire of half a mile long be so convoluted that the middle and the two ends of the wire may be brought near together, the whole being perfectly insulated. Let the wire be slightly interrupted at these three places, and the whole put into connection at pleasure with an electric machine or battery. When contact is made, three sparks will take place. Let the two end sparks be called A and C and the middle one B. As the three sparks take place close to each other, they can easily be seen at once reflected in a small plane mirror. Let now this small mirror be put in very rapid rotation round a horizontal axis so placed that the sparks (if they occur in the suitable part of the revolution) may be reflected together to the eye. Imagine the rotation to become immensely rapid:-in Mr Wheatstone's apparatus the velocity reached 800 times in a second; consequently the mirror described 1o in x 36th part of a 500 second; i. e., in 2006 of a second. But for 1° of rotation of a mirror the reflected image will describe an arc of 2°. Supposing then that all the sparks occur at the same absolute instant of time, they will be seen in one line (supposing the points of the interrupted circuit in a line), but if either spark occur later than the others by only of a second, the mirror will have revolved so much in the interval as to displace the image of that spark relatively to the others by the very palpable angular amount of 2o. In the copper wire half a mile long, the end sparks occurred simultaneously, whilst the middle spark occurred later by about one millionth of a second; giving a velocity of transmission (according

1

to Mr W.) of 288,000 miles a second, or somewhat greater than that of light. The velocity in an iron telegraph-wire, ascertained lately in America with much greater accuracy, and by a different method, is only 16,000 English miles a second; but doubts have been thrown upon the correct interpretation of these experiments. Those of M. Fizeau on the telegraphic lines of France give results more conformable to Mr Wheatstone's, namely, about 70,000 English miles per second for iron, and 120,000 for copper wire. The duration of a spark drawn immediately from the battery is insensible, but in Mr Wheatstone's experiment it lasted, of a second when transmitted by a copper wire half a mile long.

II. Electric Telegraph and Clocks.-The idea of (856.)

using the transmission of electricity to communicate telegraph signals is so obvious as hardly to deserve the name its early of an invention, the prodigious velocity of common history. electricity in wires having been established by Watson before the middle of the last century. The earliest proposal of the kind appears in the Scots Magazine for February 1753, where a correspondent from Renfrew, who signs himself C. M., proposes several kinds of telegraphs acting by the attractive power of electricity, conveyed by a series of parallel wires corresponding in number to the lezers of the alphabet, and insulated by supports of gloss jeweller's cement at every twenty yards. are to be spelt by the electricity attracting letters or by triking bells corresponding to letters. One Lege, in 1782, and even long before, proposed to convey twenty-four insulated wires in a subterranean tube, and to indicate the letters of the alphabet by means of the attraction of light bodies. In 1811 Sömmering suggested a similar application of voltaic electricity, chemical deco. position being the effect observed. Oersted first, and then Àpère (1820) suggested the use of magnetic deflections for the same purpose, which is nothing else than the needle telegraph in general use in England; but they contented themselves with the suggestion merely. MM. Gauss and Weber communicated signals at Göttingen in 1833 or 1834 to a considerable distance, and gave them the signification of letters. This was the first accomplishment of telegraphic communication by means of electricity, and it realized the fancy of Strada, quoted by Addison, of sympathetic magnets. It was, however, a mere appendage to a magnetic observatory, and its application and diffusion on a great scale seems to have required a distinct effort; for several years elapsed before we hear more of the telegraph.

VOL. I.

6 I

(858.)

(859.)

circuit.

for general adoption in a convenient form, is a matter which we need not here decide. The three independent inventors (I name them alphabetically) are Mr Morse of the United States, M. Steinheil of Munich, and Mr Wheatstone of London. The telegraph of the two last resembles in principle Oersted's and Gauss's; that of the first is entirely original, and consists in making a ribbon of paper move by clockwork, whilst interrupted marks are impressed upon it by a pen or stamp of some kind brought in contact with the ribbon by the attraction of a temporary magnet, which is excited by the circulation of the telegraphic current of electricity. In the telegraphs of MM. Wheatstone and Steinheil the needle moves only to the right or left; and by the combination of a certain number of right and left motions, either with one or with two independent needles acted on at once by distinct currents, the alphabet is easily, though somewhat tediously constructed. Such, however, is the dexterity which practice gives, that forty or even more of such complex signals are transmitted and registered per minute.1

It has already been said that we claim the exclusive invention of the electric telegraph for no one individual. But of the several inventors none probably has shown such perseverance and skill in overcoming difficulties as Mr Wheatstone. His telegraph accordingly was in general use in England before M. Steinheil was able to obtain a similar success in Germany. The telegraphs of Mr Morse are naturally preferred in America, and they have this inestimable advantage, that they preserve a permanent record of the despatches which they convey.

There is one circumstance connected with the elecThe earth- tric telegraph deserving of particular notice-I mean the apparently infinite conducting power of the earth when made to act as the vehicle of the return current. Setting all theory aside, it is an unquestionable fact, that if a telegraphic communication be made, suppose from London to Brighton, by means of a wire going thither, passing through a galvanometer, and then returning, the force of the current shown by the galvanometer at Brighton will be almost exactly doubled, if, instead of the return wire, we establish a good communication between the end of the conducting wire and the mass of the earth at Brighton. The whole resistance of the return wire is at once dispensed with! This fact was more than suspected by the ingenious M. Steinheil in 1838; but, from some cause or other, it obtained little publicity; nor does the author appear to have exerted himself to remove the reasonable prejudices

with which so singular a paradox was naturally received. A most ingenious artist, Mr Bain, established for himself the principle, and proclaimed its application somewhat later; and in 1843, perhaps the first entirely convincing experiments were made by M. Matteucci at Pisa. From this time the double wire required to move the needle telegraph was reduced to a single one. The explanation of this curious fact appears to be,—not that the electricity is conducted back by the earth to its origin at the battery,-but that the molecular disturbance polarly communicated along the conducting wire to its farther end being effectually relieved by perfect communication with a vast reservoir of neutral electricity like the earth, conduction proceeds in an uninterrupted manner, and to an unlimited extent.

Of submarine telegraphs, it is sufficient to state (860.) that the isolation is obtained by inserting the con- Submarine ducting wires in a mass of gutta percha, and that telegraph. the first on a considerable scale was sunk between Dover and Cape Gris Nez, on the French coast, in August 1851.

The applications of electricity to the measurement (861.) of time are so numerous, that I can only refer generally to the principal contrivances.

Electric

1. The simple electric clock of Mr Bain derives its (862.) maintaining power from two large plates of copper clock. and zinc (or more simply zinc and charcoal) sunk in the earth, which affords for a very long time a continuous supply of voltaic electricity. The current is conveyed into the bob of the pendulum, where it traverses a long coil of wire; and as the pendulum oscillates, the current (by a simple shifting contrivance) is reversed at each vibration. A stationary bar-magnet is placed so that when the pendulum moves, the voltaic coil of the bob embraces the magnet, and the direction of the current is such as by the electro-magnetic reaction to strengthen and maintain the vibratory movement, which is by this means perpetuated.

tic clocks.

2. Sympathetic clocks.-By means similar to those (863.) just explained, one standard clock anyhow regulated Sympathemay, by means of magneto-electric currents, convey absolutely isochronous movements to any number of affiliated clocks at any distance. Probably the first application of the kind was made by M. Steinheil.

3. American electric-registration clocks.-Mr (864.) Locke proposed to register the instant of an event American occurring in the following way: A ribbon of electricregistrapaper being put in uniform motion, as in Morse's tion clocks. telegraph, a dot is imprinted on it every second by

(865.) Chronoscopes.

(866.) Electro

as a prime

mover.

means of a simple connection with an astronomical clock. A separate marking apparatus under the control of the experimenter enables him to interpolate a dot or mark corresponding to the instant of any event happening-such, for instance, as the transit of a star. This method has the great advantage of leaving the observer at entire liberty to watch the object without having to attend to the beats of the clock, whilst it renders mistakes next to impossible. It has been successfully applied to the determination of longitudes, and more recently to all kinds of astronomical observations, by Mr Bond in America, and by Mr Airy at Greenwich (231).

4. Chronoscopes, or instruments for the measurement of excessively short intervals of time, such as the flight of military projectiles, and even the transmission of sensation and motion along nervous fibres. Such instruments have been constructed on a great variety of principles. Those of M. Pouillet, Mr Wheatstone, and Mr Siemens, deserve especial

mention.

III. Of electro-magnetism used as a moving power, we need say little. No one can witness the magnetism astonishing experiment of the sudden creation of magnetic power sufficient to sustain one or two tons by the voltaic dissolution of a few grains of zinc, without having the idea suggested of a continuous moving force. This enormous power is, however, exerted through a space so excessively minute, that its dynamical effect is always small; and, though it is, of course, possible to produce an engine by a sufficiently gigantic arrangement, the success has hitherto not been encouraging,

(867.)

(869.) Statical theory of Electricity.

The rotations of Mr Faraday and Dr Ritchie were

electro-dynamic machines on a small scale. They M. Jacobi's were (probably) first mechanically applied by M. dal machines. Negro in 1833, but more systematically by Mr. M. H. Jacobi, of St Petersburg, the following year. It is stated that the latter gentleman moved a boat on the Neva with an electro-dynamic motor equivalent to three-fourths of a horse power. He has also investigated the theory of these machines, and the most advantageous circumstances of their employment. The principle is the alternate attraction and repulsion of two temporary magnets (one of which revolves), the current of electricity being suddenly changed at the critical part of the revolution.

Electro

IV. To M. Jacobi-almost simultaneously with Mr (868.) Spencer of Liverpool-we are also indebted for one of the simplest and most elegant applications of elec- Witty Voltatype; tricity, the Galvano-plastic art, or Voltatype. In-MM. Jathis, advantage is taken of the perfectly metallic state cobi and in which the base of a metallic salt is deposited at Spencer. the negative pole of a voltaic combination. In the case, for example, of the decomposition of sulphate of copper, the sulphuric acid unites with the positive wire, or remains suspended, while the metallic copper is slowly and homogeneously deposited on the surface of any object (rendered conducting by the application of black lead or otherwise), of which it forms a perfect mould, from which a fresh cast or fac-simile in metal of the original object may be obtained by a repetition of the process. To see the veins of a leaf, or the delicat: wing of an insect, thus metallized, is certainly an ashing thing; and the applications to the usefit arts a far too numerous to be noticed here. Daniell's invention of the Constant Battery evidently suggested the Voltatype.

Having thus brought down the history of galvanic or voltaic electricity, and that of the wonderful discoveries connected with it, to our own time, I shall in this section briefly notice the more intermitting progress during the same period of our knowledge of the quantitative laws which regulate the distribution of statical electricity on bodies charged with it. (870.) "Epinus and Coulomb," says Dr Whewell,1 "were Epinus, two of the most eminent physical philosophers of Cavendish, the last century." They laid the foundations of an exact science of statical electricity; and a third, and still more eminent name, deserves to be connected with theirs, that of CAVENDISH, of whose general labours I have already given some account in the Second section of the chapter on Heat. The labours of Epinus belong rather to the period embraced in the previous Dissertation, where they have been referred to by Sir John Leslie.

and Cou

lomb.

(871.)

theory and experi

Perhaps the most elaborate of the memoirs not strictly chemical which Cavendish published were Caventhose on electricity. The Franklinian hypothesis of dish's a single fluid in excess or in defect of its average state producing the phenomena then known as elec- ments. trical, offered a tempting field to an experimental philosopher well trained in the mathematical knowledge of the day; and his paper on this subject shows extreme care in its conception and execution. He assumes, as a matter of necessity, the repulsion of matter for matter at sensible distances, considered apart from the electricity always combined with it in greater or less quantity. The indifference of matter under ordinary circumstances is held to arise from the union with it of a sufficient amount of electricity to neutralize the repulsion of the matter. In short, the electric fluid is considered as a second kind of matter repelling its own particles, and attracting those

Cavendish's electrical

experi

ments.

of other matter with a force varying inversely as some less of the distance than the cube. power Common theory and matter repels its own particles and attracts electric particles according to the same law. The limitation as to a power below the inverse cube of the distance is necessary, since were the decrease of force more rapid, a particle would not be sensibly affected by the repulsion of any portion of the fluid except what was placed close to it. The hypothesis of Cavendish and his mode of reasoning from it were in general the same as those of Epinus; but Cavendish was not aware of the researches of the Swedish philosopher until his own memoir was completed. The number of facts accurately ascertained concerning electricity was at that time too small to admit of very precise numerical comparisons, but the ordinary cases of attraction, repulsion, and induction, were perspicuously explained by the theory; and had the inverse square of the distance been assumed (as it very safely might have been) to represent the law of diminishing repulsion, several of the theorems would have assumed a much more definite character, as was shown by Robison. Cavendish first demonstrated in his paper of 1771 that electricity must be confined close to the surface of a spherical body. This memoir also includes a correct theory of the Leyden phial, a just approximation to the law of attraction as the inverse square of the distance, a theory of conduction, and o. the distribution of electricity on insulated conductors placed at a distance but connected by a fine wire or electric canal; and it was only the prelude to other researches never published, but of which some remarkable fragments exist amongst his manuscripts. Professor William Thomson, ho has partly examined these, informs me that they cont...n the experimental solution of problems such as the following: "To compare the quantities of electricity on a spherical conductor, and a plane disk of equal diameter connected by a long conducting wire." Cavendish found that the sphere holds 1.57 times the electricity of the plate, a result exactly coincident with the deductions of theory.

(872.) He makes an artificial torpedo.

I may here (for the sake of biographical connection) mention Cavendish's paper on the Torpedo, as a remarkable instance of the explanation of an obscure natural phenomenon, by the analogous effects of an artificial imitation. The experiments of Walsh on the Electrical Fishes have been cited in the preceding Dissertation. Cavendish undertook the bold task of proving, that all the external effects of the shock under varied circumstances, might be reproduced by a combination of Leyden jars duly protected. He made an artificial torpedo, consisting of 49 jars in a frame

covered with leather, and he succeeded in obtaining shocks in air as in water, exactly comparable to those obtained from the live fish by Walsh. But perhaps the most striking part of the paper is the incidental mention of several laws of electricity then certainly new, and which he had deduced from experiment. Thus, he affirms the conducting power of iron for electricity to exceed 400,000 times that of distilled water, which he states to be equivalent to the fact, that a conductor of equal diameter will transmit as much electricity if the iron be 400,000 times longer than the water-a law conformable to that of Ohm. He farther estimates the conducting power of sea-water at 100 times, but of saturated brine at 720 times that of pure water. Again, the quantity of electricity required to raise the charge on different jars or plates to the same intensity he finds to be directly as the area of the coating, and inversely as the thickness of the plate, and he applies this just conclusion with great ingenuity to explain the surprising power of the torpedo's shock by the extreme fineness of the membranes separating the columns of the electrical organs.

electrical

COULOMB (born 1736, died 18061) was a person of (873.) less genius and less mathematical attainment than Coulomb's Cavendish, yet he had very considerable geometrical experiability and much facility in applying it to the results ments. of experiments, which he conducted with the greatest ingenuity and accuracy. In the latter respect he has seldom been surpassed. His methods, and even his numbers, are still, after a lapse of more than half a century, in many cases the best we can quote. Like Cavendish, he was devoted to quantitative estimations of phenomena.

His two greatest inventions were the balance of (874.) torsion and the proof plane. In the course of his lance of strictly mechanical researches (which, as we have torsion seen in the chapter on Mechanics, Art. 339, &c., were and the numerous and important) he ascertained the laws of proof plane. torsion. Within the limits of perfect elasticity, he found that the force is as the angle of torsion of the wire or fibre, and inversely as its length. An almost indefinite minuteness may thus be attained in the measure of forces which may be balanced by the elastic torsion of a wire. We have seen (Astronomy, § 1, Art. 156) how it was applied by Michell and Cavendish to measure the gravitation of bodies. Coulomb's invention dates at least from 1784. By means of it he established (in a different and perhaps more satisfactory way than had been done by Robison in 1769) that the electric and magnetic forces vary according to the Newtonian law;2 and with the aid of the "proof plane" he obtained exact measures of the electric tension on any part of an excited body. The "proof plane" consists of a small gilt disk with an insulat