A Treatise on Electricity and Magnetism, Vol. 1 (1881) — part 20 of 28

Third Method. If by any means we can cause a succession of small bodies to detach themselves from the end of the electrode, the potential of the electrode will approximate to that of the sur- rounding air. This may be done by causing shot, filings, sand, or water to drop out of a funnel or pipe connected with the electrode. The point at which the potential is measured is that at which the stream ceases to be continuous and breaks into separate parts or drops.

Another convenient method is to fasten a slow match to the

223.] THEORY OF THE PROOF PLANE. 315

electrode. The potential is very soon made equal to that of the air at the burning end of the match. Even a fine metallic point is sufficient to create a discharge by means of the particles of the air when the difference of potentials is considerable, but if we wish to reduce this difference to zero, we must use one of the methods stated above.

If we only wish to ascertain the sign of the difference of the potentials at two places, and not its numerical value, we may cause drops or filings to be discharged at one of the places from a nozzle connected with the other place, and catch the drops or filings in an insulated vessel. Each drop as it falls is charged with a certain amount of electricity, and it is completely discharged into the vessel. The charge of the vessel therefore is continually ac- cumulating, and after a sufficient number of drops have fallen, the charge of the vessel may be tested by the roughest methods. The sign of the charge is positive if the potential of the nozzle is positive relatively to that of the surrounding air.

MEASUREMENT OF SUKFACE-DENSITY OF ELECTRIFICATION.

Theory of the Proof Plane.

223.] In testing the results of the mathematical theory of the distribution of electricity on the surface of conductors, it is necessary to be able to measure the surface-density at different points of the conductor. For this purpose Coulomb employed a small disk of gilt paper fastened to an insulating stem of gum-lac. He ap- plied this disk to various points of the conductor by placing it so as to coincide as nearly as possible with the surface of the conductor. He then removed it by means of the insulating stem, and measured the charge of the disk by means of his electrometer.

Since the surface of the disk, when applied to the conductor, nearly coincided with that of the conductor, he concluded that the surface-density on the outer surface of the disk was nearly equal to that on the surface of the conductor at that place, and that the charge on the disk when removed was nearly equal to that on an area of the surface of the conductor equal to that of one side of the disk. This disk, when employed in this way, is called Coulomb's Proof Plane.

As objections have been raised to Coulomb's use of the proof plane, I shall make some remarks on the theory of the experiment.

316 ELECTROSTATIC INSTRUMENTS. [224..

This experiment consists in bringing a small conducting body into contact with the surface of the conductor at the point where the density is to be measured, and then removing the body and determining its charge.

We have first to shew that the charge on the small body when in contact with the conductor is proportional to the surface- density which existed at the point of contact before the small body was placed there.

We shall suppose that all the dimensions of the small body, and especially its dimension in the direction of the normal at the point of contact, are small compared with either of the radii of curvature of the conductor at the point of contact. Hence the variation of the resultant force due to the conductor supposed rigidly electrified within the space occupied by the small body may be neglected, and we may treat the surface of the conductor near the small body as a plane surface.

Now the charge which the small body will take by contact with a plane surface will be proportional to the resultant force normal to the surface, that is, to the surface-density. We shall ascertain the amount of the charge for particular forms of the body.

We have next to shew that when the small body is removed no spark will pass between it and the conductor, so that it will carry its charge with it. This is evident, because when the bodies are in contact their potentials are the same, and therefore the density on the parts nearest to the point of contact is extremely small. When the small body is removed to a very short distance from the conductor, which we shall suppose to be electrified positively, then the electrification at the point nearest to the small body is no longer zero but positive, but, since the charge of the small body is positive, the positive electrification close to the small body will be less than at other neighbouring points of the surface. Now the passage of a spark depends in general on the magnitude of the resultant force, and this on the surface-density. Hence, since we suppose that the conductor is not so highly electrified as to be discharging electricity from the other parts of its surface, it will not discharge a spark to the small body from a part of its surface which we have shewn to have a smaller surface-density.

224.] We shall now consider various forms of the small body.

Suppose it to be a small hemisphere applied to the conductor so as to touch it at the centre of its flat side.

Let the conductor be a large sphere, and let us modify the form

225.] THE PROOF PLANE. 317

of the hemisphere so that its surface is a little more than a hemi- sphere, and meets the surface of the sphere at right angles. Then we have a case of which we have already obtained the exact solution. See Art. 167.

If A and B be the centres of the two spheres cutting each other at right angles, DD' a diameter of the circle of intersection, and C the centre of that circle, then if Fis the potential of a conductor whose outer surface coincides with that of the two spheres, the quantity of electricity on the exposed surface of the sphere A is

\7(AD + BD + AC-CD-BC), and that on the exposed surface of the sphere B is

the total charge being the sum of these, or V(AD+BL-CI>).

If a and /3 are the radii of the spheres, then, when a is large compared with /3, the charge on B is to that on A in the ratio of

Now let a- be the uniform surface-density on A when B is re- moved, then the charge on A is

and therefore the charge on B is

v 3 a

or, when ft is very small compared with a, the charge on the hemisphere B is equal to three times that due to a surface-density <r extending over an area equal to that of the circular base of the hemisphere.

It appears from Art. 175 that if a small sphere is made to touch an electrified body, and is then removed to a distance from it, the mean surface-density on the sphere is to the surface-density of the body at the point of contact as w2 is to 6, or as 1.645 to 1.

225.] The most convenient form for the proof plane is that of a circular disk. We shall therefore shew how the charge on a circular disk laid on an electrified surface is to be measured.

For this purpose we shall construct a value of the potential function so that one of the equipotential surfaces resembles a circular flattened protuberance whose general form is somewhat like that of a disk lying on a plane.

318 ELECTROSTATIC INSTRUMENTS. [225.

Let or be the surface-density of a plane, which we shall suppose to be that of xy.

The potential due to this electrification will be

F=— 477 0-£.

Now let two disks of radius a be rigidly electrified with surface- densities — a and -f <j ' . Let the first of these be placed on the plane of any with its centre at the origin, and the second parallel to it at the very small distance c.

Then it may be shewn, as we shall see in the theory of mag- netism, that the potential of the two disks at any point is ox/c, where &> is the solid angle subtended by the edge of either disk at the point. Hence the potential of the whole system will be V— — 4:iT(rz + a'c&.

The forms of the equipotential surfaces and lines of induction are given on the left-hand side of Fig. XX, at the end of Vol. II.

Let us trace the form of the surface for which F= 0. This surface is indicated by the dotted line.

Putting the distance of any point from the axis of z = r, then, when r is much less than a, and z is small, we find

0> = 27T— 27T- + &C.

a

Hence, for values of r considerably less than a, the equation of the zero equipotential surface is

zc 0 = — 47ro-z+2'7ro/c— 2 Tit/ h&c. ;

</c

or zn =

Hence this equipotential surface near the axis is nearly flat.

Outside the disk, where r is greater than a, a) is zero when z is zero, so that the plane of xy is part of the equipotential surface.

To find where these two parts of the surface meet, let us find at

what point of this plane -=- = 0.

When r is very nearly equal to «, the solid angle o> becomes approximately a lune of the sphere of unit radius whose angle is tan-1 {z -*- (r — a)}, that is, o> is 2 tan"1 {z -r- (r—a)}, so that

dV

— = dz

Hence, when

dV v'c

_=0, '0=«+—

,'226.] ACCUMULATORS. 319

The equipotential surface V— 0 is therefore composed of a disk- like figure of radius r0, and nearly uniform thickness ZQ, and of the part of the infinite plane of xy which lies beyond this figure.

The surface-integral over the whole disk gives the charge of electricity on it. It may be found, as in the theory of a circular current in Part IV, Art. 704, to be

Q = '

The charge on an equal area of the plane surface is 7ro-r02, hence the charge on the disk exceeds that on an equal area of the plane

in the ratio of z , STTT t

1 + 8 - log -- to unity,

where z is the thickness and r the radius of the disk, z being sup- posed small compared with r.

On Electric Accumulators and the Measurement of Capacity.

226.] An Accumulator or Condenser is an apparatus consisting of two conducting surfaces separated by an insulating dielectric medium.

A Leyden jar is an accumulator in which an inside coating of tinfoil is separated from the outside coating by the glass of which the jar is made. The original Leyden phial was a glass vessel containing water which was separated by the glass from the hand which held it.

The outer surface of any insulated conductor may be considered as one of the surfaces of an accumulator, the other being the earth or the walls of the room in which it is placed, and the intervening air being the dielectric medium.

The capacity of an accumulator is measured by the quantity of electricity with which the inner surface must be charged to make the difference between the potentials of the surfaces unity.

Since every electrical potential is the sum of a number of parts found by dividing each electrical element by its distance from a point, the ratio of a quantity of electricity to a potential must have the dimensions of a line. Hence electrostatic capacity is a linear quantity, or we may measure it in feet or metres without ambiguity.

In electrical researches accumulators are used for two principal purposes, for receiving and retaining large quantities of electricity in as small a compass as possible, and for measuring definite quan- tities of electricity by means of the potential to which they raise the accumulator.

320 ELECTROSTATIC INSTRUMENTS. [227-

For the retention of electrical charges nothing has been devised more perfect than the Leyden jar. The principal part of the loss arises from the electricity creeping along the damp uncoated surface of the glass from the one coating to the other. This may be checked in a great degree by artificially drying the air within the jar, and by varnishing the surface of the glass where it is exposed to the atmosphere. In Sir W. Thomson's electroscopes there is a very small percentage of loss from day to day, and I believe that none of this loss can be traced to direct conduction either through air or through glass when the glass is good, but that it arises chiefly from superficial conduction along the various insulating stems and glass surfaces of the instrument.

In fact, the same electrician has communicated a charge to sulphuric acid in a large bulb with a long neck, and has then her- metically sealed the neck by fusing it, so that the charge was com- pletely surrounded by glass, and after some years the charge was found still to be retained.

It is only, however, when cold, that glass insulates in this way, for the charge escapes at once if the glass is heated to a temperature below 100°C.

When it is desired to obtain great capacity in small compass, accumulators in which the dielectric is sheet caoutchouc, mica, or paper impregnated with paraffin are convenient.

227.] For accumulators of the second class, intended for the measurement of quantities of electricity, all solid dielectrics must be employed with great caution on account of the property which they possess called Electric Absorption.

The only safe dielectric for such accumulators is air, which has this inconvenience, that if any dust or dirt gets into the narrow space between the opposed surfaces, which ought to be occupied only by air, it not only alters the thickness of the stratum of air, but may establish a connexion between the opposed surfaces, in which case the accumulator will not hold a charge.

To determine in absolute measure, that is to say in feet or metres, the capacity of an accumulator, we must either first ascertain its form and size, and then solve the problem of the distribution of electricity on its opposed surfaces, or we must compare its capacity with that of another accumulator, for which this problem has been solved.

As the problem is a very difficult one, it is best to begin with an accumulator constructed of a form for which the solution is known.

228.] MEASUREMENT OF CAPACITT. 321

Thus the capacity of an insulated sphere in an unlimited space is known to be measured by the radius of the sphere.

A sphere suspended in a room was actually used by MM. Kohl- rausch and Weber, as an absolute standard with which they com- pared the capacity of other accumulators.

The capacity, however, of a sphere of moderate size is so small when compared with the capacities of the accumulators in common use that the sphere is not a convenient standard measure.

Its capacity might be greatly increased by surrounding the- sphere with a hollow concentric spherical surface of somewhat greater radius. The capacity of the inner surface is then a fourth proportional to the thickness of the stratum of air and the radii of the two surfaces.

Sir W. Thomson has employed this arrangement as a standard of capacity, but the difficulties of working the surfaces truly spherical, of making them truly concentric, and of measuring their distance and their radii with sufficient accuracy, are considerable.

We are therefore led to prefer for an absolute measure of capacity a form in which the opposed surfaces are parallel planes.

The accuracy of the surface of the planes can be easily tested, and their distance can be measured by a micrometer screw, and may be made capable of continuous variation, which is a most important property of a measuring instrument.

The only difficulty remaining arises from the fact that the planes must necessarily be bounded, and that the distribution of electricity near the boundaries 'of the planes has not been rigidly calculated. It is true that if we make them equal circular disks, whose radius is large compared with the distance between them, we may treat the edges of the disks as if they were straight lines, and calculate the distribution of electricity by the method due to Helmholtz, and described in Art. 202. But it will be noticed that in this case part of the electricity is distributed on the back of each disk, and that in the calculation it has been supposed that there are no conductors in the neighbourhood, which is not and cannot be the case in a small instrument.

228.] We therefore prefer the following arrangement, due to Sir W. Thomson, which we may call the Guard-ring arrangement, by means of which the quantity of electricity on an insulated disk may be exactly determined in terms of its potential.

VOL. I.

322

ELECTROSTATIC INSTRUMENTS.

[228.

a

A

B

& o(^y

G

a

n ,

Fig. 21.

The Guard-ring Accumulator.

Bb is a cylindrical vessel of conducting- material of which the outer surface of the upper face is accurately plane. This upper

surface consists of two parts, a disk A, and a broad ring BB surrounding the disk, separated from it by a very small interval all round, just sufficient to prevent sparks passing1. The upper surface of the disk is accurately in the same plane with that of the guard-ring. The disk is supported by pillars of insulating material GG. C is a metal disk, the under surface of which is accurately plane and parallel to BB. The disk C is considerably larger than A. Its distance from A is adjusted and measured by means of a micrometer screw, which is not given in the figure.

This accumulator is used as a measuring instrument as follows : — Suppose C to be at potential zero, and the disk A and vessel Bb both at potential V. Then there will be no electrification on the back of the disk because the vessel is nearly closed and is all at the same potential. There will be very little electrification on the edges of the disk because BB is at the same potential with the disk. On the face of the disk the electrification will be nearly uniform, and therefore the whole charge on the disk will be almost exactly represented by its area multiplied by the surface-density on a plane, as given in Art. 124.

In fact, we learn from the investigation in Art. 201 that the charge on the disk is

j^2 + ^/2 H"*-W a ) \ SA SA A + aY

where R is the radius of the disk, R' that of the hole in the guard- ring, A the distance between A and C, and a a quantity which

cannot exceed R-R1^ .

If the interval between the disk and the guard-ring is small compared with the distance between A and <?, the second term will be very small, and the charge on the disk will be nearly

F-

22Q.] COMPARISON OF CAPACITIES, 323

Now let the vessel Bb be put in connexion with the earth. The charge on the disk A will no longer be uniformly distributed, but it will remain the same in quantity, and if we now discharge A we shall obtain a quantity of electricity, the Value of which we know in terms of V, the original difference of potentials and the measur- able quantities R, JKf and A.

On the Comparison of the Capacity of Accumulators.

229.] The form of accumulator which is best fitted to have its capacity determined in absolute measure from the form and dimen- sions of its parts is not generally the most suitable for electrical experiments. It is desirable that the measures of capacity in actual use should be accumulators having only two conducting surfaces, one of which is as nearly as possible surrounded by the other. The guard-ring accumulator, on the other hand, has three independent conducting portions which must be charged and discharged in a certain order. Hence it is desirable to be able to compare the capacities of two accumulators by an electrical process, so as to test accumulators which may afterwards serve as secondary standards.

I shall first shew how to test the equality of the capacity of two guard-ring accumulators.

Let A be the disk, B the guard-ring with the rest of the con- ducting vessel attached to it, and C the large disk of one of these accumulators, and let A'} B', and C' be the corresponding parts of the other.

If either of these accumulators is of the more simple kind, having only two conductors, we have only to suppress B or B', and to suppose A to be the inner and C the outer conducting surface, C, in this case being understood to surround A.

Let the following connexions be made.

Let B be kept always connected with C", and I? with C, that is, let each guard-ring be connected with the large disk of the other condenser.

(1) Let A be connected with B and C' and with /, the electrode of a Ley den jar, and let A' be connected with B' and C and with the earth.

(2) Let A, B, an£ C' be insulated from J.

(3) Let A be insulated from B and C", and A from Bf and £

(4) Let B and C" be connected with B' and C and with the earth.

(5) Let A be connected with A'.

324 ELECTROSTATIC INSTRUMENTS. [229.

(6) Let A and A' be connected with an electroscope E. We may express these connexions as follows : —

(1) 0 = C=£'=A' | A = £=C'=J.

(2) 0 = C=£'=A' | A = 3=C'\J.

(3) 0 = C=ff\A' | A\£=C'.

(4) 0 = C=ff\A' | ^|£=C'=0.

(5) 0 = C=£'\A' = ^|_B=C"=0.

(6) 0 = 0= If | A'=E = A | .5=C'=0.

Here the sign of equality expresses electrical connexion, and the vertical stroke expresses insulation.

In (l) the two accumulators are charged oppositely, so that A is positive and A* negative, the charges on A and A/ being uniformly distributed on the upper surface opposed to the large disk of each accumulator.

In (2) the jar is removed, and in (3) the charges on A and A' are insulated.

In (4) the guard-rings are connected with the large disks, so that the charges on A and A', though unaltered in magnitude, are now distributed over their whole surface.

In (5) A is connected with Af. If the charges are equal and of opposite signs, the electrification will be entirely destroyed, and in (6) this is tested by means of the electroscope E.

The electroscope E will indicate positive or negative electrification according as A or A' has the greater capacity.

By means of a key of proper construction, the whole of these operations can be performed in due succession in a very small fraction of a second, and the capacities adjusted till no electri- fication can be detected by the electroscope, and in this way the capacity of an accumulator may be adjusted to be equal to that of any other, or to the sum of the capacities of several accumulators, so that a system of accumulators may be formed, each of which has its capacity determined in absolute measure, i.e. in feet or in metres, while at the same time it is of the construction most suitable for electrical experiments.

This method of comparison will probably be found useful in determining the specific capacity for electrostatic induction of different dielectrics in the form of plates or disks. If a disk of the dielectric is interposed between A and C, the disk being con- siderably larger than A, then the capacity of the accumulator will

22Q.] SPECIFIC INDUCTIVE CAPACITY. 325

be altered and made equal to that of the same accumulator when A and C are nearer together. If the accumulator with the dielectric plate, and with A and C ^t distance #, is of the same capacity as the same accumulator without the dielectric, and with A and C at distance #', then, if a is the thickness of the plate, and K its specific dielectric inductive capacity referred to air as a standard,

• • ~~ /

a + x — x

The combination of three cylinders, described in Art. 127, has been employed by "Sir W. Thomson as an accumulator whose capa- city may be increased or diminished by measurable quantities.

The experiments of MM. Gibson and Barclay with this ap- paratus are described in the Proceedings of the Royal Society, Feb. 2, 1871, and Phil. Trans., 1871, p. 573. They found the specific in- ductive capacity of paraffin to be 1.975, that of air being unity.

P.AET II,

ELECT RO KINEMATICS.

CHAPTEK I.

THE ELECTRIC CURRENT.

230.] WE have seen, in Art. 45, that when a conductor is in electrical equilibrium the potential at every point of the conductor must be the same.

If two conductors A and B are charged with electricity so that the potential of A is higher than that of B, then, if they are put in communication by means of a metallic wire C touching both of them, part of the charge of A will be transferred to B, and the potentials of A and B will become in a very short time equalized.

231.] During this process certain phenomena are observed in the wire C, which are called the phenomena of the electric conflict or current.

The first of these phenomena is the transference of positive electrification from A to B and of negative electrification from B to A. This transference may be also effected in a slower manner by bringing a small insulated body into contact with A and B alternately. By this process, which we may call electrical con- vection, successive small portions of the electrification of each body are transferred to the other. In either case a certain quantity of electricity, or of the state of electrification, passes from one place to another along a certain path in the space between the bodies.

Whatever therefore may be our opinion of the nature of elec- tricity, we must admit that the process which we have described constitutes a current of electricity. This current may be described

232.] THE VOLTAIC BATTERY. 327

as a current of positive electricity from A to B, or a current of negative electricity from B to A, or as a combination of these two currents.

According to Fechner's and Weber's theory it is a combination of a current of positive electricity with an exactly equal current of negative electricity in the opposite direction through the same substance. It is necessary to remember this exceedingly artificial hypothesis regarding the constitution of the current in order to understand the statement of some of Weber's most valuable ex- perimental results.

If, as in Art. 36, we suppose P units of positive electricity transferred from A to B, and N units of negative electricity trans- ferred from B to A in unit of time, then, according to Weber's theory, P = TV, and P or N is to be taken as the numerical measure of the current.

We, on the contrary, make no assumption as to the relation between P and N, but attend only to the result of the current, namely, the transference of P + N of positive electrification from A to B, and we shall consider P --N the true measure of the current. The current, therefore, which Weber would call 1 we shall call 2.

On Steady Currents.

232.] In the case of the current between two insulated con- ductors at different potentials the operation is soon brought to an end by the equalization of the potentials of the two bodies, and the current is therefore essentially a Transient current.

But there are methods by which the difference of potentials of the conductors may be maintained constant, in which case the current will continue to flow with uniform strength as a Steady Current.

The Voltaic Battery.

The most convenient method of producing a steady current is by means of the Voltaic Battery.

For the sake of distinctness we shall describe DanielFs Constant Battery : —

A solution of sulphate of zinc is placed in a cell of porous earth- enware, and this cell is placed in a vessel containing a saturated solution of sulphate of copper. A piece of zinc is dipped into the sulphate of zinc, and a piece of copper is dipped into the sulphate of copper. Wires are soldered to the zinc and to the copper above

328 THE ELECTRIC CURRENT. [233.

the surface of the liquid. This combination is called a cell or element of DanielPs battery. See Art. 272.

233.] If the cell is insulated by being placed on a non-con- ducting stand, and if the wire connected with the copper is put in contact with an insulated conductor A, and the wire connected with the zinc is put in contact with H, another insulated conductor of the same metal as A, then it may be shewn by means of a delicate electrometer that the potential of A exceeds that of B by a certain quantity. This difference of potentials is called the Electromotive Force of the Darnell's Cell.

If A and B are now disconnected from the cell and put in communication by means of a wire, a transient current passes through the wire from A to £, and the potentials of A and B become equal. A and B may then be charged again by the cell, and the process repeated as long as the cell will work. But if A and B be connected by means of the wire C, and at the same time connected with the battery as before, then the cell will main- tain a constant current through C, and also a constant difference of potentials between A and B. This difference will not, as we shall see, be equal to the whole electromotive force- of the cell, for part of this force is spent in maintaining the current through the cell itself.

A number of cells placed in series so that the -zinc of the first cell is connected by metal with the copper of the second, and so on, is called a Voltaic Battery. The electromotive force of such a battery is the sum of the electromotive forces of the cells of which it is composed. If the battery is insulated it may be charged with electricity as a whole, but the potential of the copper end will always exceed that of the zinc end by the electromotive force of the battery, whatever the absolute value of either of these potentials may be. The cells of the battery may be of very various construction, containing different chemical substances and different metals, provided they are such that chemical action does not go on when no current passes.

234.] Let us now consider a voltaic battery with its ends insulated from each other. The copper end will be positively or vitreously electrified, and the zinc end will be negatively or resinously electrified.

Let the two ends of the battery be now connected by means of a wire. An electric current will commence, and will in a very short time attain a constant value. It is then said to be a Steady Current.

ELECTROLYSIS. 329

Properties of the Current.

235.] The current forms a closed circuit in the direction from copper to zinc through the wires, and from zinc to copper through the solutions.

If the circuit be broken by cutting any of the wires which connect the copper of one cell with the zinc of the next in order, the current will be stopped, and the potential of the end of the wire in connexion with the copper will be found to exceed that of the end of the wire in connexion with the zinc by a constant quantity, namely, the total electromotive force of the circuit.

Electrolytic Action of the Current.

236.] As long as the circuit is broken no chemical action goes on in the cells, but as soon as the circuit is completed, zinc is dissolved from the zinc in each of the Darnell's cells, and copper is deposited on the copper.

The quantity of sulphate of zinc increases, and the quantity of sulphate of copper diminishes unless more is constantly supplied.

The quantity of zinc dissolved and also that of copper deposited is the same in each of the Daniell's cells throughout the circuit, what- ever the size of the plates of the cell, and if any of the cells be of a different construction, the amount of chemical action in it bears a constant proportion to the action in the DanielPs cell. For instance, if one of the cells consists of two platinum plates dipped into sulphuric acid diluted with water, oxygen will be given off at the surface of the plate where the current enters the liquid, namely, the plate in metallic connexion with the copper of Daniell's cell, and hydrogen at the surface of the plate where the current leaves the liquid, namely, the plate connected with the zinc of Daniell's cell.

The volume of the hydrogen is exactly twice the volume of the oxygen given off in the same time, and the weight of the oxygen is exactly eight times the weight of the hydrogen.

In eveiy cell of the circuit the weight of each substance dissolved, deposited, or decomposed is equal to a certain quantity called the electrochemical equivalent of that substance, multiplied by the strength of the current and by the time during which it has been flowing.

For the experiments which established this principle, see the seventh and eighth series of Faraday's Experimental Researches;

330 THE ELECTRIC CURRENT. [237.

and for an investigation of the apparent exceptions to the rule, see Miller's Chemical Physics and WiedemamVs Galvanismus.

237.] Substances which are decomposed in this way are called Electrolytes. The process is called Electrolysis. The places where the current enters and leaves the electrolyte are called Electrodes. Of these the electrode by which the current enters is called the Anode, and that by which it leaves the electrolyte is called the Cathode. The components into which the electrolyte is resolved are called Ions: that which appears at the anode is called the Anion, and that which appears at the cathode is called the Cation.

Of these terms, which were, I believe, invented by Faraday with the help of Dr. Whewell, the first three, namely, electrode, elec- trolysis, and electrolyte have been generally adopted, and the mode of conduction of the current in which this kind of decomposition and transfer of the components takes place is called Electrolytic Conduction.

If a homogeneous electrolyte is placed in a tube of variable section, and if the electrodes are placed at the ends of this tube, it is found that when the current passes, the anion appears at the anode and the cation at the cathode, the quantities of these ions being- electrochemically equivalent, and such as to be together equivalent to a certain quantity of the electrolyte. In the other parts of the tube, whether the section be large or small, uniform or varying, the composition of the electrolyte remains unaltered. Hence the amount of electrolysis which takes place across every section, of the tube is the same. Where the section is small the action must therefore be more intense than where the section is large, but the total amount of each ion which crosses any complete section of the electrolyte in a given time is the same for all sections.