Thomas Graham.

Elements of chemistry, including the applications of the science in the arts (Volume 2) online

. (page 40 of 67)
Online LibraryThomas GrahamElements of chemistry, including the applications of the science in the arts (Volume 2) → online text (page 40 of 67)
Font size
QR-code for this ebook

mixture of chlorine and hydrogen is brought by the action of


light, is not permanent ; on the contrary, the resistance to
combination overcome by the influence of the light, is soon
restored when the gas is allowed to stand in the dark.

The resistance to combination which prevents the union of
the gases until the action is assisted by light, may be increased
by various circumstances, especially by the presence of foreign
gases, even in very small quantity. An excess of 10 3 00 of
hydrogen above that contained in the normal mixture, reduces
the action from 100 to 38. Oxygen, in quantity amounting
to only 10 g 00 of the total volume of gas, diminishes the action
from 100 to 4-7 ; and -j-J-J-Q reduces it from 100 to 1-3. An
excess of T ^^-o of chlorine reduces the action from 100 to
60-2; and -J^. from 10 to 41 ' 3 - A sma11 quantity of
hydrochloric acid gas does not produce any appreciable
diminution ; y^o ^ tne non-insolated mixture reduces the
action from 100 to 55.

The increase in the rate at which combination goes on up
to a certain point under the influence of light, appears to arise,
not from any peculiar property of light, but rather from the
mode of action of chemical affinity itself. Chemical induction
is in fact observed in cases in which there is nothing but pure
chemical action to produce the alteration. Thus, when a
dilute aqueous solution of bromine mixed with tartaric acid is
left in the dark, hydrobromic acid is formed ; and, by deter-
mining the amount of free bromine present in the liquid at
different times, it is found that the rate at which the produc-
tion of hydrobromic acid goes on is not uniform, but increases
up to a certain point, according to a law similar to that which
is observed in photo-chemical induction.

These phenomena seem to point to the conclusion that the
affinity between any two bodies is in itself a force of constant
amount, but that its action is liable to be modified by opposing
forces, similar to those which affect the conduction of heat or
electricity, or the distribution of magnetism in steel. We
overcome these resistances when we accelerate the formation
of a precipitate by agitation, or a decomposition by insolation.


Optical and Chemical Extinction of the Chemical Rays.
When light passes through any medium; part of it is lost by
reflection at the surface, another portion by absorption within
the medium, so that the quantity of emergent light is only a
fraction of the incident light. This is true with regard to the
chemical as well as to the luminous rays. By passing light
from a constant source through cylinders with plate-glass
ends filled with dry chlorine, it is found that, with a given
length of cylinder, the quantity of the chemical rays trans-
mitted, when no chemical action takes place, is to the quan-
tity in the incident light in a constant ratio ; in other words,
the absorption of the chemical rays is proportional to the in-
tensity of the light. It is also found that the quantity of
chemical rays transmitted varies proportionally to the density
of the absorbing medium.

But further, when light passes through a medium in which
it excites chemical action, it is found that, in addition to the
optical extinction already spoken of, a quantity of light is lost
proportional to the amount of chemical action produced. The
depth of pure chlorine at C. and 0'76 mm. pressure, through
which the light of a coal-gas flame must pass in order to be
reduced to ^ is found to be 173-3 millimeters. Hence,
since the quantity of light absorbed varies as the density, the
depth of chlorine diluted with an equal volume of air, or other
chemically inactive gas, required to produce the same amount
of extinction, would be 346*6 mm. But when the sensitive
mixture of equal volumes of chlorine and hydrogen is used,
the depth of the mixture which the light must penetrate to be
reduced to y^-, is found to be only 234 mm. Hence, it appears
that light is absorbed in doing chemical work.

With light from other sources, results are obtained similar
in character, but differing in amount. Diffuse morning light
reflected from the zenith of a cloudless sky is reduced to T J F
by passing through 45*6 mm. of chlorine, and through 73*5
mm. of the sensitive mixture ; diffuse evening light is reduced
to T ^ by passing through 19*7 mm. of chlorine and through


57*4 mm. of the standard mixture. Hence it appears that
the chemical rays of diffuse morning light are absorbed by
chlorine much more quickly than those of lamp-light ; and
those of evening light with still greater facility. From this
we may conclude that the chemical rays reflected at different
times and hours, possess, not only quantitative but also quali-
tative differences, similar to the various coloured rays of the
visible spectrum. It is a fact well known to photographers,
that the amount of light photometrically estimated gives no
measure of the time in which a given photochemical effect is
produced. For the taking of pictures, a less intense morning
light is always preferred to a bright evening light.


Measurement of the Force of Electric Currents. There are
two methods by which the forces of electric currents are com-
pared with each other, viz., the chemical or electrolytic, and
the electromagnetic methods.

Faraday has shown that the amount of chemical work done
is the same in all parts of the circuit ; that, if two decomposing
cells be introduced, one containing dilute sulphuric, the other
hydrochloric acid, the quantity of hydrogen evolved is the
same in both, and equal to the hydrogen evolved (by true
current action) in each cell of the battery ; moreover, that
the quantities of different elements eliminated in any part of
the circuit, are always in the ratio of their equivalent weights.
The voltameter (I. 290) affords, therefore, a true and exact
measure of the amount of the chemical or electrical force
developed by the battery. But its indications are not always
sufficiently rapid. In fact, in using this instrument, it is
necessary to wait till a measurable quantity of gas is collected.
It will, therefore, indicate the relative quantity of electricity



which has passed through the circuit in a certain finite inter-
val, say in a minute; but it gives no information of any
variations that may have taken place during that interval ;
moreover, it can only be used to measure currents of con-
siderable strength.

The Tangent-compass. To supply these deficiencies, and
obtain exact and instantaneous indications of the relative
forces of electric currents, recourse is had to the electro-
magnetic method, which consists in observing the deflection
of a magnetic needle produced by the current. Instruments
for this purpose are called Galvanometers or Rheometers. The
effect of a coil of wire in intensifying the effect of the current
upon a magnetic needle, is described at page 290. Vol. I., of
this work. But the kind of instrument there described,
though commonly called a galvanometer, is really only a
galvanoscope, or multiplier. It indicates with great delicacy
the existence and direction of an electric current, but it is not
constructed for quantitative determinations.

In the true galvanometer (Fig. 41) the current, instead of
passing through a long coil of wire placed close to the needle,
is made to pass through a broad
circular band of brass or copper,
p Q, of considerable dimensions, in
the centre of which is placed a mag-
netic needle, n, the length of which
is very small in comparison with
the diameter of the circular con-
ductor, so that the distance of the
extremity of the needle from the
conductor P Q, and consequently
the force exerted upon it by the cur-
rent, is sensibly the same at all
angles of deflection. The instrument



is so placed that the plane of the circle P Q coincides with the
magnetic meridian. To determine the relation which exists
under these circumstances between the deflection of the
needle and the force of the current, let P Q (Fig. 42) repre-
sent the circular conductor seen from above ; a z the direction
of the needle under the influence of the current. The extre-
mity of the needle is then acted upon by two forces, viz., the
force of terrestrial magnetism acting
parallel to P Q, and the force of
the current acting at right angles to
that direction. Let these forces be
represented in magnitude and direc-
tion by the lines a b, a c. Draw
also the line fa d perpendicular to
a z, and bf, c d, perpendicular to df.
Then the lines af, a d represent the
resolved portions of the forces a b,
a c, which act at right angles to the
needle, and tend to turn it one way
or the other. In order, therefore,
that the needle may be at rest, a d
must be equal to a/, or

a c . cos c a d = a b, sin a bf.

Now the angle cad is equal to v,

the angle of deflection of the needle

from the meridian, because a c is

perpendicular to P Q, and a d to az; and the angle a bf

is also equal to v, because a b is parallel to P Q, and bfto a z.

Hence the preceding equation becomes

a c . cos v = a b . sin v ;
therefore a c = a b . tan v.

Or, if we denote the force of the earth's magnetism by M, and
that of the electric current by E, we have

E = M tan v.


Consequently, since the magnetic force of the earth is constant
at the same place (at least for short intervals of time), the
magnetic force of the current is proportional to the tangent of the
angle of deflection : hence the name of the instrument.

Comparison between the chemical and magnetic actions of
the current. By introducing into the same voltaic curcuit, a
voltameter and a tangent-compass, it is found that the chemical
action of the current is directly proportional to its magnetic
action. The tangent-compass affords, therefore, a measure of
the chemical as well as of the magnetic force of the current,
the quantity of chemical or electrical force in the circuit
being proportional to the tangent of the angle of deflection of
the needle.

If m milligrammes of hydrogen are evolved in a second
in the voltameter, when the galvanometer exhibits a de-
flection of 45, and therefore a current force = 1 (since
tan 45 = 1), then, when the same galvanometer shows a
deflection = a, the quantity of hydrogen evolved in t
seconds will be m . t . tan a. The quantity of any other
element eliminated in the same circuit, will be found by
multiplying this quantity by the equivalent weight of that

With a tangent -compass, the diameter of whose conductor
measures one decimeter, it is found that, when the deflection
is 45, one milligramme, or 11*2 cubic centimeters (at C.
and Bar. O76 met.) of hydrogen is eliminated in 32'3 seconds.
Hence with any other circular current whose radius is r deci-
meters and force = tan a, the time t in which 1 milligramme
of hydrogen is evolved, or 9 milligrammes of water are
decomposed, is


t =

tan at

Ohm's Formula?. The amount of electrical or chemical
power developed in the voltaic circuit, or, in other words,


the quantity of electricity which passes through a transverse
section of the circuit, in a unit of time, evidently depends
upon two conditions ; viz., the power, or electromotive force
of the battery, and the resistance offered to the passage of the
current by the conductors, liquid or solid, which it has to
traverse. With a given amount of resistance, the power of
the battery is proportional to the quantity of electricity deve-
loped in a given time ; and by a double or treble resistance,
we mean simply that which, with a given amount of exciting
power in the battery, reduces the quantity of electricity deve-
loped, or work done, to one-half or one-third. If, then, we
denote the electromotive force of the battery by E, and the
resistance by 72, we have, for the quantity of electricity pass-
ing through the circuit in a unit of time, the expression :

= s <'>

This is called Ohm's law, from the name of the distinguished
mathematician who first announced it. It must be under-
stood, not as a theorem, but as a definition. To say that the
strength of the current varies directly as the electromotive
force, and inversely as the resistance, is simply to define
what we mean by electromotive force and what we mean by

Let us now endeavour, by means of the formula (1), to
estimate the effect produced on the strength of the current
by increasing the number and size of the plates of the battery.
The resistance R consists of two parts ; viz. that which the
current experiences in passing through the cells of the battery
itself, and that which is offered by the external conductor
which joins the poles. This conductor may consist either wholly
of metal, or partly of metal and partly of electrolytic liquids.

* It must be remembered that we are here merely comparing the strength
of electric currents one with the other, not reducing the current force to ab-
solute mechanical measure, or even comparing it with the electro-static forces
of attraction and repulsion. (See page 506.)


Let the resistance within the battery be r, and the external
resistance r' ; then, in the one-celled battery, we have

Now suppose the battery to consist of n cells perfectly similar ;
then the electromotive force becomes nE } the resistance within
the battery nr\ if, then, the external resistance remains the
same, the strength of the current will be denoted by

nE E .

q '"


If T^ be small, this expression has nearly the same value

as - - ; that is to say, if the circuit be closed by a good
r + V

conductor, such as a short thick wire, the quantity of elec-
tricity developed by the compound battery of n cells, is
sensibly the same as that evolved by a single cell of the same
dimensions. But if / is of considerable amount, as when the
circuit is closed by a long thin wire, or when an electrolyte is
interposed, the strength of the current increases considerably
with the number of plates. In fact, the expression (3) is
always greater than (2) ; for

nE E = (n - 1) J5V

n r + / r + r' (n r + /) (7- + r') '

a quantity which is necessarily positive when n is greater
than unity.

Suppose, in the next place, that the size of the plates is
increased, while their number remains the same. Then,
according to the chemical theory, an increase in the surface of
metal acted upon must produce a proportionate increase in the
quantity of electricity developed, provided the conducting
power of the circuit is sufficient to give it passage. According
to the theory which attributes the development of the elec-



tricity to the contact of dissimilar metals, an increase in the
size of the plates does not increase the electromotive force,
but it diminishes the resistance within the cells of the battery
by offering a wider passage to the electricity. Hence in the
single cell, if the surface of the plates, and therefore the trans-
verse section of the liquid, be increased in times, the expression
for the strength of the current becomes

E mE

r + mr"

If r' be small, this expression is nearly the same as ,,

that is to say, the quantity of electricity in the current in-
creases very nearly in the same ratio as the size of the plates ;
but when the external resistance is considerable, the advantage
gained by increasing the size of the plates is much less.

We may conclude, then, that when the resistance in the
circuit is small, as in electro-magnetic experiments, a small
number of large plates is the most advantageous form of
battery ; but in overcoming great resistances, power is gained
by increasing the number rather than the size of the plates.

Electric Resistance of Metals. The preceding principles
enable us to determine the manner in which the resistance of
a metallic wire varies with its length. For this purpose
suppose a one-celled battery (Daniell's) to be used, which
maintains a constant action during the time of the experiment.
First let the current be made to pass directly through the
tangent- compass, and afterwards let wires, of uniform thick-
ness and of the lengths of 5, 10, 40, 70, and 100 meters, be
interposed in the circuit, and the resulting deflections ob-
served. Now, as the force of the battery is constant, the
resistance is inversely as the strength of the current. But
the total resistance is made up of that of the interposed wires,
together with that of the battery itself, and that of the con-
ductor of the tangent-compass. These last two resistances



we may suppose to be equal to that of a wire of the same
thickness as the above, and of a certain unknown length, x.
Instead, therefore, of the lengths of wire 5, 10, 40, &c., we
must substitute x + 5, x + 10, x + 40, &c. An experiment
of this kind* gave the following results :

Length of Wire.

Observed Deflection.

Tangent of Deflection.

x meters

62 0'


x + 5

40 20


x + 10

28 30


x + 40

9 45


x + 70



x + 100

4 15


Now, let us assume, as most probable, that the resistance
of a wire increases in direct proportion to its length, then,
according to Ohm's law, the first two experiments give :

x : x + 5 = 0-849 : 1-880

whence, x = 4*11. And, by combining in a similar manner
the first experiment with all the others, we obtain for x the
several values 4-06, 4-03, 4-14, 4-09, the mean of the whole
being 4-08. Substituting this value for x in the preceding
table, and calculating the corresponding deflections on the
supposition that the strength of the current varies inversely
as the resistance, that is as the length of the conductor, we
obtain the following results :

Length of




4-08 meters

62 0'

62 0'


40 18

40 20

+ 2'


28 41

28 30

- 11


9 56

9 45

- 11


5 57


+ 3


4 14

4 15

+ 1

* Miiller, Lchrbuch der Pliysik. 1853, ii. 177.

NN 2


From the results of this and similar experiments, it is
inferred that the resistance of a conductor of uniform thickness
varies directly as its length.

The Rheostat or Current-regulator. The various forms of
the so-called constant battery, Daniell's for example (I. 284),
attain their end but imperfectly, a galvanometer included in
the circuit always exhibiting more or less variation. A really
constant current can only be obtained by interposing in the
circuit a conducting wire of variable length, so that the
resistance may be increased or diminished as the action of the
battery becomes stronger or weaker. Various instruments
have been contrived for this purpose. The one most used,
invented by Professor Wheatstone, is represented in fig. 43.
A and B are two cylinders of the Fig. 43.

same dimensions the first of
dry wood, the second of brass
placed with their axes parallel
to each other. The wooden
cylinder A has a fine screw
cut on its surface, and around
it, following the thread of the
screw, is coiled a thin bcass
wire. One extremity of this wire
is attached to a brass ring, v, at the nearer end of the wooden
cylinder, and the other to the farther extremity of the brass
cylinder. The ring v and the nearer end of the brass cylin-
der are connected with the wires of the battery through
the medium of the screw-joints CD. A movable handle,
7i, serves to turn the cylinders alternately round their axes.
By turning B to the right, the wire is uncoiled from A and
coiled upon B ; and the contrary when A is turned to the left.
The number of coils of wire upon A are indicated by a scale
placed between the cylinders, the fractions of a turn being
measured by an index moving round the ring v, which is
graduated accordingly. As the coils of the wire are insulated


on the wooden cylinder, but not on the brass, it is evident
that the path of the current will be longer, and therefore the
resistance greater, in proportion to the number of coils of wire
upon the wooden cylinder.

By means of the rheostat and the tangent-compass, the
resistances afforded by different conductors to the passage of
the current may be measured with great facility. Suppose
that when the wire of the rheostat is completely uncoiled
from the wooden cylinder (the index then standing at 0), a
tangent-compass introduced into the circuit shows a deflection
of 46. Then let a copper wire four yards long and -^th of
an inch thick, be introduced into any part of the same circuit.
The galvanometer-needle will then exhibit a smaller deflection,
say 37. On removing the wire, the galvanometer will again
exhibit its former deflection of 46. Now let the rheostat wire
be coiled round the wooden cylinder till the needle returns to
37, and suppose that to produce this effect twenty turns
of the rheostat wire are necessary. This length of the
rheostat wire produces a resistance equal to that of the wire
under examination. Next let a similar experiment be made
with a wire of the same length but of twice the thickness, and
consequently having a transverse section four times as great
as that of the former. It will be found that five turns of the
rheostat wire, or one-fourth of the former length, are sufficient
to produce a resistance equal to that of the second wire. By
experiments thus conducted it is found that: The resistance
of a wire or any other conductor of given length varies inversely
as its transverse section. And comparing this result with that
which was established at page 503, we find that : Conductors
of the same material offer equal resistances, when their lengths
are to one another in the same proportion as their transverse

In a similar manner, the relative conducting powers of
different metals may be ascertained. Taking the resistance
of pure copper as the unit, it is found that that of iron is

N N 3


7 '02, of brass 3-95, of German silver 15-47. The conducting
powers are of course inversely as these numbers (II. 441).

Heating Power of the Voltaic Current. The degree of heat
excited in a metallic wire by the passage of the current,
increases with the strength of the current and with the
resistance of the wire. To determine the numerical relations
of this phenomenon, the wire to be heated is formed into a
spiral and enclosed within a vessel containing strong alcohol,
or some other non-conducting liquid, in order that the cur-
rent may pass entirely through the wire, and not through the
liquid itself. The rise of temperature in the liquid is noted
by a delicate thermometer ; the strength of the current mea-
sured by the tangent-compass ; and the resistance of the wire
afterwards determined in the manner above described. By
this method Lenz* has shown that:

TJie quantity of heat evolved in a given time is directly propor-
tioned to the resistance of the wire, and to the square of the quan-
tity of electricity wh ich passes throuyh it.

The same result has been obtained by Joule f, both for
wires and liquid conductors ; by E. Becquerel for liquids ; and
by RiessJ for the heat produced by the discharge of the
electricity accumulated in a Leyden jar.

Reduction of the Force of the Current to absolute mechanical
Measure: This important determination has been made the
subject of an extensive research by Weber and Kohlrausch.
To understand the results obtained by these philosophers, it is
necessary to define exactly the several units of measurement
adopted :

a. The unit of. electric fluid is the quantity which, when
concentrated in a point, and acting on an equal quantity of

* Fogg. Ann. Ixi. 18. f Phil. Mag. [3], xix. 210.

J Pogg. Ann. xl. 335 ; xliil 47 ; xlv. 1.

Abhandhmgen derMathematisch-physischen Classe der Konigl. Sachsis-
chcn Gescllsch. d. Wiss. Leipzig. 1856.


the same fluid also concentrated in a point, and at the unit of
distance, exerts a repulsion equal to the unit of force.

b. The unit of electrochemical intensity is the force of the
current which, in a unit of time, decomposes a unit of weight
of water, or an equivalent quantity of any other electrolyte.

c. The unit of electromagnetic force, is the force of a
current which when it traverses a circular conductor
whose area is equal to the unit of surface, and acts upon a

Online LibraryThomas GrahamElements of chemistry, including the applications of the science in the arts (Volume 2) → online text (page 40 of 67)