EFFECTS OF THE CURRENT
The term “electric current,” in the present state of our knowledge, should
be regarded as denoting the existence of a state of things in which
certain definite experimental effects are produced, for some of which
there certainly is no analogy exhibited in ordinary hydraulic currents.
The following are the most important of these effects:
1. Thermal effect;
2. Magnetic effect;
3. Chemical effect.
It is rather to these effects than to any imaginary current flow in the
conductor that the mind of the reader should be directed.
With this preliminary caution, which should never be lost sight of, the
use of familiar words and expressions connected with the flow of water in
pipes is justified in order to avoid roundabout and cumbrous phrases
which, though perhaps more nearly in accord with present knowledge of the
facts, would not tend to clearness or conciseness.
The three most important effects of the current just mentioned, may be
presented in more detail as follows:
1. The _Thermal effect:_--
The conductor along which the current flows becomes heated. The
rise of temperature may be small or great according to
circumstances, but some heat is always produced.
2. The _Magnetic effect;_
The space both outside and inside the substance of the
conductor, but more especially the former, becomes a “magnetic
field” in which delicately pivoted or suspended magnetic needles
will take up definite positions and magnetic materials will
become magnetized.
[Illustration: FIG. 87.--Lenz’s apparatus for measuring the heat given off
by an electric current. It consisted of a wide mouthed stoppered bottle
fixed upside down, with its stopper, _b_ in a wooden box; the stopper was
perforated so as to give passage to two thick platinum wires, connected at
one end with binding screws, _s_, while their free ends were provided with
platinum cones by which the wires under investigation could be readily
affixed; the vessel contained alcohol, the temperature of which was
indicated by a thermometer fitted in a cork inserted in a hole made in the
bottom of the vessel. The current is passed through the platinum wires,
and its strength measured by means of a galvanometer interposed in the
circuit. By observing the increase of temperature in the thermometer in a
given time and knowing the weight of the alcohol, the mass of the wire,
the specific heat, and the calorimetric values of the vessel, and of the
thermometer, compared with alcohol, the heating effect which is produced
by the current in a given time can be calculated.]
3. The _Chemical effect;_--
If the conductor be a liquid which is a chemical compound of a
certain class called _electrolytes_, the liquid will be
decomposed at the places where the current enters and leaves
it.
=Thermal Effect.=--If a quantity of electricity were set flowing in a
closed circuit and the latter offered no _resistance_, it would flow
forever, just as a wagon set rolling along a circular railway would never
stop if there were no _friction_.
[Illustration: FIG. 88.--The Seebeck effect: If in a complete metallic
circuit having junctions of dissimilar metals, the junctions are at
different temperatures, then generally a steady current will flow in the
circuit as long as the differences of the temperatures of the junction is
maintained. To demonstrate this, a piece of copper K bent in the shape
seen in the figure, was placed on a block of bismuth A B, carrying a
pivoted magnetic needle N S; as soon as the equality of temperatures was
altered by either heating or cooling one of the junctions of the two
metals, the needle indicated a current which continued to flow as long as
the difference of temperature was maintained at the junctions. The
movement of the needle indicated the direction in which the current
flowed. If, for instance, the north junction B were heated, the N pole
moved eastwards, showing that at the heated junction the current flows
from the bismuth to the copper, at the cold junction from the copper to
the bismuth.]
When matter in motion is stopped by friction, the energy of its motion is
converted into heat by the friction thus causing the matter to come to
rest. Similarly, when electricity in motion, that is, an electric current
is stopped by resistance, the energy of its flow is transformed into heat
by the resistance of the circuit.
If the terminals of a battery be joined by a short thick wire of low
resistance, most of the heat will be developed in the battery, whereas, if
a thin wire of high resistance be used it will become hot, while the
battery itself will remain comparatively cool.
To investigate the development of heat by a current, Joule and Lenz used
instruments on the principle of fig. 87, in which a thin wire joined to
two stout conductors is enclosed within a glass vessel containing alcohol,
into which is placed a thermometer. The resistance of the wire being
known, its relation to the other resistances can be calculated. Joule
found that the number of heat units developed in a conductor is
proportional to:
1. The resistance;
2. The square of the current strength;
3. The time that the current lasts.
Joules’ law may be stated as follows:
_The heat generated in a conductor by an electric current is proportional
to the resistance of the conductor, the time during which the current
flows, and the square of the strength of the current._
The quantity of heat in calories may be calculated by use of the
equation,
calories per second = volts × amperes × .24. (1)
The total number of calories H developed in t seconds will be
given by
H = P.D. × C × t × .24. (2)
EXAMPLE--If a current of 10 amperes flows in a wire whose
terminals are at a potential difference of 12 volts, how much
heat will be developed in 5 minutes?
Substituting in equation (2):
10 × 12 × (60 × 5) × .24 = 8640 calories.
Since by Ohm’s Law potential difference = I × R substituting IR
for P.D. in (2)
H = I^{2} R × t × .24
=Use of Heat from Electric Current.=--In the transmission of electricity
from place to place, it is very desirable that none of the energy be
expended in heating the conductor. Hence copper wires of the proper size
must be used.
In wiring a building for electric lights, the insurance rules require that
the wires be of a certain size and that they be put up in a certain
manner. Otherwise they will not insure a building against fire.
It is often desirable, however, to use the electric current for the
purpose of producing heat. The carbons of the arc and incandescent lamps
are intensely heated that they may produce light. Coils of German silver
wire or other high resistance wire are heated by the passage of a current
through them. In this manner the electric stove is made.
Soldering coppers, smoothing irons, and baking ovens are heated in a
similar manner.
=Magnetic Effect.=--An electric current flowing in a wire causes it to be
surrounded by a _magnetic field_, which consists of _lines of force_
encircling the wire. The field is strongest near the wire and diminishes
gradually in strength at increasing distances therefrom. The presence of
this magnetic field is shown by various experiments and the subject is
fully explained in chapter IX on _magnetism_.
=Chemical Effect.=--Pats van Trostwyk (1789) pointed out that an electric
discharge was capable of decomposing water; to show this he used gold
wires, which he allowed to dip in water, connecting one of them with the
inner, and another with the outer coating of a Leyden jar, and passing the
discharge through the water. The gas bubbles collected proved to consist
of oxygen and hydrogen gas.
Nicholson and Carlisle (1800) dipped a copper wire which was connected
with one of the poles of a voltaic pile into a drop of water, which
happened to be on the plate connected with the other pole; gas bubbles
appeared, and the drop of water became smaller and smaller.
This experiment was repeated in a somewhat different manner, the
brass wires from a pile being brought under a tube filled with
water and closed at the top. Gas bubbles were produced by the
wire in connection with the negative pole of the pile, and the
water was observed to diminish gradually. At the positive wire,
on the contrary, no gas came off, but the metal lost its
metallic lustre, became dark, and finally crumbled away. The gas
which had collected in the tube proved to be hydrogen; while on
examining the black mass it was found that the constituents of
brass, viz., copper and zinc, had become oxidized.
[Illustration: FIG. 89.--An electrolytic cell. The parts are: A, cell; B,
electrolyte; C, positive electrode or cathode; D, negative electrode or
cathode.[9]]
=Electrolysis.=--_Electric analysis_ or more briefly _electrolysis_ was
the term applied by Faraday to the process of decomposing a liquid by the
passage of a current of electricity through it.
The vessel containing the liquid is known as an _electrolytic cell._ In
fig. 89, A is the cell, which may be of glass or of any other suitable
material, and B is the liquid which is to be electrolyzed. Current enters
by the _positive electrode_ C, also known as the _anode_, traverses the
liquid, and leaves by the _negative electrode_, or _cathode_, D.
[Illustration: FIG. 90.--Modern apparatus for decomposing water by
electrolysis. Platinum electrodes P and P′ are placed at the bottom of two
upright tubes O and H, and are connected to the terminals T and T′ by
platinum wires, which are fused through the glass of the tubes. These
tubes have glass stop cocks S and S′ at their upper ends, and at their
lower ends are connected by a short glass tube, from the center of which
rises the large central tube which expands with a bulb at its upper end,
which is open at the top. The three tubes can be filled with acidulated
water from the central tube, the previously contained air being allowed to
escape through the stop cocks, which are afterwards closed. If it be so
filled, and the terminal T be attached to the positive and T′ to the
negative pole of a suitable battery, bubbles of gas will be observed to
rise from the plates P and P′, and finding their way to the top of the
respective tubes, will displace the liquid, which will be driven into the
open central tube. The gas rising from the anode P is oxygen (O), and that
rising from the cathode P′ is hydrogen (H). If the tubes are graduated,
the latter will be found to occupy about twice the volume of the former.
The proportion is theoretically 2 to 1; however, on account of the
different solubilities of the two gases in water, oxygen being the more
soluble of the two, is deficient in quantity.]
The passage of current through the water splits up its molecules into
their constituent atoms of oxygen and hydrogen, the former being given off
in bubbles at the anode, and the latter at the cathode.
When current is passed through a solution of copper sulphate between
platinum electrodes, the liquid is decomposed, atoms of copper being
deposited at the cathode, bubbles of oxygen being given off at the anode,
and sulphuric acid being formed in the liquid, which latter becomes more
and more acid as the copper is withdrawn.
[Illustration: FIG. 91.--Grotthuss’ theory of electrolysis. Grotthuss in
(1806), announced his theory that the molecules in an electrolyte have
their individual electro-positive and electro-negative atoms charged
positively and negatively respectively. In an ordinary liquid, for
instance in water, the molecules are arranged indifferently, like row 1,
with their positive and negative ends pointing in all directions. When the
charged plates A and B connected to the + and - poles of a battery are
inserted in the water, the molecules under the action of the laws of
electrostatic action turn as shown in row 2, so that all the hydrogen or
shaded ends (+) are turned towards the (-) plate B and all the oxygen or
unshaded ends (-) towards the (+) plate A. All along the row the
electrical forces are supposed to tear the molecules asunder, depositing H
on B and O on A. The atoms in the middle of the liquid, however,
recombine, for the hydrogen atoms in their journey towards B meet the
oxygen atoms travelling in the opposite direction, and we get the state of
affairs represented in row 3. The next step is to rotate once more the
atoms into the positions shown in row 2, and so on. In this way the theory
accounts for the products only appearing at the electrodes and not in the
body of the liquid.]
If, however, the anode be of copper instead of platinum, no sulphuric acid
will be formed, neither will oxygen be given off at the anode. As copper
is deposited at the cathode, an equal quantity will be dissolved at the
anode, so that the original constitution of the liquid is maintained.
The atoms separated from each other by the electric current were called
_ions_ by Faraday; those going to the anode being _anions_, and those
going to the cathode being _kathions_.
Anions are generally regarded as _electro-negative_, because they move as
if attracted to the positive electrode, while kathions are regarded as
_electro-positive_.
In order to explain the transfer of electricity and the transfer of matter
through the electrolyte, Grotthuss put forward the hypothesis that when
two metal plates at different potentials are placed in a cell, the effect
produced in the liquid is that the molecules of the liquid arrange
themselves in innumerable chains, as shown in fig. 91, in which every
molecule has its atoms pointing in a certain direction, the
electro-positive atom being attracted towards the cathode and the
electro-negative towards the anode. An interchange then takes place all
along the line, the free atoms appearing at the electrodes, and every atom
discharging a minute charge of electricity upon the electrode at which it
is liberated.
=Electro-chemical Series.=--This is an arrangement of the metals in a
series in such a manner that the most electro-positive is at one end and
the most electro-negative at the other.
The order of the metals varies with the electrolyte in which the metals
are tested.
The following table shows such series for the most common metals, in three
different solutions:
_Sulphuric acid._ _Hydrochloric acid._ _Caustic potash._
Zinc Zinc Zinc
Cadmium Cadmium Tin
Tin Tin Cadmium
Lead Lead Antimony
Iron Iron Lead
Nickel Copper Bismuth
Bismuth Bismuth Iron
Antimony Nickel Copper
Copper Silver Nickel
Silver Antimony Silver
Gold
Platinum
Faraday stated several laws of electrolysis, as follows:
1. _The quantity of an ion liberated in a given time is proportional to
the quantity of electricity that has passed through the voltameter[10] in
that time._
2. _The quantity of an ion liberated in a voltameter is proportional to
the electro-chemical equivalent of the ion._
3. _The quantity of an ion liberated is equal to the electro-chemical
equivalent of the ion multiplied by the total quantity of electricity that
has passed._
=Electric Osmose.=--Porret observed that if a strong current be led into
certain liquids as if to electrolyze them, a porous partition being placed
between the electrodes, the current mechanically carries part of the
liquid through the porous diaphragm, so that the liquid is forced to a
higher level on one side than on the other. This phenomenon is known as
_electric osmose_.
=Electric Distillation.=--Closely connected with the preceding phenomenon
is that of the _electric distillation of liquids_. It was noticed by
Beccaria that _an electrified liquid evaporates more rapidly than one not
electrified_.
Gernez has recently shown that in a bent closed tube, containing
two portions of liquid, one of which is made highly + and the
other highly -, the liquid passes over from + to -. This
apparent distillation is not due to difference of temperature,
nor does it depend on the extent of surface exposed, but is
effected by a slow creeping of the liquid along the interior
surface of the glass tubes. Bad conductors, such as turpentine,
do not thus pass over.
[Illustration: FIG. 92.--Effect of the electric current on a frog’s legs;
discovered in 1678 by Galvani.]
=Muscular Contractions.=--It was discovered in 1678 that when a portion of
muscle of a frog’s leg, hanging by a thread of nerve bound with a silver
wire, was held over a copper support so that both nerve and wire touched
the copper, the muscle immediately contracted.
More than a century later Galvani’s attention was drawn to the
subject by his observation of spasmodic contractions in the legs
of freshly killed frogs under the influence of the “return
shock” experienced every time a neighboring electric machine was
discharged.
The limbs of the frog, prepared as directed by Galvani, are
shown in fig. 92. After the animal has been killed the hind
limbs are detached and skinned; the crural nerves and their
attachments to the lumbar vertebrae remaining. For some hours
after death the limbs retain their contractile power. The frog’s
limbs thus prepared form an excessively delicate galvanoscope.
=Electroplating.=--This is the process of depositing a layer or coating of
a rarer metal upon the surface of a baser, or of a metal upon any
conducting surface, by electrolysis.
The electric current used may be obtained from a battery or other source.
The battery has its positive plate connected to a rod extending across a
trough or tank containing the plating bath.
Suspended from the rod are anodes of gold, silver, or copper or whatever
metal from which a deposit is desired. The other plates of the battery or
the negative elements, are connected with another rod across the trough,
to which are suspended the articles to be plated.
=Electrotyping.=--This is the process by which, type, wood cuts, etc., are
reproduced in copper by the process of electroplating. A mould is first
made of the set type in wax; this mould is next coated with black lead to
give it a metallic surface, as the wax is an insulator; the mould is then
subjected to the process of electro deposition, resulting in the formation
of a film of copper on the prepared surface.
The copper shell is removed from the mould by applying hot water; the
shell is then backed up with electrotype metal to render it strong enough
for use.
Almost all the illustrations in this book, for example, are printed from
electrotype copies, and not from the original wood blocks, which would not
wear so well.