THE CHEMICAL AND HEAT EFFECTS OF ELECTRIC CURRENTS
(1) THE CHEMICAL EFFECT OF AN ELECTRIC CURRENT
=280. Electroplating.=--If two carbon rods (electric light carbons
answer very well) are placed in a solution of _copper sulphate_ (Fig.
260) and then connected by wires to the binding posts of an electric
battery, one of the rods soon becomes covered with a coating of
_metallic copper_ while bubbles of gas may be seen upon the other
carbon. If a solution of _lead acetate_ is used in the same way a
deposit of _metallic lead_ is secured, while a solution of _silver
nitrate_ gives silver.
[Illustration: FIG. 260.--Two carbons placed in a solution of copper
sulphate.]
[Illustration: FIG. 261.--An electroplating bath.]
This process of depositing metals upon the surface of solids by an
electric current is called _electroplating_. Everyone has seen
_electroplated_ articles such as silver plated knives, forks, and
spoons, and nickel-plated rods, handles, etc. _Copper electrotype_
plates such as are used in printing school books are made by this
process. In practical electroplating a solution of the metal to be
deposited is placed in a tank; across the top of this tank are placed
copper rods to act as conducting supports. From one of these rods, the
cathode, objects to be plated are hung so as to be immersed in the
liquid. From other rods, the anodes, are hung plates of the metal to be
deposited. These are dissolved as the current deposits a coating upon
the articles, thus keeping the solution up to its proper strength. (See
Fig. 261.)
[Illustration: FIG. 262.--The current is carried through the solution by
ions.]
=281. Electrolysis.=--A solution from which a deposit is made by an
electric current is called an _electrolyte_. The plates or other objects
by which the current enters or leaves the electrolyte are called the
_electrodes_. The electrode by which the current enters is called the
_anode_ (_an_ = in) while the electrode by which it leaves is the
cathode (_cath_ = away). The process by which an electric current
decomposes a solution and deposits a substance upon an electrode is
called _electrolysis_. The _current_ always flows within the cell from
_anode to the cathode_. (See Fig. 262.) The metal goes with the current
and is found deposited upon the cathode.
=282. Theory of Electrolysis.=--The action going on in an _electrolytic_
cell has been carefully studied. The _theory of electrolysis_, which is
supported by much experimental evidence, supposes that many of the
molecules in a _dilute_ solution of a substance "split up" into two
parts called "ions," one ion having a positive, the other a negative
charge. In a dilute solution of sulphuric acid, the _positive_ ion is of
hydrogen, while the _negative_ ion is the (SO_{4}) or sulphion. These
ions bearing electric charges are believed to be the _carriers of the
electric current_ through the electrolyte.
The positive ions move with the current from the anode to the cathode,
while the negative ions apparently are repelled by the cathode and
appear upon the anode. Evidence of the accumulation of the two kinds of
ions at the two electrodes is furnished by the _electrolysis_ of water,
described below.
=283. Electrolysis of Water.=--Two glass tubes (Fig. 263), _H_ and _O_,
are attached at the bottom to a horizontal glass tube. To the latter is
also connected an upright tube _T_. At the lower ends of _H_ and _O_ are
inserted, fused in the glass, platinum wires, _A_ and _C_. The tubes are
filled with a weak solution of sulphuric acid. The tops of _H_ and _O_
are closed with stopcocks, _T_ being open; a current of electricity is
sent in at _A_ and out at _C_. A movement of the ions at once begins,
the positive hydrogen ions appearing at _C_. These accumulate as bubbles
of hydrogen which rise to the top of _H_ and displace the liquid. At the
same time bubbles of oxygen appear at _A_. These rise in _O_ and also
displace the liquid which rises in _T_. After the action has continued
some time it may be noticed that the volume of hydrogen is just twice
that of the oxygen. This was to have been expected since the formula
for water is H_{2}O. The nature of the gas in _H_ or _O_ may be tested
by opening the stopcock and allowing the gas to escape slowly. The
hydrogen gas can be lighted by a flame while the oxygen gas will cause a
spark upon a piece of wood to glow brightly, but does not burn itself.
[Illustration: FIG. 263.-Electrolysis of water; oxygen collects in _O_,
hydrogen in _H_.]
=284. Evidence that ions are necessary to conduct a current in a liquid=
is furnished by the following experiment. A quart jar is carefully
cleaned, and half filled with distilled water. Two pieces of zinc 5 cm.
square are soldered to pieces of rubber-insulated No. 14 copper wire.
The zincs are placed in the distilled water (Fig. 264) and the wires are
connected to a 110 volt circuit with a 16 candle-power incandescent lamp
in _series_ with the cell, as in the figure. If the zincs have been
carefully cleansed and the water is pure, no current flows as is shown
by the lamp remaining dark. If a minute quantity of sulphuric acid or of
common salt is placed in the water the lamp at once begins to glow. Ions
are now present in the liquid and conduct the current. That some
substances in solution do not form ions may be shown by adding to
another jar of pure water some glycerine and some cane sugar, substances
resembling the acid and salt in external appearance but which do not
_ionize_ when dissolved as is shown by the lamp remaining dark after
adding the glycerine and sugar. The acid and salt are of _mineral_
origin while the glycerine and sugar are _vegetable_ products. This
experiment illustrates the principle that the water will conduct only
when it contains ions.
[Illustration: FIG. 264.--The current passes only when ions are present
in the liquid.]
=285. Laws of Electrolysis.=--These were discovered by Faraday in 1833,
and may be stated as follows: _I. The mass of a substance deposited by
an electric current from an electrolyte is proportional to the intensity
of current which passes through it._
_II. The mass of any substance deposited by a current of uniform
intensity is directly proportional to the time the current flows._
These laws have been used as a basis for defining and measuring the unit
of current flow, the _ampere_. (See Art. 264.)
=286. Instances of Electrolysis.=--(a) Medicines, especially those
containing a mineral substance, are sometimes introduced into the human
body by electrolysis. (b) Water and gas pipes are sometimes much
weakened by the effects of electric currents in the earth, especially
return currents from street railways. Such currents use the metal pipes
as a conductor. At the place where the current leaves the metal and
enters the ground, it removes metallic ions from the pipe. This process
continuing, the pipe becomes weakened and at length breaks. (c) _Copper_
is purified by the use of electric currents that remove the copper from
ore or from other metals and deposit it upon electrodes. _Electrolytic_
copper is the purest known. (d) _Aluminum_ is obtained by the use of
large currents of electricity that first heat the material used until it
melts and then deposit the metal from the fluid material by
electrolysis. These results are called chemical effects of the current
since by the use of electric currents substances are changed chemically,
that is, they are separated into different chemical substances.
Important Topics
1. Electrolysis, electroplating, anode, cathode, ion.
2. Theory of electrolysis--evidence: (a) electrolysis of water; (b)
conductivity of acid and water.
3. Laws of electrolysis.
4. Practical use of electrolysis.
Exercises
1. A dynamo has an E.M.F. of 10 volts. What is the resistance in the
circuit when 20 amperes are flowing?
2. How much silver will be deposited in an hour by this current?
3. Name five objects outside of the laboratory that have been acted upon
by electrolysis. How in each case?
4. Why is table ware silver plated? Why are many iron objects nickel
plated?
5. How is the electrolysis of water pipes prevented?
6. Two grams of silver are to be deposited on a spoon by a current of 1
ampere. Find the time required.
7. How long will it take to deposit 20 g. of silver in an electroplating
bath if a current of 20 amperes is used?
8. If 1000 g. of silver are deposited on the cathode of an electrolytic
reduction plant in 10 minutes, what is the current intensity employed?
(2) THE STORAGE BATTERY AND ELECTRIC POWER
=287. Differences Between Voltaic and Storage Cells.= Voltaic cells in
which electric currents are produced by the chemical action between
metal plates and an electrolyte are often called _primary batteries_. In
voltaic cells one or both plates and the electrolyte are used up or lose
their chemical energy in producing the current and after a time need to
be replaced by new material, the _chemical energy_ of the electrolyte
and of one of the plates having been _transformed_ into electrical
energy.
A different proceeding obtains with another type of cell. This is
called a _storage battery_, or an accumulator. In these cells, the same
_plates_ and electrolyte are _used_ without change _for extended
periods_, sometimes for a number of years. For this reason storage
batteries have displaced many other types of cells, and they are now
used (a) to operate many telephone, telegraph, and fire-alarm circuits,
(b) to work the spark coils of gas and gasoline engines, (c) to help
carry the "peak" load upon lighting and power circuits and (d) to
furnish power for electric automobiles. Since a storage battery can
deliver an electric current only after an electric current from an
outside source has first been sent through it, they are often called
_secondary batteries_.
=288. Construction and Action of a Storage Cell.=--The common type of
storage cells consists of a _number of perforated_ plates made of an
alloy of lead and a little antimony. (See Figs. 265, 266, 267.) Into the
perforations is pressed a paste of red lead and litharge mixed with
sulphuric acid. The plates are placed in a strong solution (20 to 25 per
cent.) of sulphuric acid. The plates are now ready to be charged. This
is accomplished by sending a direct current from an electric generator
through the cell. The hydrogen ions are moved by the current to one set
of plates and change the paste to _spongy_ metallic lead. The sulphions
move to the other set of plates and change the paste to lead oxide. This
electrolytic action causes the two plates to become quite different
chemically so that when the cell is fully charged it is like a voltaic
cell, in having plates that are different chemically. It has, when fully
charged, an E.M.F. of about 2.2 volts. The several plates of a cell
being in parallel and close together, the cell has but small internal
resistance. Consequently a large current is available.
[Illustration: FIG. 265.--The positive plate of a storage cell.]
[Illustration: FIG. 266.--The negative plate of a storage cell.]
[Illustration: FIG. 267.--A complete storage cell.]
[Illustration: FIG. 268.]
About 75 per cent. of the energy put into the storage cell in charging
can be obtained upon _discharging_. Therefore the _efficiency_ of a good
storage cell is about 75 per cent. Fig. 268 represents a storage battery
connected to charging and discharging circuits. The lower is the
charging circuit. It contains a dynamo and a resistance (neither of
which are shown in the figure) to control the current sent into the
cell. The charging current enters the positive pole and leaves by the
negative pole. The current produced by the cell, however, flows in the
_opposite_ direction through it, that is, out from the positive and in
at the negative pole. This current may be controlled by a suitable
resistance and measured by an ammeter. Storage cells have several
advantages: (a) They can be charged and discharged a great many times
before the material placed in the perforations in the plates falls out.
(b) The electrical energy used in charging the plates _costs less_ than
the plates and electrolyte of voltaic cells. (c) Charging storage cells
takes much _less labor_ than replacing the electrolyte and plates of
voltaic cells. (d) Storage cells produce _larger currents_ than voltaic
cells. The two principal _disadvantages_ of storage cells are that (a)
they are _very heavy_, and (b) their initial _cost_ is _considerable_.
[Illustration: FIG. 269.--The Edison storage cell.]
[Illustration: FIG. 270.--The plates of the Edison storage cell.]
=289. The Edison storage cell= (Figs. 269 and 270) has plates of iron
and nickel oxide. The electrolyte is a strong solution of potassium
hydroxide. These cells are lighter than lead cells of the same capacity
and they are claimed to have a longer life.
=290. Energy and Power of a Storage Cell.=--In a storage cell, the
electrical energy of the charging current is transformed into _chemical_
energy by the action of electrolysis. It is this chemical energy that is
transformed into the energy of the electric current when the cell is
discharged. The _capacity_ of storage cells is rated in "ampere hours,"
a 40 ampere hour cell being capable of producing a current of 1 ampere
for 40 hours, or 5 amperes for 8 hours, etc. The production and
extensive use of electric currents have made necessary accurate methods
for measuring the _energy_ and _power_ of these currents. To illustrate
how this is accomplished, let us imagine an electric circuit as
represented in Fig. 268. Here four storage cells in series have an
E.M.F. of 8 volts and in accordance with Ohm's law produce a current of
2 amperes through a resistance in the circuit of 4 ohms. Now the work
done or energy expended by the current in passing through the resistance
between the points _M_ and _N depends_ upon three factors (1) the E.M.F.
or _potential difference_; (2) the _current intensity_ and (3) the
_time_. The energy is measured by their product. That is, _electrical
energy_ = _potential difference_ × _current intensity_ × _time_. This
represents the electrical energy in _joules_, or
Joules = volts × amperes × seconds, or
_j_ = _E_ × _I_ × _t_.
In the circuit represented in Fig. 268 the energy expended between the
points _M_ and _N_ in 1 minute (60 seconds) is 8 × 2 × 60 = 960 joules.
=291. Electric Power.=--Since power refers to the _time rate_ at which
work is done or energy expended, it may be computed by dividing the
electrical energy by the time, or the _electrical power_ = _volts_ ×
_amperes_. The power of 1 joule per second is called a _watt_.
Therefore,
Watts = volts × amperes, or
Watts = _E_ × _I_.
Other units of power are the _kilowatt_ = 1000 watts and the
_horse-power_ = 746 watts. In the example given in Art. 290 the power of
the current is 8 × 2 = 16 watts, or if the energy of the current
expended between the joints _M_ and _N_ were converted into mechanical
horse-power it would equal 16/746 of a horse-power. Electrical energy is
usually sold by the _kilowatt-hour_, or the amount of electrical energy
that would exert a power of 1000 watts for one hour, or of 100 watts for
10 hours, or of 50 watts for 20 hours, etc.
Important Topics
1. The storage battery, its construction, electrolyte, action, uses,
advantages, disadvantages.
2. Electric energy, unit value, how computed?
3. Electric power, three units, value, how computed, how sold?
Exercises
1. In what three respects are voltaic and storage cells alike? In what
two ways different?
2. Name the four advantages of storage cells in the order of their
importance. Give your reasons for choosing this order.
3. Why are dry cells more suitable for operating a door-bell circuit,
than a storage battery? Give two reasons.
4. The current for a city telephone system is provided by a storage
battery. Why is this better than dry cells at each telephone?
5. An incandescent lamp takes 0.5 ampere at 110 volts. What power is
required to operate it? How much _energy_ will it transform in 1 minute?
6. How long would it take for this lamp to use a kilowatt hour of
energy?
7. A street car used 100 amperes at 600 volts pressure. What power was
delivered to it? Express also in kilowatts and horse-power.
8. An electric toaster takes 5 amperes at 110 volts. If it toasts a
slice of bread in 2 minutes, what is the cost at 10 cents a kilowatt
hour?
9. An electric flat iron takes 5 amperes at 110 volts. Find the cost of
using it for 2 hours at 12 cents a kilowatt hour.
10. A 1/4 kilowatt motor is used to run a washing-machine for 5 hours.
What is the expense for this power at 10 cents a kilowatt hour?
11. What is the efficiency of a motor that takes 7390 watts and develops
9 horse-power?
12. How many horse-power are there in a water-fall 212 ft. high over
which flows 800 cu. ft. of water per second? Express this power in
kilowatts.
13. What horse-power must be applied to a dynamo having an efficiency of
go per cent. if it is to light 20 arc lamps in series, each taking 10
amperes at 60 volts?
(3) THE HEAT EFFECT OF ELECTRIC CURRENTS
=292. The Production of Heat by an Electric Current.=--When no chemical
or mechanical work is done by an electric current its energy is employed
in overcoming the resistance of the conducting circuit and is
transformed into _heat_. This effect has many practical applications and
some disadvantages. Many devices employ the heating effect of electric
currents, (a) the electric furnace, (b) electric lights, (c) heating
coils for street cars, (d) devices about the home, as flat irons,
toasters, etc. Sometimes the heat produced by an electric current in the
wires of a device such as a transformer is so large in amount that
especial means of cooling are employed. Unusually heavy currents have
been known to melt the conducting wires of circuits and electrical
devices. Hence all circuits for electric power as well as many others
that ordinarily carry small currents are protected by _fuses_. An
_electric fuse_ is a short piece of wire that will melt and break the
circuit if the current exceeds a determined value. The fuse wire is
usually enclosed in an incombustible holder. Fuse wire is frequently
made of lead or of an alloy of lead and other easily fusible metals.
(See Figs. 271 and 272.)
[Illustration: FIG. 271.--A type of enclosed fuse.]
[Illustration: FIG. 272.--A link fuse (above); plug fuses (below).]
=293. Heat Developed in a Conductor.=--A rule for computing the amount
of heat produced in an electric circuit by a given current has been
accurately determined by experiment. It has been found that 1 _calorie_
of heat (Art. 142), is produced by an expenditure of 4.2 joules of
electrical (or other) energy. In other words, 1 joule will produce 1/4.2
or 0.24 calorie. Now the number of joules of electrical energy in an
electric circuit is expressed by the following formula:
Joules = volts × amperes × seconds, or since 1 joule = 0.24 calorie,
Calories = volts × amperes × seconds × 0.24 or
_H_ = _EI_ × _t_ × 0.24 (1)
By Ohm's law, _I_ = _E_/_R_ or _E_ = _I_ × _R_, substituting in equation
(1) _IR_ for its equal _E_ we have
_H_ = _I²R_ × _t_ × 0.24 (2)
Also since _I_ = _E_/_R_ substitute _E_/_R_ for _I_ in equation (1) and
we have
_H_ = _E²_/_R_ _t_ × 0.24 (3)
To illustrate the use of these formulas by a problem suppose that a
current of 10 amperes is flowing in a circuit having a resistance of 11
ohms, for 1 minute. The heat produced will be by formula (2) = (10)² ×
11 × 60 × 0.24 equals 15,840 calories.
[Illustration: FIG. 273.--A carbon filament incandescent lamp.]
[Illustration: FIG. 274.--A tungsten lamp.]
=294. The Incandescent Lamp.=--One of the most common devices employing
the heat effect of an electric current is the _incandescent lamp_. (See
Fig. 273.) In this lamp the current is sent through a carbon filament,
which is heated to incandescence. In order to keep the filament from
burning as well as to prevent loss of heat by convection, it is placed
in a glass bulb from which the air is exhausted. Two platinum wires
fused in the glass connect the carbon filament with the grooved rim and
the end piece of the base. The end piece and rim connect with the socket
so that an electric current may flow through the filament of the lamp.
The carbon incandescent lamp has a low efficiency. It takes 0.5 ampere
of current at 110 volts or in other words it requires 55 watts to cause
a 16-candle-power lamp to glow brightly, hence 1 candle power in this
lamp takes 55/16 = 3.43 watts.
The _efficiency of electric lamps_ is measured by the _number of watts
per candle power_. This is a peculiar use of the term efficiency, as the
larger the number the less efficient is the lamp. More efficient lamps
have been devised with filaments of the metals _tantalum_ and _tungsten_
(Fig. 274). These give a whiter light than do carbon lamps, and consume
but about 1.25 watts per candle power.
COMPARATIVE "EFFICIENCY" OF ELECTRIC LAMPS
------------------+----------+----------------+------------
|Watts per | | Watts per
Name of lamp | candle | Name of lamp | candle
| power | | power
------------------+----------+----------------+------------
Carbon filament | 3 to 4 |Arc lamp | 0.5 to 0.8
Metallized carbon | 2.5 |Mercury arc | 0.6
Tantalum | 2.0 |Flaming arc | 0.4
Tungsten | 1.0 to |Nitrogen-filled | 0.6 to 0.7
| 1.5 | tungsten |
------------------+----------+----------------+------------
Incandescent lamps are connected in parallel (see Fig. 254) to wires
that are kept at a constant difference of potential of 110 or 115 volts.
It is customary to place not more than twelve lamps upon one circuit,
each circuit being protected by a fuse and controlled by one or more
switches.
=295. The Arc Light.=--The electric _arc_ light (see Fig. 275) is
extensively used for lighting large rooms, also in stereopticons and
motion picture machines. The light is intense, varying from 500 to 1700
candle power. The so-called mean spherical candle power of the arc light
is about 510. The candle power in the direction of greatest intensity is
about 1200. It is produced at an expenditure of about 500 watts. It is
therefore more efficient than the incandescent lamp, often taking less
than 0.5 watt per candle power produced. The arc light was first devised
by Sir Humphrey Davy in 1809, who used two pieces of charcoal connected
to 2000 voltaic cells. The arc light requires so much power that its
production by voltaic cells is very expensive. Consequently it did not
come into common use until the dynamo had been perfected. Fig. 276 shows
the appearance of the two carbons in an arc light. If a direct current
is used the positive carbon is heated more intensely, and gives out the
greater part of the light. The positive carbon is consumed about twice
as fast as the negative and its end is concave, the negative remaining
pointed.
[Illustration: FIG. 275.--An electric arc light.]
[Illustration: FIG. 276.--The appearance of a pair of used carbons.]
With alternating currents, the rods are equally consumed and produce
equal amounts of light. In the stereopticon, the carbons are usually
placed at right angles as in Fig. 277. In the stereopticon as well as
in outdoor lighting the direct current is more effective, although the
alternating current is often used, since the latter can be produced and
distributed more cheaply than can direct currents. In arc lamps, placing
an inner glass globe (Fig. 278) about the carbons, decreases the
consumption of the carbons materially. The carbon rods of _enclosed_ arc
lamps often last 60 to 100 hours.
[Illustration: FIG. 277.--A right-angle electric arc lamp for a
stereopticon.]
[Illustration: FIG. 278.--An enclosed arc lamp.]
The reason why an _open_ arc lamp needs to be "retrimmed" oftener
than the _enclosed_ lamp, that is, have new carbons placed in it,
is because the carbons "burn" freely, that is unite with the oxygen
of the air. In the enclosed arc lamp, the supply of oxygen in the
inner globe is limited and is soon consumed, therefore the carbons
last many times longer in such lamps.
Some carbon rods have soft cores containing calcium salts. These
vaporize in the arc producing the _flaming arc light_ of a bright yellow
color, and give more light than the ordinary lamp.
Important Topics
1. Heat effects of electric currents, uses and applications.
2. Computation of the heat developed in a circuit. Three formulas.
3. Electric lamps; incandescent and arc; construction, uses,
efficiency.
Exercises
1. Sketch a circuit containing 10 incandescent lamps in parallel. If
each lamp when hot has a resistance of 220 ohms, and the E.M.F. is 100
volts, what current will flow?
2. What will it cost to use these lights for 3 hours a day for 30 days
at 10 cents a kilowatt hour?
3. How much heat will these lamps produce per minute?
4. How could you connect 110-volt lamps to a street car circuit of 660
volts? Explain this arrangement and draw a diagram.
5. A certain arc lamp required 10 amperes of current at 45 volts
pressure. What would it cost at 10 cents per kilowatt hour if used 3
hours a day for 30 days?
6. Show a diagram of 3 arc lamps in series. If each takes 45 volts and
10 amperes, how much E.M.F. and current will they require?
7. If an electric toaster uses 5 amperes at 115 volts, how much heat
will this develop in half an hour?
9. How much heat is developed in an electric toaster in 2 minutes, if it
uses 5 amperes at 100 volts?
10. How many B.t.u.'s are given off in an electric oven that takes 10
amperes at 110 volts for 1 hour? (1 B.t.u. equals 252 calories.)
11. An electric heater supplies heat at the rate of 700 B.t.u.'s an
hour. How much power does it require?
12. How many watts are required to operate 120 incandescent lamps in
parallel if each takes 0.5 amperes at 110 volts?
13. An electric lamp takes 12 amperes at a P.D. of 110 volts. How many
B.t.u.'s are radiated from it each second? How many calories?
14. If a 110-volt incandescent lamp is submerged for 10 minutes in 400
gr. of cold water while a current of 0.5 amperes is flowing, how many
degrees centigrade will the water be warmed?
15. In an electric furnace a current of 3000 amperes is used at a P.D.
of 10 volts. Find the heat developed in 1 minute.
16. How many candle power should a 20-watt tungsten lamp give if its
efficiency is one watt per candle power?
17. What is the "efficiency" of a 40-watt tungsten lamp if it gives 34
candle power?
Review Outline: Current Electricity
Produced by--Chemical action; voltaic and storage cells.
Three { Magnetic, electromagnet, uses and applications.
Principal { Chemical, electrolysis, applications.
Effects: { Heat, lighting and heating devices.
Theories: (a) of voltaic cells, (b) of electrolysis.
Units: Ampere, ohm, volt, watt, joule, kilowatt, horse power.
Measurement--(a) magnetic effect; galvanometer, ammeter, voltmeter,
wattmeter, Wheatstone bridge, construction
and use.
(b) chemical effect; voltameter.
Laws: (a) Right hand rules, for conductor and helix.
(b) Resistance, Conductors in series and parallel.
(c) Ohm's law, heat law, power law, 3 forms for each.
(d) Cells in parallel and series.
Problems: Upon applications of the laws and formulas studied.
Devices. { Voltaic cells; wet, dry, and Daniell.
and { Electrolysis and the storage battery.
Instruments: { Measuring instruments, electric bell, sounder,
{ heating and lighting devices.
Terms: Anode, cathode, electrolyte, ion, circuit switch, current,
e.m.f., resistance, potential.