quantity in coulombs = current in amperes × time in seconds, or simply
Again:
One ampere-hour is simply another way of saying 3,600 coulombs. Of course 3,600 coulombs of electricity may be obtained in any desired time. It all depends on the rate of flow or the current strength in amperes.
For instance, 2 amperes in 1⁄2 hour, or 4 amperes in 1⁄4 hour will also give one ampere-hour of 3,600 coulombs.
It is well to keep the distinction between coulombs and amperes in mind, as even in text books very lately published these units are confounded. To illustrate further the difference between coulombs and amperes, the following example is given.
It is sometimes estimated that the quantity of electricity in a flash of lightning is 1⁄10 coulomb, and the duration of the discharge 1⁄20000 part of a second. What is the current in amperes?
Now since
solving (1) for the current,
substituting the given values in (2),
Power.—The term power means the rate at which work is done; it is usually expressed as the number of foot pounds done in one minute, that is
Power exerted for a certain time produces work.
Ques. What is the mechanical unit of power?
Ans. The horse power.
Ques. What is one horse power?
Ans. 33,000 foot pounds per minute.
The unit is due to James Watt as being the power of a strong London draught horse to do work during a short interval and used by him to measure the power of his steam engines. One horse power = 33,000 ft. lbs. per minute = 550 ft. lbs. per sec. = 1,980,000 ft. lbs. per hour.
Ques. What is one horse power hour?
Ans. Work done at the rate of one horse power for one hour.
Ques. What is the electrical unit of power?
Ans. The watt.
Ques. What is a watt?
Ans. It is the power due to a current of one ampere flowing at a pressure of one volt. One watt = one ampere × one volt. It is equal to one joule per second.
Ques. What is a kilowatt?
Ans. 1,000 watts.
The Watt-Hour.—The elements which may be measured are, however, not only the volume of current, the unit of which is the ampere, and time, the unit of which is the hour, but also the pressure, the unit of which is the volt.
It is evident that a perfect system of electrical measurement should take account of the total amount of energy consumed, and should depend not only upon the volume of current, but also upon the pressure at which the current is applied.
The basis of such a system if provided in a unit which is the product of the two units of current and pressure, and which is termed a volt-ampere or watt.
The watt-hour represents the amount of work done by an electric current of one ampere strength flowing for one hour under a pressure of one volt.
EXAMPLE—An incandescent lamp taking one-half an ampere of current on a circuit having a pressure of 100 volts, or a lamp taking one ampere on a circuit having a pressure of 50 volts, would each be consuming 50 watts of energy, and this multiplied by the number of hours would give the total number of watt-hours for any definite time.
The watt, then, is an accurate and complete unit of measurement and is generally applicable to all forms of electrical consumption.
A watt of electrical energy corresponds to 1⁄746 of a horse power of mechanical energy; hence, if a lamp or motor require energy equivalent to 1⁄746 of a horse power for one hour, it might be said to take one watt-hour.
Mechanical Equivalent of Heat.—The eminent English physicist, James Prescott Joule, worked for more than forty years in establishing the relation between heat and mechanical work; he stated the doctrine of the conservation of energy and discovered the law, known as Joule’s law, for determining the relation between the heat, current pressure, and time in an electric circuit.
Ques. What is heat?
Ans. A form of energy.
Heat is produced in the agitation of the molecules of matter—the energy expended in agitating these molecules is transformed into heat.
Ques. How is heat measured?
Ans. In British thermal units (B.t.u.).
Ques. What is the British thermal unit?
Ans. The quantity of heat required to raise the temperature of 1 lb. of pure water 1° Fahr., at or near 39.1° F., the temperature of maximum density.
Ques. What is the mechanical equivalent of heat?
Ans. The number of foot pounds of mechanical energy equivalent to one British thermal unit.
Joule’s experiments 1843-50 gave the figure 772 ft. lbs. which is known as Joule’s equivalent. Later experiments gave higher figures, and the present accepted value is 778 ft. lbs., that is: 1 B.t.u. = 778 ft. lbs.
Electrical Horse Power.—It is desirable to establish the relation between watts and foot pounds in order to determine the capacity of an electric generator or motor in terms of horse power.
One watt is equivalent to one joule per second or 60 joules per minute. One joule in turn, is equivalent to .7374 ft. lbs., hence 60 joules equal:
Since one horse power = 33000 ft. lbs. per minute, the electrical equivalent of one horse power is
or,
Again, one kilowatt or 1000 watts is equivalent to
The Farad.—The measure constructed to hold a gallon of water may be called the gallon measure. The capacity of a condenser which would contain a charge of one coulomb under one volt pressure is the farad. It may seem strange that there is a unit of quantity and another of capacity to hold that quantity, when in the case of water the term “gallon” may suffice for the measure and the liquid it can hold. Electricity in this respect, however, corresponds to a compressible fluid or a gas.
A gallon measure may hold a gallon of gas or ten; it depends entirely upon the pressure. So a condenser of a certain size may hold any number of coulombs, according to the electrical pressure.
The farad being inconveniently large for practical use, one-millionth of a farad, called a microfarad, is generally adopted.
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:
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.
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.
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:
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,
The total number of calories H developed in t seconds will be given by
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):
Since by Ohm’s Law potential difference = I × R substituting IR for P.D. in (2)
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.
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.
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.
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.
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.
Magnetism.—The ancients applied the word “magnet,” magnes lapes, to certain hard black stones which possess the property of attracting small pieces of iron, and as discovered later, to have the still more remarkable property of pointing north and south when hung up by a string. At this time the magnet received the name of lodestone or “leading stone.” It is commonly, though incorrectly, spelled loadstone.
Ques. Describe two kinds of magnetism.
Ans. Magnets have two opposite kinds of magnetism or magnetic poles, which attract or repel each other in much the same way as would two opposite kinds of electrification.
Ques. What is the nature of each kind of magnetism?
Ans. One has a tendency to move toward the north and the other toward the south.
Ques. Where is the magnetism the strongest?
Ans. In two regions called the poles.
Ques. Describe the distribution of magnetism in a long shaped magnet.
Ans. The strongest magnetism resides in the ends, while all around the magnet half way between the poles there is no attraction at all.
Ques. How are the poles designated?
Ans. They are called the north pole and the south pole.
Ques. What is the distinguishing feature of each?
Ans. The north pole points approximately to the earth’s geographical north, while the south pole of a magnet points approximately to the earth’s geographical south.
The north pole is the positive (+) pole and the south pole is the negative. The north and south poles were formerly called in France, the austral and boreal poles respectively.
Magnetic Field.—When a straight bar magnet is held under a piece of card board upon which iron filings are sprinkled, the filings will arrange themselves in curved lines radiating from the poles. If a horse shoe magnet be held at right angles to the plane of the card board, the filings will arrange themselves in curved lines, as shown in fig. 108. These lines are called magnetic lines of force or simply lines of force; they show that the medium surrounding a magnet is in a state of stress, the space so affected being called the magnetic field.
Ques. What is the extent and character of the magnetic field?
Ans. The influence of a magnet is supposed to extend in all directions indefinitely, however, the effect is very slight beyond a comparatively limited area.
Magnetic Force.—This is the force with which a magnet attracts or repels another magnet or any piece of iron or steel. The force varies with the distance, being greater when the magnet is nearer and less when the magnet is farther off. The following are the laws relating to magnetic force:
1. Like magnetic poles repel one another; unlike magnetic poles attract one another.
2. The force exerted between two magnetic poles varies inversely as the square of the distance between them.
Magnetic Circuit.—The path taken by magnetic lines of force is called a magnetic circuit; the greater part of such a circuit is usually in magnetic material, but there are often one or more air gaps included. The total number of lines of force in the circuit is known as the magnetic flux.
Ques. How is magnetic flux measured?
Ans. By a unit called the maxwell.
Named after James Clerk Maxwell the Scottish physicist.
Ques. What is the maxwell?
Ans. The amount of magnetism passing through every square centimetre of a field of unit density.
Ques. What is the unit of field strength?
Ans. The gauss.
Ques. What is a gauss?
Ans. The intensity of field which acts on a unit pole with a force of one dyne. It is equal to one line of force per square centimetre. Named after Karl Friedrich Gauss, the German mathematician.
The Magnetic Effect of the Current.—Hans Christian Oerstead, the Danish scientist, discovered in 1819 that a magnet tends to set itself at right angles to a wire carrying an electric current. He also found that the way in which the needle turns, whether to the right or left of its usual position, depends: 1, upon the position of the wire that carries the current, whether it be above or below the needle, and 2, on the direction in which the current flows through the wire.