The elements of the theory of magnetism have been based on Sir Wm.
Thomson's papers in the Philosophical Transactions; and the description of
the apparatus used in magnetic observatories has been drawn from the recently
published work of the Astronomer Royal. The account of electrical units
given in the Appendix is mainly founded on the Report of the Electrical
Committee of the British Association for the year 1863.
M. Deschanel's descriptions of apparatus, of which some very elaborate
examples occur in the present volume, left little to be desired in point of
clearness. In no instance has it been found necessary to resort to the mere
verbal rendering of unintelligible details.
CHAPTER XLII. ELECTRICAL INDUCTION.
CHAPTER XLIII. MEASUREMENT OF ELECTRICAL FORCES.
Coulomb's torsion-balance, 566. Repulsion inversely as square of distance, 567. Correc-
tions, and fallacious objections, 568. Attraction, 569. Force proportional to charge,
570. Electricity resides on external surface, proof-plane, 571. Limitations of the
rule, 572. Ice-pail experiment, 573. No force within a conductor; Faraday's cubical
box; Rigorous proof of law of inverse squares, 574. Electrical density and distribu-
tion, 575. Coulomb's experiments, 576. Alternate contact, 577. Density on points
and edges, power of points, 578. Dissipation of charge, 579,. ⚫ pp. 559-572.
CHAPTER XLIV. ELECTRICAL MACHINES.
Early history, 580. Ramsden's machine, 581. Limit of charge, 582. Nairne's machine,
583. Winter's, 584. Armstrong's hydro-electric, 585. Electrophorus, 586.
machine, 587. Voss machine, 588, .
Spark, 589. Brush; Crooked Spark; Polar tension, 590. Duration of Spark; Wheat-
stone's revolving mirror, 591. Discharge in rarefied gases, 592. Colour of spark,
Spangled tubes, 594. Shock; Tickling sensation, 595. Mechanical and
physical properties of the spark, 596. Chemical properties, 597. Wind from points,
598. Electric watering-pot, 599,
Force at a point due to attraction and repulsion, 600. Difference of potentials defined as
work, 601. Force is equal to rate of variation of potential, 602. Lines of force, 603.
Equipotential surfaces; Force inversely as distance between them, 604. Potential of
a conductor, 605. Energy of a charged conductor is QV; Gravitational analogy,
606. Tubes of force; Force inversely as section; Analogy of liquid flow, 607.
Force proportional to number of tubes per unit area, 608. Force just outside a
conductor is 4p, 609. Relation of induction to lines and tubes of force, 610.
Potential equal to sums of quotients of quantity by distance, 611. Potential of
CHAPTER XLVII. ELECTRICAL CONDENSERS.
Condensation; Collecting and condensing plate, 616. Calculation of capacity of con-
denser, 617. Discharge of condenser by jointed discharger, 618. Invention of
Leyden jar, 619. Leyden jar; Energy of its charge, 620. Residual charge; Frank-
lin's jar with movable coatings, 621. Discharge of insulated jar by alternate con-
tacts, 622. Condensing power, 623. Influence of the dielectric; Specific inductive
capacities, 624. Faraday's determinations, 625. Polarization of the dielectric; Its
doubly refracting property discovered by Dr. Kerr, 626. Two classes of condenser,
627. Condensers for galvanic electricity, 628. Volta's condensing electroscope, 629.
Leyden battery, 630. Lichtenberg's figures, 631. Charge by cascade, 632,
pp. 606-623.
CHAPTER XLVIII. EFFECTS PRODUCED BY THE DISCHARGE OF
CONDENSERS.
Shock to a number of persons, 633. Universal discharger; Heating of metallic threads,
634. Electric portrait, 635. Velocity of electricity; Watson's experiment; Wheat-
stone's determination; Trials with Atlantic cable, 636. Unit-jars of Lane and
Harris, 637. Puncture of card and of glass, 638. Firing mines and gun-cotton, 639,
Franklin's experiment, 646, 647. Duration of lightning tested by revolving sectors, 648.
Thunder, 649. Shock by influence, 650. Lightning conductors, 651, 652. Ordinary
electricity of the atmosphere, 653. Methods of obtaining indications; Arrow, point,
burning match, insulated ball, water jet, 654. Interpretation of indications. They
measure density of electricity on earth's surface. This is induced by electricity over-
head, 655. Results of observation; Automatic records at Kew; Eye observations at
Windsor, Nova Scotia; Brussels; Kreuznach, 656, 657. Conjectures regarding the
sources of atmospheric electricity; Relations between electricity and rain; Lord
Rayleigh's experiments with jets, 658. Volta's theory of hail, 659. Theories regard-
ing waterspouts, 660,
pp. 640-652.
Magnets, natural and artificial, 661. Force greatest at ends, middle portion neutral, 662.
Lines formed by filings, 663. Curve of force-intensity, 664. Magnetic needle;
Meridian, 665. Declination, 666. Dip, 667. Mutual action of poles, 668. Names
of poles, 669. Magnetic induction, 670. Polarity of broken pieces of a magnet, 671.
Two opposite (imaginary) magnetic fluids; Magnetic potential and lines of force, 672.
Direction of magnetization; Magnetic field, 673. Ideal simple magnet; Strength of
pole; Moment of magnet, 674. Compound magnet of uniform magnetization; In-
tensity of magnetization, 675. Magnetization and moment of actual magnets, 676,
pp. 653-663.
CHAPTER LII. EXPERIMENTAL DETAILS.
The earth's force simply directive, 677. Horizontal, vertical, and total intensities, 678.
Torsion-balance, 679. Declination theodolite, and declination magnet, 680. Dip
circles, 681. Measurement of horizontal intensity, 682. Bifilar magnetometer and
balance magnetometer, 683. Results of observation; Map showing magnetic merid-
ians and lines of equal dip, 684. The earth as a magnet; Biot's hypothesis, 685.
Changes of declination and dip, 686. Magnetic storms, 687. Ship's compass, 688.
Methods of magnetization; Consequent points; Lifting power, 689. Molecular
changes accompanying magnetization, 690. All bodies either paramagnetic or dia-
magnetic, 691. Reason of setting of soft-iron needle in a uniform field, 692. Fara-
day's apparatus, 693. Magneto-crystallic action, 694, .
Voltaic electricity, 695. Galvanic element, 696. Galvanic battery, 697. Galvani's dis-
coveries, 698. Voltaic pile, 699. Couronne de tasses, 700. Trough battery, 701.
Wollaston's battery, 702. Hare's deflagrator, 703. Polarization of plates, 704.
Daniell's battery, 705. Bunsen's battery, 706. Amalgamated zinc, 707. Dry pile;
Bohnenberger's electroscope, 708. Bichromate battery, 708A,
pp. 686-696.
Ersted's experiment, 709. Ampère's rule, 710.
711. Reaction of magnet on current, 712.
Sine galvanometer, 714. Tangent galvanometer, 715. Multiplier, 716. Differential
galvanometer, 717. Astatic needle, 718. Mirror galvanometer, 719. Calibration
of galvanometer, 720. General law for magnetic force due to a current, 721. Swing
due to instantaneous current, 722. The galvanometer a true measurer of current, 723.
Needle deflected by motion of a charged body, 724,
Electrolysis, 725. Transport of elements, 726. Discovery of Potassium, 727. Electrolysis
of salts, 728. Voltameters, 729. Quantitative laws of electrolysis, 730. Chemical
relations of electro-motive force, 731. Secondary batteries, 732. Faure's accumulator,
733. Applications of accumulators, 734. Electro-metallurgy, 735. Electro-gilding
and electro-plating, 736. Electrotype, 737. Applications of electrotype, 738.
Statement of Ohm's law, 739. Electro-motive force, 740. Resistance, 741. Rheostat,
742. Resistance coils, 743. Specific resistances, 744. Resistance of liquids, 745.
Choice of galvanometer, 746. Divided circuits, 747. Arrangement of cells in
battery, 748. Distribution of potential in voltaic circuit, 749. Measurement of
resistance of battery, 750. Measurement of electro-motive force of battery, 751. By
electrometer, 752. By Latimer Clark's potentiometer, 753. Simultaneous deter-
mination of electro-motive force and resistance, 754. Clark's standard cell, 755.
Wheatstone's bridge, 756. Loop test, 757. Conjugate branches, 758. Conjugate
branches when there are several batteries, 759. Investigation of condition of con-
jugateness, 760. Thomson's method for resistance of galvanometer, 761. Mance's
method for resistance of battery, 762, .
CHAPTER LVII. RELATIONS BETWEEN ELECTRICITY AND HEAT.
Heating of wires, 763. Joule's law, 764. Relation of heat in circuit to chemical action
in battery, 765. Distribution of heat in circuit, 766. Mechanical work done by
current, 767. Thermo-electric currents, 768, 769. Thermo-electric order, 770.
Source of thermo-electric force, .771. Thermo-electric diagram, 772. Pair, 773.
Neutral point, 774. General formula, 775. Circuit of three metals, 776. Nomen-
clature, 776A. Thermopiles, 777. Observation of temperature by two junctions,
778. Pyro-electricity, 779. Effect of light on selenium, 780, pp. 741-749*.
NOTE-Mathematical investigation relating to thermo-electricity, pp. 749*-751.
Electro-dynamics; Ampère's stand, 781. Three elementary laws, 782. Continuous rota-
tion produced by circular current, 783. Action of indefinite rectilinear current, 784,
785. Action upon rectangular current, 786. Sinuous currents, 787. Mutual action
of two elements of currents; Maxwell's rule; Magneto-electric rule, 788. Action of
the earth on currents, 789. Solenoids, 790. Dip of solenoid, 791. Solenoids behave
like magnets, 792. Astatic circuits, 793. Ampère's theory of magnetism, 794.
Rotation of magnet under influence of current, 795. Magnetization by currents, 796.
Electro-magnets, 797. Residual magnetism, 798,
CHAPTER LIX. INDUCTION OF CURRENTS.
Induced currents, 799. Produced by commencement or cessation, 800. By variations of
strength, 801. Of distance, 802. By movement of magnet or by change of its
strength, 803. Lenz's law, 804. Rule derived from lines of magnetic force, 805.
Quantitative rules based on tubes of force, 806. Deduced from principle of energy,
807. Superposition of tubes of force, 808. Uniform field, 809. Movement of tubes
of force with change of magnetization, 810. Unit of resistance, 811. Induction by
means of terrestrial magnetism, 812. British Association experiment, 813. Self-in-
duction, 814. Ruhmkorff's coil, 815. Its spark, 816. Discharge in rarefied gases, 817.
Experiments of Gassiot and De la Rue, 818. Action of magnets on the discharge,
819. Magneto-electric machines, 820. Clarke's, 821. Holmes and Alliance machines,
822. Siemens' armature, 823. Wilde's machine, 824. Dynamo-electric machines,
825. Ladd's, 826. Siemens', 827. Gramme's, 828. Wheatstone's telegraphic cur-
rents, 829. Arago's rotations, 830. Copper dampers and Faraday's cube, 831.
Electro-medical machines, 832. Lines of force involve no hypothesis, 833,
CHAPTER LX. ELECTRIC TELEGRAPHS.
History, 834. Batteries, 835. Wires; No return wire needed, 836. Single-needle tele-
graph, 837. Dial telegraphs; Breguet's, 838, 839. Alarum, 840. Wheatstone's
universal telegraph, 841. Morse's telegraph and alphabet, 842. Relay, 843. Hughes'
printing telegraph, 844. Bain's electro-chemical telegraph, 845. Casselli's autographic
telegraph, 846. Cowper's writing telegraph, 847. Submarine telegraphy; Retarda-
tion; Thomson's receiving instruments, 848. Wheatstone's automatic system with
specimen, 849. Duplex telegraphy, 850. Quadruplex telegraphy, 850A, pp. 806-831.
CHAPTER LXI. MISCELLANEOUS APPLICATIONS OF ELECTRICITY.
Electromotors, 851, 852. Quantitative relations, 853. Economy in transmission, 854.
Transformation of currents, 854A. Froment's engine, 855. Electric pen, 856.
trolled clocks, 857. Telephone, 858. Microphone, 859. Microphonic transmitters,
860. Induction balance, 861. Electric light; Behaviour of the carbons, 862. Char-
acter of the light and difficulties, 863. Foucault's regulator, 864. Jablochkoff's
system, 865. Incandescent lamps, 865A,
pp. 832-846.
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