TABLE IV.-ELECTRO-MAGNETISM.
Given a battery, a helix of wire, and a bar of soft iron, then
(1.) The amount of magnetism varies with the strength of the current. (2.) The amount of magnetism is independent of the thickness of the
(3.) The amount of magnetism is practically independent of the diameter of the coil.
(4.) The amount of magnetism varies with the diameter of the soft-iron rod.
(5.) The attractive force of electro-magnets is proportional to the square of the force of the current by which they are magnetised.
TABLE V.-ATTRACTION OF MAGNETS.
(1.) A horse-shoe soft-iron bar, 131⁄2 inches across the poles, diameter of pole 24 inches, with 1690 feet of copper wire in ten equal helices, excited by a battery with one vol. of SO, in 50 of water.
In 1838, a boat 28 feet long, 73 feet wide, drawing 23 feet of water, and carrying 14 persons, was propelled at 1 mile per hour by an electromagnetic machine, excited by 320 pairs of plates, each of 36 square inches, excited by sulphate of copper.
In 1839, the same boat was propelled at 3 miles per hour by an improved machine, excited by 64 plates, each 36 square inches, excited by sulphuric and nitric acids.
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Centigrade thermometric scale, 66
Chemical action, heat from, 62; of
light, 123; of electricity, 137
Chemical analysis by galvanism, 184
Chemical electricity, 177
Cleavage, effect of, on electricity, 138
Coal, a convenient fuel, 359
Coil, induction, 206
Cold, convection of, 341
Colour, 116; spectrum of, 119; from
absorption, 277; of polarised light,
326
Coloured light from white, 97, 116
Combustion, nature of, 358
Commutators, 218, 259
Compass, declination of, 235
Compound echoes, 26
Compression, effect of, on magnetic
polarity, 255
Condenser, 174
Conduction of heat, 71; of elec-
tricity, 139, 153; connected with
convection and radiation, 274;
effects of, 333; varieties of, 334
connected with specific heat, 336 ;
nature of, 337
Conservation of force in sound, 35
Constant battery, 183
Continuance of sound, 40
Continuous current derived from in-
duction, 259
Convection of heat, 71, 80; connected
with conduction and radiation,
274; causes of, 340; examples of,
340; nature of, 340; of cold, 341
Currents, secondary, 205; tertiary,
208; reversal of, 218; continuous,
derived from induction, 259
Curved polarisation, 325
Darkness produced from light, 304
Daniell's battery, 183
Declination of a compass, 235; line
of no, 235
Decomposition of light by a prism,
97, 116, 309
Definition of a magnet, 230; of tem-
perature, 66
Deflection of a magnet, 234
Density, effect of, on velocity, 33
Dia-magnetic substances, 249
Dia-magnetism, 248; a polar force,
250; of liquids, 252; of gases,
253; nature of, 253; causes affect-
ing, 253
Differential thermometer, 291
Diffusion of sound, 22
Dip of a magnet, 234
Discharge, universal, 173
Dispersion, 303; by refraction, 313
Double sounds, 18; by refraction, 99
Drum of ear, 53
Duration of a sound, 53
Dynamical electricity, 177
Dynamical theory of heat, 69
Ear, description of, 53
Earth, a conductor of electricity,
146; magnetism by, 229
Earth currents, 216
Echo, 18; at Enville, 26; com-
pound, 26
Effects of heat, 60; of electricity,
135; of galvanism, 184; of mag-
netism, 227; of conduction, 333
Electric spark, 141; machine, 166;
light, 187
Electricity, nature of, 9, 142; two
kinds of, 134; effects of, 135;
effects of cleavage on, 138; con-
duction of, 139; transference of,
145; polarisation by, 150; solids
the best conductors of, 153; velo-
city of, 154; induction of, 156;
measurement of, 164; apparatus
for, 165; voltaic, 177; chemical,
177; dynamical, 177; compared
with magnetism, 225; from mag-
netism, 228; magnetism from,
229; compared with magnetism,
232; derived from magnetism,
Electrolysis, 185
Electro-magnet, 240
Electrometers, 164, 171, 172
Electrophorus, 173
Electroscopes, 133, 170
Elements of light, 121
Elementary colours, 119
Enville, echo at, 26'
Equatorial position of magnet, 249
Estimation of vibrations, 47
Ethereal medium of light, 106;
limited theory of, 107; objections
to, 109; objections considered, 345
Euler's theory of colour, 121
Eustachian tube, 53
Examples of electricity, 132; of
electrolysis, 185; of radiation,
272, 275; of reflection, 287, 289;
of refraction, 314; of absorption,
282; of transmission, 319; of
convection, 340
Exchange of heat, 78
Expansion by heat, 51
Explosion, nature of, 20
Eye, description of, 130
Fahrenheit thermometric scale, 66
Faraday's net, 147
Feeling, nature of, 6
Fluorescence, 129
Force of galvanism, 190; primary,
347
Fraunhofer's lines, 98
Fresnel's rhomb, 330
Friction, heat from, 63
Galvanic batteries, varieties of, 181;
effects of, 184.
Galvanic force, estimation of, 201;
relay of, 222
Galvanism, nature of, 11; origin of,
177, 180; action of, 181; chemical
analysis by, 184; deflection of
magnets by, 187; force of, 190,
measurement of, 193
Galvanometers, 179, 195, 198
Gases, effect of heat on, 59; specific
heat of, 76; dia-magnetism of,
253
Generation of sound-waves, 38
Greenwich Observatory, apparatus
at, 245
Grove's battery, 181
Hearing, nature of, 6
Heat, nature of, 8, 69; expansion
by, 61; increase of temperature
by, 62; sources of, 62; from the
sun, 62; from chemical action,
62; from friction, 63; from
mechanical action, 63; from per-
cussion, 63; measurement of, 64;
dynamical theory of, 69; similar
to light, 70; transference of, 70;
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