Practical Physics

11. How far from a screen must a 4-candle-power light be placed to give the same illumination as a 16-candle-power electric light 3 m. away? 12. If two plane surfaces placed 1 m. and 2 m. respectively from a given light receive perpendicularly the same quantity of light, how must their areas compare? State the law involved.

13. If two foot-candles are desired for reading, at what distance from the book must a 32-candle-power lamp be placed?

CHAPTER XIX. 1. An object 5 cm. long is 50 cm. from a concave mirror of focal length 30 cm. Where is the image, and what is its size? 2. Describe the image formed by a concave lens. Why can it never be larger than the object?

3. What is the focal length of a lens if the image of an object 10 ft. away is 3 ft. from the lens?

4. If the object in Problem 3 is 6 in. long, how long will the image be?

5. A beam of sunlight falls on a convex mirror through a circular hole in a sheet of cardboard, as in Fig. 487. Prove that when the diameter of the reflected beam rq is twice the diameter of the hole np, the distance mo from the mirror to the screen is equal to the focal length oF of the mirror.

6. If a rose R is pinned upside down in a brightly illuminated box, a real image may be formed in a glass of water W by a concave mirror C (Fig. 488). Where must the eye be placed to see the image?

7. How far is the rose from the mirror in the arrangement of Fig. 488?

8. A candle placed 20 cm. in front of a concave mirror has its image formed 50 cm. in front of the mirror. Find the radius of the mirror.

n m

FIG. 487. Determination of focal length of a convex mirror

FIG. 488. Image of object at center of curvature

9. The parabolic mirror used as an objective in one of the telescopes at the Mount Wilson observatory is 100 in. in diameter and has a focal el length of about 50 ft. What magnification is obtained when it is used with a 2-inch eyepiece; with a 1-inch eyepiece? What is gained by the use of a mirror of such enormous diameter?

ake &

10. A compound microscope has a tube length of 8 in., an objective of focal length in., and an eyepiece of focal length 1 in. What is its magnifying power?

11. If the focal length of the eye is 1 in., what is the magnifying power of an opera glass whose objective has a focal length of 4 in.?

12. Explain as well as you can how a telescope forms the image which you see when you look into it.

13. The magnifying power of a microscope is 1000, the tube length is 8 in., and the focal length of the eyepiece is in. What is the focal length of the objective?

CHAPTER XX. 1. If a soap film is illuminated with red, green, and yellow strips of light, side by side, how will the distance between the yellow fringes compare with that between the red fringes? with that between the green fringes? (See table on page 403.)

2. What will be the apparent color of a red body when it is in a room to which only green light is admitted?

3. Will a reddish spot on an oil film be thinner or thicker than an adjacent bluish portion?

4. Explain the ghastly appearance of the face of one who stands under the light of a Cooper-Hewitt mercury-vapor arc lamp.

5. Draw a figure to show how a spectrum is formed by a prism, and indicate the relative positions of the red, the yellow, the green, and the blue in this spectrum.

6. Why is a rainbow never seen during the middle part of the day? 7. If you look at a broad sheet of white paper through a prism, it will appear red at one edge and blue at the other, but white in the middle. Explain why the middle appears uncolored.

8. Can you see any reason why the vibrating molecules of an incandescent gas might be expected to give out a few definite wave lengths, while the particles of an incandescent solid give out all possible wave lengths?

9. Can you see any reason why it is necessary to have the slit narrow and the slit and screen at conjugate foci of the lens in order to show the Fraunhofer lines in the experiment of § 480?

CHAPTER XXI. 1. How are ultra-violet waves detected? What apparatus is used to reveal infra-red waves?

2. Explain how the heat of the sun warms the earth.

3. What is electric resonance? How may it be demonstrated?

4. Describe the construction of an X-ray tube. Describe as well as you can the action within it when in use.

Aberration, chromatic, 409
Absolute temperature, 134
Absolute units, 6

INDEX

Absorption of gases, 102 ff.; of light
waves, 414; and radiation, 419
Acceleration, defined, 75; of gravity,

Back E. M. F. in motors, 303
Baeyer, von, 417

Balance, 7

Balance wheel, 141

Ball bearings, 145, 146
Balloon, kite, 44, 45; dirigible, 44;
helium, 45

Barometer, mercury, 30; von Guer-
icke's, 31; the aneroid, 31; the self-
registering, 32, 38

Batteries, primary, 272 ff.; storage,
281, 283

Battleship, 152

Bearings, ball, 145, 146; roller, 146
Beats, 332, 348

Becquerel, 441; portrait of, 446
Bell, Alexander Graham, 316; por-
trait of, 316
Bell, electric, 259
Bicycle pedal, 146
Binocular vision, 398
Boiler, steam, 191

Boiling points, definition of, 183;
effect of pressure on, 183

Boyle's law, stated, 36; explained, 51
British thermal unit, 152
Brittleness, 92

Brooklyn Bridge, 143
Brownian movements, 52
Bunsen, 376

Caisson, 46

Calories, 152; developed by electric
currents, 289

Camera, pinhole, 390; photographic,
390

Candle power, of incandescent lamps,
285; of arc lamps, 286; defined,
375

Canner, steam-pressure, 184
Capacity, electric, 240
Capillarity, 96 ff.
Capstan, 117

Carburetor, 198, 199

Cartesian diver, 43
Cathode, defined, 248
Cathode rays, 436
Cells, galvanic, 245; primary, 272 ff.;
local action in, 272; theory of,
273; Daniell, 275; Weston, 277;
Leclanché, 277; dry, 278; com-
binations of, 279, 280; storage, 281,

283

Center of gravity, 68
Centrifugal force, 84
Charcoal, absorption by, 102
Charles, law of, 136

Chemical effects of currents, 248
Cigar lighter, platinum-alcohol, 103
Clermont, 135

Clouds, formation of, 174
Clutch, automobile, 196
Coefficient of expansion of gases, 136;
of liquids, 138; of solids, 140
Coefficient of friction, 145
Cohesion, 92; properties of solids
depending on, 92; in liquids, 93;
in liquid films, 93

Coils, magnetic properties of, 252 ff.;
currents induced in rotating, 294
Cold storage, 202
Color, and wave length, 402; of
bodies, 404; compound, 405; com-
plementary, 406; of pigments, 407;
of thin films, 408

Curie, 441, 442, 444; portrait of, 446
Currents, wind and ocean, 207; elec-
tric, defined, 245; effects of elec-
tric, 248 ff.; magnetic fields about,
252; measurement of electric,
256 ff.; induced electric, 290 ff.
Curvature, of a liquid surface, 97;
of waves, 369; defined, 370; of a
mirror, 386; center of, 463

Daniell cell, 275

Davy safety lamp, 205
Declination, 222

Densities, table of, 8, 9

Density, defined, 8; formula for, 9;
of air, 26; maximum, of water,
138; of saturated vapor, 171; of
electric charge, 234
Descartes, 43

Dew, formation of, 174
Dew point, 175
Dewar flask, 209
Differential, automobile, 197
Diffusion, of gases, 50, 52; of liquids,
54; of solids, 55; of light, 359
Digester, 184
Dipping needle, 223
Discord, 347
Dispersion, 403

Dissociation, 249, 273
Distillation, 185
Diving bell, 45
Diving suit, 46

Doppler effect, in sound, 326; in
light, 416
Dry cell, 278
Ductility, 92
Dynamo, principle of, 290; rule for,
293; alternating-current, 296; four-
pole direct-current, 300; series-
wound, shunt-wound, and com-
pound-wound, 301; defined, 302
Dyne, 86

Eccentric, 191
Echo, 327

Edison, 356; portrait of, 316
Efficiency, defined, 147; of simple
machines, 147; of water motors,
148, 149; of steam engines, 193;
of electric lights, 285 ff.; of trans-
formers, 312
Elasticity, 90; limits of, 91

Electric charge, unit of, 227; distri-
bution of, 233; density of, 234
Electric iron, 269

Electric motor, principle of, 292;
construction of, 301; defined, 302
Electricity, static, 225 ff.; electron
theory of, 229, 438 ff.; current of,
244 ff.

Electrolysis of water, 248
Electromagnet, 247, 255
Electromotive force, defined, 263;
of galvanic cells, 266; induced,
291; strength of induced, 294;
curve of alternating, 297; curve of
commutated, 299; back, in motors,
303; in secondary circuit, 307; at
make and break, 308
Electron theory, 229, 438 ff.
Electrophorus, 242

Electroplating, 249
Electroscope, 227, 232

Electrostatic voltmeter, 239
Electrotyping, 250

Energy, defined, 122; potential and
kinetic, 123; transformations of,
124, 157, 162, 163; formulas for,
125, 126; conservation of, 155;
from sun, 157; expenditure of
electric, 284; stored in atoms, 445
Engine, steam, 189; steam, defined,
191; compound steam, 193, 298;
gas, 191, 194

English equivalent of metric units, 5
Equilibrant, 60

Equilibrium, stable, 69; neutral, 71;
unstable, 71
Erg, 106
Ether, 367

Evaporation, 53; effect of tempera-
ture on, 168; of solids, 168; effect
of air on, 171, 172; cooling effect
of, 176; freezing by, 178; effect of
air currents on, 178; effect of sur-
face on, 179; and boiling, 184
Expansion, of gases, 136; of liquids,

138; of solids, 139; unequal, of
metals, 142; cooling by, 155; on
solidifying, 165

Eye, 392; pupil of, 392; nearsighted,
393; farsighted, 393

Fahrenheit, 131
Falling bodies, 72-78

Faraday, 251, 290; portrait of, 290
Fields, magnetic, 219

Films, contractility of, 95; color of,
408

Fire syringe, 155
Fireless cooker, 206
Float valve, 452
Floating dry dock, 448
Floating needle, 100
Flotation, law of, 22
Focal length, of convex lens, 378;
of convex mirror, 385, 463
Fog, formation of, 174
Foley, 387

Foot-candle, 376

Force, beneath liquid, 11; definition
of, 57; method of measuring, 57;
composition of, 59; resultant of,
59; component of, 61, 62; centrifu-
gal, 84; lines of, 218; fields of, 219
Formula for lenses and mirrors, 388
Foucault, 358
Foucault currents, 309

Franklin, 236; portrait of, 230; kite
experiment of, 231

Fraunhofer lines, 414
Freezing mixtures, 188

Freezing points, table of, 164; of
solutions, 187

Friction, 144 ff.

Frost, formation of, 174
Fundamentals, defined, 341; in pipes,
349, 350

Fuse, electric, 269

Fusion, heat of, 161, 162

Galileo, 72, 73, 128, 132; portrait
of, 72
Galvani, 245

Galvanic cell, 245
Galvanometer, 256, 257

Gas engine, 191, 194
Gas heating coil, 213
Gas mask, 103

Gas meter, 46; dials of, 48
Gay-Lussac, law of, 136
Geissler tubes, 437

Gilbert, 225; portrait of, 222
Gliding, principle of, 78-80
Governor, 192

Gram, of mass, 4; of force, 57; of
force, variation of, 58
Gramophone, 355

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