Phosphorous, Electrical Reduction of.
Phosphorous is reduced from bone phosphate by the heat of the electric arc. The phosphate mixed with charcoal is exposed to the heat of the voltaic are, and reduction of the phosphorous with its volatilization at once ensues. The phosphorous as it volatilizes is condensed and collected.
Photo-electricity.
The development of electrical properties by exposure to light. Crystals of fluor spar are electrified not only by heat (see Pyro-electricity) but also by exposure to sunlight or to the light of the voltaic arc.
[Transcribers note: Although first observed in 1839 by Becquerel, it was not explained until 1905 by Albert Einstein with the introduction of photons.]
Photo-electric Microscope.
A projection, solar or magic-lantern microscope worked by the electric light.
Photo-electro-motive Force.
Electro-motive force produced in a substance by the action of light.
411 STANDARD ELECTRICAL DICTIONARY.
Photometer.
An apparatus for measuring the intensity of light emitted by a given lamp or other source of illuminating power. They may be cla.s.sified into several types.
Calorimetric or Heat Photometers act by measuring relatively the heat produced by the ether waves (so-called radiant heat) emitted by the source. The accuracy of the instrument is increased by pa.s.sing the rays through an alum solution. A thermopile, or an air thermometer, may be used to receive the rays.
Chemical Photometers. In these the light falls upon sensitized photographic paper. The depth of coloration is used as the index of illuminating power.
Direct Visual Photometers. These include Rumford"s Shadow Photometer, Bunsen"s Bar Photometer, and Wheatstone"s Bead Photometer, in which the light is estimated by direct visual comparison of its effects.
Optical Photometers. These include Polarization Photometers, in which the light is polarized; Dispersion Photometers, in which a diverging lens is placed in the path of the rays of light so as to reduce the illuminating power in more rapid ratio than that of the square of the distance.
Selenium Photometers, in which the variations in resistance of selenium as light of varying intensity falls upon it is used as the indicator of the intensity of the light.
Jet Photometers, for gas only, in which the height of a flame under given conditions, or the conditions requisite to maintain a flame of given height, is used to indicate the illuminating power.
The subject of photometers has acquired more importance than ever in view of the extensive introduction of the electric light. (See Candle, Standard--Carcel--Viol?"s Standard--and Photometers of various kinds.)
Photometer, Actinic.
A photometer whose registrations are produced by the action of the light being tested upon sensitized paper or plates, such as used in photography. Some efforts at self-registering photometers have been based on actinic registration of the height of a flame of the gas to be tested.
Photometer, Bar.
A photometer in which the two lights to be compared are fixed at or opposite to the ends of a bar or scale of known length, generally 60 or 100 inches. The bar is divided by the rule of the inverse square of the distances, so that if a screen is placed on any part of the bar where it receives an equal amount of light from both sources, the figure on the bar will indicate the relative illuminating power of the larger lamp or light in terms of the smaller. The divisions of the bar are laid out on the principle that the illuminating power of the two sources of light will vary inversely with the square of their distance from the screen.
412 STANDARD ELECTRICAL DICTIONARY.
The screen used is sometimes the Bunsen disc. This is a disc of paper with a spot of paraffine wax in the centre melted thoroughly into the paper or with a ring of paraffine wax surrounding the untouched centre.
When this disc is equally illuminated on both sides the spot is nearly invisible. Inequality of illumination brings it out more visibly.
Sometimes a Leeson disc is used. This consists of three pieces of paper, two thin ones between which a thicker piece, out of which a star is cut, is laid. When equally illuminated on both sides the star appears equally bright on both sides.
The bar photometer is the standard form. A candle or pair of candles may be burned at one end and an incandescent lamp at the other, or a gas flame may first be rated by candles and used as a standard.
Synonyms--Bunsen"s Photometer--Translucent Disc Photometer.
Fig. 259. BAR PHOTOMETER.
Photometer. Calorimetric.
A photometer in which the radiant energy, so called radiant heat, is used as the measurer of the light.
In one type a differential air thermometer is used, one of whose bulbs is blackened. On exposing this bulb to a source of light it will become heated, and if lights of the same character are used the heating will be in proportion to their illuminating power quite closely. The heating is shown by the movements of the index. By careful calibration the instrument may be made quite reliable.
Photometer, Dispersion.
A photometer in which the rays from one of the lights under comparison are made more divergent by a concave lens. In this way a strong light, such as all arc lamp can be photometered more readily than where only the natural divergence of the beam exists. The law of the variation of the intensity of light with the square of the distance is abrogated for a law of more rapid variation by the use of a concave lens.
The diagram, Fig. 260, ill.u.s.trates the principle. E represents a powerful light, an arc light, to be tested. Its distance from the screen is e. Its light goes through the concave lens L and is dispersed as shown over an area A1, instead of the much smaller area A, which the same rays would otherwise cover. Calling l the distance of the lens from the screen, f its focus, and c the distance of the standard candle from the screen when the shadows are of equal intensity, we have the proportion.
Illuminating power of lamps: ditto of standard candle:: (l (e-l) + fe)2 : (c f)2
413 STANDARD ELECTRICAL DICTIONARY.
Fig. 260. DIAGRAM OF PRINCIPLE OF THE DISPERSION PHOTOMETER.
The cut, Fig. 261, gives a perspective view of Ayrton"s Dispersion Photometer. C is the standard candle, L the concave lens, R the rod for producing the two shadows on the screen S.
Fig. 261. AYRTON"S DISPERSION PHOTOMETER.
The mirror M is fixed at an angle of 45? with the stem on which it rotates. The light of the arc lamp is received by the mirror and is reflected through the lens. The candle holder slides along a graduated bar C, and at D is an index plate to show the angle at which the spindle carrying the mirror is set.
414 STANDARD ELECTRICAL DICTIONARY.
Dr. J. Hopkinson in his dispersion photometer uses a double convex lens.
This gives a focal image of the arc-lamp between the lens and screen, whence the rays diverge very rapidly, thus giving the desired dispersion effect.
It is princ.i.p.ally for arc lamps that dispersion photometers are used.