Fig. 256. ELECTRIC PEN.
Pentane Standard, Harcourt's.
A standard of illuminating power; in it the combustible substance is a
gas made by mixing one cubic foot of air with three cubic inches of
liquid pentane, measured at 60° F. or, if measured as gases, 20 volumes
of air to 7 of pentane. It is burned at the rate of 0.5 cubic foot per
hour from a cylindrical tube one inch in diameter, closed at the top by
a disc 0.5 inch thick with a hole 0.25 inch in diameter, through which
the gas issues. It gives a flame 2.5 inches high.
The pentane used is the distillate of petroleum which boils at 50° C.
(122° F.) ; it has a specific gravity at 15° C. (60° F.) of from 0.628
to 0.631. It is almost pure pentane (C5H12).
As long as the rate of consumption is between 0.48 and 0.52 cubic foot
per hour the flame gives practically the same light.
407 STANDARD ELECTRICAL DICTIONARY.
Perforator.
An apparatus used in automatic high speed telegraphy for perforating
strips of paper. These are then used by drawing between a roller and
contact spring for making and breaking the telegraphic circuit for the
production of a record, such as the Morse record, at the distant
receiving station.
The perforated strip has different classes of holes punched in it to
represent dots or dashes. It is fed by machinery very rapidly, so that
the message is transmitted with the highest speed. Several operators may
simultaneously prepare the paper strips, and thus in conjunction with
its rapid feeding in the transmitter, far surpass the time of ordinary
direct transmission.

Fig. 257. PERFORATOR FOR
WHEATSTONE'S AUTOMATIC TELEGRAPH.
Perforators may be entirely mechanical but are sometimes pneumatic,
compressed air being used to operate them. The holes they make are on
different levels of the paper strip, as shown in the cut.
Period.
The time required for the completion of one complete element of periodic
motion. This may be a complete alternation (See Alternation, Complete)
of an alternating current, or of an oscillatory discharge.
Periodicity.
The rate of succession of alternations or of other fixed phases; the
rate of recurrence of phenomena.
408 STANDARD ELECTRICAL DICTIONARY.
Permanency.
In electric current conductors the property of possessing conductivity
unaffected by lapse of time. Generally the permanency of conductors is
very high. In some cases a slow annealing takes place which causes a
gradual change with the lapse of time. Annealed German silver wire has
been found to increase in conductivity at about .02 per cent. in a year.
(Matthiessen.) Wire, whether annealed or not, is left in a strained
condition after the drawing operations, and such a change is consonant
with this fact. The figure only applies to the samples tested by
Matthiessen.
Permanent State.
In a telegraph line or other current conductor, the condition when a
uniform current strength obtains over the whole line. When a current is
started it advances through the line with a sort of wave front gradually
increasing in strength. At the further end some time may elapse before
it attains its full intensity. When its does the permanent state
prevails. Until then the variable state, q. v., exists in the line.
Permeameter.
An apparatus for determining the permeability of samples of iron. It
consists of a large slotted block of iron. A coil is placed within the
slot. A hole is drilled through one end, and a rod of the iron to be
tested is passed through this hole and through the coil to the bottom of
the slot. The lower end of the rod must be accurately faced off. The
current is turned on, upon which the rod adheres to the bottom of the
slot. The force required to detach it is determined with a spring
balance. The permeation through its face is proportional to the square
of the force required.

Fig. 258. PERMEAMETER.
Permeance.
The multiplying or the conducting power for magnetic lines of force
possessed by a given mass of material. It varies with the shape and size
of the substance as well as with the inducing force. It is distinguished
from permeability, as the latter is a specific quality proper to the
material, and expressed as such; the permeance is the permeability as
affected by size and shape of the object as well as by its material.
409 STANDARD ELECTRICAL DICTIONARY.
Pflüger's Law.
A law of electro-therapeutics. It states that stimulation of a nerve is
only produced by successive appearance of the kathelectrotonic state,
and disappearance of the anelectrotonic state.
Phantom Wires.
The extra transmission circuits obtained in multiplex telegraph systems.
A single line arranged for four separate simultaneous transmissions by
quadruplex apparatus is said to establish three phantom wires.
Phase.
In wave motion, oscillating motion, simple harmonic motion, or similar
periodic phenomena, the interval of time passed from the time the moving
particle moved through the middle point of its course to the instant
when the phase is to be stated.
Pherope.
An apparatus for the electric transmission of pictures. (See Telephote.)
[Transcriber's note: Precursor of the contemporary Fax and scanner.]
Philosopher's Egg.
An ellipsoidal vessel mounted with its long axis vertical and with two
vertical electrodes, the upper one sliding, and arranged to be attached
to an air pump. A discharge through it when the air is exhausted takes
the general shape of an egg.
Phonautograph.
An apparatus for registering the vibrations of a stylus, which is
mounted on a diaphragm and is acted on by sound waves.
It is virtually a resonating chamber, over one of whose ends a parchment
diaphragm is stretched. To the centre of the parchment a needle or
stylus is attached. A cylinder covered with soot is rotated in contact
with the point of the stylus. As the chamber is spoken into the
diaphragm and stylus vibrate and the vibrations are marked on the
cylinder. It is of some electric interest in connection with telephony.
Phone.
Colloquial abbreviation for telephone.
Phonic Wheel.
A form of small motor of very simple construction. It consists of a
toothed wheel of soft iron. A bar electro-magnet is fixed with one pole
facing the teeth of the wheel. By a tuning fork make and break a
succession of impulses of rapid frequency and short duration are sent
through the magnet. The teeth act as armatures and are successively
attracted by the magnet. The regulated speed is one tooth for each
impulse, but it may rotate at one-half the speed, giving two teeth for
each impulse, or at certain other sub-multiples of its regular speed. It
is the invention of Paul Lecour.
410 STANDARD ELECTRICAL DICTIONARY.
Phonograph.
An apparatus for reproducing articulate speech. It is not electric,
except as it may be driven by electricity.
It consists of a cylinder of wax-like material which is rotated and
moved slowly, longitudinally, screw fashion, at an even speed. A glass
diaphragm carrying a needle point is supported with the point barely
touching the wax. If the diaphragm is agitated, as by being spoken
against, the needle is driven back and forwards cutting a broken line or
groove following the direction of the thread of a screw in the wax, the
depth of which line or groove continually varies.
This imprints the message. If the needle is set back and the cylinder is
rotated so as to carry the needle point over the line thus impressed,
the varying depth throws the needle and diaphragm into motion and the
sound is reproduced.
The cylinder is rotated often by an electric motor, with a centrifugal
governor.
[Transcriber's note; Due to T. A. Edison, 1877, fifteen years before
this book.]
Phonozenograph.
An apparatus for indicating the direction of the point where a sound is
produced. It operates by a microphone and telephone in conjunction with
a Wheatstone bridge to determine the locality.
Phosphorescence.
The emission of light rays by a substance not heated, but whose
luminosity is due to the persistence of luminous vibration after light
has fallen upon it.
A phosphorescent body, after exposure to light, is luminous itself.
Phosphorescence may be induced by rubbing or friction, by heat, by
molecular bombardment, as in Crookes' tubes, and by static discharge of
electricity, as well as by simple exposure to light.
Another form of phosphorescence may be due to slow chemical combustion.
This is the cause of the luminosity of phosphorous.
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 classified 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 passing 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--Violé'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, illustrates 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.