A rod is supported in a vertical position. Back of it is a screen of
white paper. The two lights to be compared are arranged in front of the
rod and at a little distance from each other. They are shifted about
until the two shadows appear of equal darkness. The relative intensity
of the lights varies inversely with the square of their distances from
the shadows cast respectively by them on the screen.
The cut, Fig. 262, shows the simplest type of the shadow photometer. In
the cut, Fig. 263, a shadow photometer for testing incandescent lamps is
shown. In it E is the lamp under trial supported by a clamp H. A is an
ampere meter in circuit with the lamp, and V is a voltmeter. A candle C
can be moved along a graduated scale G G. R is the vertical rod, and S
is the screen on which the shadows fall.
Photophore.
An instrument for medical examination of the cavities of the body. It
includes an incandescent lamp mounted in a tube with a concave mirror
and convex lens.
Photo-voltaic Effect.
The change in resistance of some substances effected by light. Selenium,
of all substances, is most susceptible to this effect. (See Selenium.)
Piano, Electric.
A piano whose manual or key-board operates to close electric circuits,
whereby electro-magnets are caused to operate to drive the hammers
against the strings.
Pickle.
An acid solution for cleaning metal surfaces before electro-plating,
galvanizing or other deposition of metal upon them.
Picture, Electric.
A picture produced by passing a strong discharge through a piece of gold
leaf clamped or firmly pressed upon a sheet of paper. The gold leaf is
cut out of the desired shape, or else a stencil of paper overlays it.
The discharge dissipates the gold, and produces a purple colored
reproduction of the design upon the paper. The design is due to the
deposition of an exceedingly thin film of metallic gold.
Synonym--Electric Portrait.
Pile.
A galvanic or voltaic battery. It is sometimes restricted to a number of
voltaic couples connected. It should be only applied to batteries with
superimposed plates and no containing vessel such as the Dry Pile, q.
v., or Volta's Pile, q. v.
Pilot Transformer.
In alternating current distribution a small transformer placed at any
part of the system and connected to a voltmeter in the central station,
to indicate the potential difference of the leads.
Pilot Wires.
Wires brought from distant parts of electric light or power mains, and
leading to voltmeters at the central station, so that the potential of
distant parts of the system can be watched. The wires can be very small,
as they have but little current to transmit.