Secondary Plates, Colors of.
In a secondary battery of the lead plate type, the color of the plates
is a good indication of the condition of the battery. The negative plate
should be brown or deep-reddish, the other should be slate-colored.
Secondary Poles.
Poles sometimes found in magnets existing in positions intermediate
between the end or true poles.
Synonym--Consequent Poles.
Seebeck Effect.
The production of a current by heating the junction of two different
metals forming part of a circuit, or the thermo-electric production of
current, is stated as the Seebeck effect, having been discovered by that
investigator.
Selenium.
A non-metallic element. It is interesting electrically on account of the
changes its electric resistance undergoes when it is subjected to light.
In one set of experiments it was found that diffused light caused the
resistance to fall in the ratio of 11 to 9. Full sunlight reduced it to
one-half. Of the spectrum colors red was most powerful and the ultra red
region still more strongly affected its resistance.
The effect produced by exposure to light is instantaneous, but on
removal to the dark only slowly disappears.
A vessel of hot water was found to have no effect, showing that short
ether waves are essential to the effect.
Selenium Cell.
A selenium resistance box. Vitreous selenium is made by keeping ordinary
selenium for some hours at a temperature of about 220º C. (428º F.)
after fusing. It is placed in an electric circuit as part of the
conductor.
Its resistance can then be determined. It decreases in sunlight to about
one-half its resistance in the dark.
The selenium cell is used in the Photophone, q. v. Otherwise it is
little more than a subject of experiment.
Selenium Eye.
A model eye in which selenium in circuit with a battery and galvanometer
takes the place of the retina of the human eye.
Self-repulsion.
When a body is electrified each molecule repels its neighbor and the
condition in question is thus designated. An electrified soap-bubble
expands in virtue of self-repulsion.
Semi-conductors.
Substances which conduct static electricity poorly, but quite
appreciably and beyond the extent of leakage. The following are
examples: Alcohol and ether, powdered glass, flowers of sulphur, dry
wood, paper, ice at 0º C. (32º F.)