Download Radiation Light and Illumination: A Series of Engineering Lectures Delivered at Union College PDF

TitleRadiation Light and Illumination: A Series of Engineering Lectures Delivered at Union College
Author
LanguageEnglish
File Size17.4 MB
Total Pages328
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Page 164

146 RADIATION, LIGHT, AND ILLUMINATION.

increase of resistance at low current, due to the arc stream not

completely filling the vapor tube, gives for the vacuum arc the

approximate equation:
I

e = e +
d*>

7 7 ^"d
ald bi

^

where

ld
= diameter of arc tube, cm.,

I = length of arc, cm.,
i = current.
For the mercury arc, it is :

e = 13 volts,
a - 1.68,
Z>
= 0.29 for mercury anode,
= 0.167 for graphite or metal anode,

c = 0.52.

Arc Length and Efficiency.

66. The arc most frequently employed for illumination is
the plain carbon arc. In this the arc flame or the vapor stream

gives no useful light, but the light is given by the black-body
radiation of the incandescent carbon terminal, mainly the

positive terminal, which is hottest, and is given at high efficiency
due to the very high temperature of the radiator. The light
of the carbon arc thus is incandescent light, and not lumines-
cence. In the alternating carbon arc, alternately, the two ter-
minals are positive and negative, and, as relatively little heat is

produced at the negative terminal, the average temperature of

the carbon terminals of an alternating arc is lower, and the

efficiency of light production therefore less. Thus, while direct-

current carbon arcs reach efficiencies corresponding to specific

consumptions -of from 1 to 1.5 watts per mean spherical candle

power, alternating carbon arcs show only from 2.5 to 3 watts

per candle power, or even still higher specific consumption.

Thus, the only excuse for the use of the alternating carbon arc

is the much greater simplicity and convenience of the electric
generating apparatus, the stationary transformer, compared to
the arc machine with the direct-current arc, and with the de-

velopment of the constant-current mercury-arc rectifier; this

Page 328

362'
I

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