Tracking down the origin of Arc plasma science. I. Early pulsed and oscillating discharges Page: 12 of 26
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copper, silver... The semi-metals bismuth and zink received the same impression as the proper
metals; being melted about as much as iron." [27]
Today we know that a higher discharge current causes the number of arc spots operating
simultaneously to increase rather than a change in the character of individual spots. The number of spots, or
the current per spot, also depends on the material and its surface conditions. Priestley observes:
"When the battery was charged very high, the central spot was the most irregular, many of the
dots which composed it spreading into the outer circle, and some dots appearing beyond the outer
circle...I imagined that...two or more concentric circles might be produced, if a greater quantity
of coated glass was used, or perhaps if the explosion was received upon metals that were more
easily fused than brass... upon tin, I frst observed a second outer circle... it consisted ofvery fine
points hardly visible, except when held in an advantageous light... (Plate I, Jig. 5, No. 2)" [27], cf.
Fig. 5.
Formation of arc craters is associated with formation of macroparticles. Referring to his
experiment with a brass electrode, Priestley continues [27]:
"Beyond this central spot was a circle of black dust, which was easily wiped off " Using
gold, "there were... hollow bubbles of the metal, which must have been raised when it was in a
state of fusion. These looked very beautiful when examined with a microscope in the sun, and
where easily distinguished from the cavities... The hole progress seems to have been first a fusion,
then an attraction of the liquid metal, which help to form the bubbles; and lastly the bursting of
the bubbles, which left the cavities."
He investigated the nature of the black dust from brass in another contribution [28]. He discharged
a bank of parallel capacitors, a "battery of thirty-two square feet," through a brass chain.
"I had before observed that the electric sparks betwixt each link to be most intensely bright, so as,
sometimes, to make the whole chain appear like one flame in the dark; but the appearance of the
chain in the instant of the shock, as it hung freely in the air, was exceedingly beautiful; the sparks
being largest and brightest at the bottom, and smaller by degrees, towards the top, where they
were scarcely visible; the weight of the lower links having brought them so much nearer
together. " ([28] pp.281-282)12
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Anders, Andre. Tracking down the origin of Arc plasma science. I. Early pulsed and oscillating discharges, article, January 14, 2003; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc785933/m1/12/?q=leyden: accessed April 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.