During the last six years Dr. Warren de la Rue has been investigating,
in conjunction with Dr. Hugo Muller, the various and highly interesting
phenomena which accompany the electric discharge. From time to time the
results of their researches were communicated to the Royal Society, and
appeared in its Proceedings. Early last year Dr. De la Rue being requested
to bring the subject before the members of the Royal Institution, acceded
to the pressing invitation of his colleagues and scientific friends.
The discourse, which was necessarily long postponed on account of the
preparations that had to be made, was finally given on Friday, the 21st of
January, and was one of the most remarkable, from the elaborate nature of
the experiments, ever delivered in the theater of that deservedly famous
institution.
Owing to the great inconvenience of removing the battery from his
laboratory, Dr. de la Rue, despite the great expenditure, directed Mr. S.
Tisley to prepare, expressly for the lecture, a second series of 14,400
cells, and fit it up in the basement of the Royal Institution. The
construction of this new battery occupied Mr. Tisley a whole year, while
the charging of it extended over a fortnight.
The "de la Rue cell," if we may so call one of these elements, consists of
a zinc rod, the lower portion of which is embedded in a solid electrolyte,
viz., chloride of silver, with which are connected two flattened silver
wires to serve as electrodes. When these are united and the silver chloride
moistened, chemical action begins, and a weak but constant current is
generated.
The electromotive force of such a cell is 1.03 volts, and a current
equivalent to one volt passing through a resistance of one ohm was found to
decompose 0.00146 grain of water in one second. The battery is divided
into "cabinets," which hold from 1,200 to 2,160 small elements each. This
facilitates removal, and also the detection of any fault that may occur.
It will be remembered that in 1808 Sir Humphry Davy constructed his battery
of 2,000 cells, and thus succeeded in exalting the tiny spark obtained in
closing the circuit into the luminous sheaf of the voltaic arc. He also
observed that the spark passed even when the poles were separated by a
distance varying from 1/40 to 1/30 of an inch. This appears to have been
subsequently forgotten, as we find later physicists questioning the
possibility of the spark leaping over any interpolar distance. Mr. J.
P. Gassiot, of Clapham, demonstrated the inaccuracy of this opinion by
constructing a battery of 3,000 Leclanché cells, which gave a spark of
0.025 inch; a similar number of "de la Rue" cells gives an 0.0564 inch
spark. This considerable increase in potential is chiefly due to better
insulation.
The great energy of this battery was illustrated by a variety of
experiments. Thus, a large condenser, specially constructed by Messrs.
Varley, and having a capacity equal to that of 6,485 large Leyden jars,
was almost immediately charged by the current from 10,000 cells. Wires of
various kinds, and from 9 inches to 29 inches in length, were instantly
volatilized by the passage of the electricity thus stored up. The current
induced in the secondary wire of a coil by the discharge of the condenser
through the primary, was also sufficiently intense to deflagrate wires of
considerable length and thickness.
It was with such power at his command that Dr. De la Rue proceeded to
investigate several important electrical laws. He has found, for example,
that the positive discharge is more intermittent than the negative,
that the arc is always preceded by a streamer-like discharge, that its
temperature is about 16,000 deg., and its length at the ordinary pressure
of the atmosphere, when taken between two points, varies as the square
of the number of cells. Thus, with a battery of 1,000 cells, the arc was
0.0051 inch, with 11,000 cells it increased to 0.62 inch. The same law was
found to hold when the discharge took place between a point and a disk; it
failed entirely, however, when the terminals were two disks.
It was also shown that the voltaic arc is not a phenomenon of conduction,
but is essentially a disruptive discharge, the intervals between the
passage of two successive static sparks being the time required for the
battery to collect sufficient power to leap over the interposed resistance.
This was further confirmed by the introduction of a condenser, when the
intervals were perceptibly larger.
Faraday proved that the quantity of electricity necessary to produce a
strong flash of lightning would result from the decomposition of a single
grain of water, and Dr. de la Rue's experiments confirm this extraordinary
statement. He has calculated that this quantity of electricity would be
5,000 times as great as the charge of his large condenser, and that a
lightning flash a mile long would require the potential of 3,500,000 cells,
that is to say, of 243 of his powerful batteries.
In experimenting with "vacuum" tubes, he has found that the discharge is
also invariably disruptive. This is an important point, as many physicists
speak and write of the phenomenon as one of conduction. Air, in every
degree of tenuity, refuses to act as a conductor of electricity. These
experiments show that the resistance of gaseous media diminishes with the
pressure only up to a certain point, beyond which it rapidly increases.
Thus, in the case of hydrogen, it diminishes up to 0.642 mm., 845
millionths; it then rises as the exhaustion proceeds, and at 0.00065 mm.,
8.6 millionths, it requires as high a potential as at 21.7 mm., 28.553
millionths. At 0.00137 mm., 1.8 millionth, the current from 11,000 cells
would not pass through a tube for which 430 cells sufficed at the pressure
of minimum resistance. At a pressure of 0.0055 mm., 0.066 millionth, the
highest exhaust obtained in any of the experiments, even a one-inch spark
from an induction coil refused to pass. It was also ascertained that there
is neither condensacian nor dilatation of the gas in contact with the
terminals prior to the passage of the discharge.
These researches naturally led to some speculation about the conditions
under which auroral phenomena may occur. Observers have variously stated
the height at which the aurora borealis attains its greatest brilliancy
as ranging between 124 and 281 miles. Dr. de la Rue's conclusions fix
the upper limit at 124 miles, and that of maximum display at 37 miles,
admitting also that the aurora may sometimes occur at an altitude of a few
thousand feet.
The aurora was beautifully illustrated by a very large tube, in which the
theoretical pressure was carefully maintained, the characteristic roseate
tinge being readily produced and maintained.
In studying the stratifications observed in vacuum tubes, Dr. de la Rue
finds that they originate at the positive pole, and that their steadiness
may be regulated by the resistance in circuit, and that even when the least
tremor cannot be detected by the eye, they are still produced by rapid
pulsations which may be as frequent as ten millions per second.
Dr. de la Rue concluded his interesting discourse by exhibiting some of the
finest tubes of his numerous and unsurpassed collection.--_Engineering_
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