INTERESTING ELECTRICAL RESEARCHES.
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.