The secondary battery was the only available means of propelling vessels by electrical power, and considering that these batteries might be made to serve the purpose of keel ballast, their weight, which was still considerable, would not be objectionable. The secondary battery was not an entirely new conception. The hydrogen gas battery suggested by Sir Wm. Grove in 1841, and which was shown in operation, realized in the most perfect manner the conception of storage, only that the power obtained from it was exceedingly slight. The lecturer, in working upon Sir Wm. Grove's conception, had twenty-five years ago constructed a battery of considerable power in substituting porous carbon for platinum, impregnating the same with a precipitate of lead peroxidized by a charging current. At that time little practical importance attached however to the object, and even when Plante, in 1860, produced his secondary battery, composed of lead plates peroxidized by a charging current, little more than scientific curiosity was excited. It was only since the dynamo machine had become an accomplished fact that the importance of this mode of storing energy had become of practical importance, and great credit was due to Faure, to Sellon, and to Volckmar for putting this valuable addition to practical science into available forms. A question of great interest in connection with the secondary battery had reference to its permanence. A fear had been expressed by many that local action would soon destroy the fabric of which it was composed, and that the active surfaces would become coated with sulphate of lead, preventing further action. It had, however, lately been proved in a paper read by Dr. Frankland before the Royal Society, corroborated by simultaneous investigations by Dr. Gladstone and Mr. Tribe, that the action of the secondary battery depended essentially upon the alternative composition and decomposition of sulphate of lead, which was therefore not an enemy, but the best friend to its continued action.
In conclusion, the lecturer referred to electric nomenclature, and to the means for measuring and recording the passage of electric energy. When he addressed the British Association at Southampton, he had ventured to suggest two electrical units additional to those established at the Electrical Congress in 1881, viz.: the watt and the joule, in order to complete the chain of units connecting electrical with mechanical energy and with the unit quantity of heat. He was glad to find that this suggestion had met with a favorable reception, especially that of the watt, which was convenient for expressing in an intelligible manner the effective power of a dynamo machine, and for giving a precise idea of the number of lights or effective power to be realized by its current, as well as of the engine power necessary to drive it; 746 watts represented 1 horse-power.
Finally, the watt meter, an instrument recently developed by his firm, was shown in operation. This consisted simply of a coil of thick conductor suspended by a torsion wire, and opposed laterally to a fixed coil of wire of high resistance. The current to be measured flowed through both coils in parallel circuit, the one representing its quantity expressible in amperes, and the other its potential expressible in volts. Their joint attractive action expressed therefore volt-amperes or watts, which were read off upon a scale of equal divisions.
The lecture was illustrated by experiments, and by numerous diagrams and tables of results. Measuring instruments by Professors Ayrton and Perry, by Mr. Edison and by Mr. Boys, were also exhibited.
ON THE PREPARATION OF GELATINE PLATES.
[Footnote: Being an abstract of the introductory lecture to a course on photography at the Polytechnic Institute, November 11.]
By E. HOWARD FARMER, F.C.S.
Since the first announcement of these lectures, our Secretary has asked me to give a free introductory lecture, so that all who are interested in the subject may come and gather a better idea as to them than they can possibly do by simply leading a prospectus. This evening, therefore, I propose to give first a typical lecture of the course, and secondly, at its conclusion, to say a few words as to our principal object. As the subject for this evening's lecture I have chosen, "The Preparation of Gelatine Plates," as it is probably one of very general interest to photographers.