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A soft rubber washer separates the bottles at the holes, making a water-tight joint, and also acting as a support to hold in place a thin film of gold-beater’s skin, through which the liquids must pass by the process of endosmose and exosmose, from one bottle to the other. All the boitles are filled with a dilute solution of sulphate of zinc ; within bottle I. is placed a piece of sulphate of copper about the size of a pea, which changes the solution to a blue colour; the copper electrode at the bottom is connected by an insulated wire, which passes through the cork to the back of the key, K. At the bottom of bottle II. is a small chunk of zinc, which collects any deposit of copper, should any pass through from bottle I. In the top of bottle III. is suspended the zinc electrode, which measures about 11 inches long by J wide and 1 thick; its con ducting wire as shown passes directly to the galvanometer. It will be noticed this arrangement gives a very constant battery which cannot polarize, as each electrode is completely isolated, and the separation of the bottles with the gold-beater’s skin also prevents an easy mixture of the solutions. Each elec trode is never endangered, but is kept immersed in a solution favourable to retain it in perfect condition. The main or Fuller battery, M, has been somewhat modified, but consists of a zinc electrode inserted in the porous cup, in which has been placed a teaspoonful of mercury and a dilute solu tion of sulphuric acid and water. In the glass jar are four carbon rods about one inch square, arranged to fit in each corner of the jar, connected by a ring of wire at the top to one conducting wire, which passes out through the top of the cell. The jar is filled with the usual bichromate of potash solution, known as electropom. A metal screw cap secures a rubber cover to the top of the jar, and thereby prevents the evaporation of the solution. Six cells are employed, and arc plainly seen, located on a shelf at the right in the figure. The amount of resistance inserted in the series is a trifle more than the resistance of the lamp while hot, and consists of a length of 5} feet of German silver wire T86 of an inch in diameter, divided into equal sections connected together at one end by a metal link. One section lies upon the millimetre scale, parallel with, and directly under, brass rod No. 2; the other also lies on the board under brass rod No. 1. The section of re sistance wire under rod No. 2 is electrically connected thereto by the hinged metal pointed foot and adjustable collar, which may be adjusted to any point on the rod over the milli meter scale, and is secured by a set screw. The position of this collar is never changed except when a new lamp is to be inserted in the circuit. The section of resistance wire under rod No. 1 is electrically connected to tho latter by a sliding collar provided with a spring, at the end of which is a grooved brass wheel about 8 of an inch in diameter, which bears directly upon the wire. In order to intelligently understand the operation, we will de tail the two different circuits of the batteries. The circuit of the Fuller battery, M, is from the positive or carbon pole of the battery to brass bar No. 1, through the collar, spring and wheel, to the German silver wire, to the lamp, and then back to the negative or zinc pole of the battery. It will be seen that by sliding the collar on rod No. 1, the amount of resistance in this circuit is easily increased or diminished. The circuit of battery is from the positive or copper pole through the key to the resistance wire, thence through the pointed foot and fixed collar to brass bar No. 2, through the galvanometer, and back to the negative or zinc pole of the battery. In the lamp circuit a switch is inserted for turning the current on and off when testing. It will be noticed that there is a section of the resistance wire (the amount between the end connecting with key, and the pointed foot under rod 2) through which the current from each battery flows ; and although the current from the main battery has a circuit independent of the galvanometer, it is in this section of wire that both currents are brought into juxtaposition and the electro-motive force of the main battery compared with that of the constant battery. The variation is at once noticed on the galvanometer, and is easily regulated, as will be hereafter de scribed. In operating the lamp, the switch in the lamp circuit is first turned on, then the key, K, is pressed, which brings the current of the standard battery, S, into opposition to the current from the main battery, through the galvanometer, G. If the electro motive force of the main battery is too weak, the needle of the galvanometer will be sent to the right of zero a few degrees by battery, S ; but such movement is at once overcome by diminish ing the resistance in the main circuit through the sliding of the collar on rod No. 1 toward the left, in the direction of Key, K. As quickly as the resistance is cut out by this movement, so is the needle of the galvanometer forced back to the left until it reaches the zero point ; then the batteries are exactly balanced, and the light obtained is equal to that of a standard candle ; at this zero reading we have also a constant number of volts of electro-motive force. A bunch of twenty lamps accompany the apparatus, and may be seen resting upon the base-board near the galvanometer. Although all of the lamps may possess the same electrical re sistance, they will not emit an equal amount of light; hence in inserting a new lamp it is necessary to locate the hinged pointed metal foot under the brass rod, No. 2, ata different point on the milimetre scale, to correspond with the number marked on the label attached to each lamp. Aroceedings of Socteties. London and Provincial PHOrOGKAPIIIC Association. A meeting of this Society was held on Thursday, the 14th inst., Mr. W. Cobb in the chair.