The Boy's Playbook of Science Part 24

_Second Experiment._

To ascertain the pa.s.sage of a current of voltaic electricity, the instrument called the galvanometer needle is provided, which consists of a coil of copper wire surrounding a magnetic needle, so as to leave the latter freedom of motion from right to left, or _vice versa_. When this coil is made part of the voltaic circle it becomes magnetic, and reacting on the magnetized needle, deflects it to one side or the other, according to the direction of the current. (Fig. 180.)

[Ill.u.s.tration: Fig. 180. A galvanometer needle, consisting of a coil of covered copper wire, the ends of which terminate at the binding screws.

The magnetic needle is suspended on a point in the centre, and the coil is surrounded with a graduated circle.]

_Third Experiment._

If a number of simple voltaic circles, such as the one described in the first experiment, are connected together, they form a voltaic battery, in which of course the quant.i.ty of electricity is greatly increased.

Batteries of all kinds, from the original Volta's pile, consisting of round zinc and copper plates soldered together with interposed cloth moistened with dilute sulphuric acid, or his _couronne des ta.s.ses_, consisting of zinc and silver wires soldered together in pairs, and placed in gla.s.s cups containing dilute acid, to the improved batteries of Cruikshank, Wilkinson, Babington, Wollaston, and the still more perfect arrangements of Daniell, Mullins, Shillibeer, and Grove, have been from time to time recommended for their own peculiar features.

Amongst these several inventions, none will be found more useful than the _constant_ battery of Daniell for electrotyping, silvering, gilding, and other purposes, and Grove's battery for all the more brilliant results, such as the deflagration of the metals or the production of the electric light. The construction of the Daniell and Grove batteries will therefore be described. The former consists of a cylindrical vessel made of copper, in which is suspended or placed (as it is open at the top) a membranous, brown-paper, canvas, or porous earthenware tube, containing an amalgamated rod of zinc. To charge this arrangement, a strong solution of sulphate of copper, with some sulphuric acid, is poured into the copper vessel, which is provided usually with a sort of [Page 196]

colander at the top to hold crystals of sulphate of copper, and in the porous tube containing the zinc rod is poured dilute sulphuric acid. A number of these cylinders of copper, twenty inches high and three inches and a half in diameter, arranged in wooden frames to the number of twenty, afford a quant.i.ty of electricity sufficient to demonstrate all the usual phenomena. (Fig. 181.)

[Ill.u.s.tration: Fig. 181. A A. Copper cylindrical vessel with colander to hold the crystals of sulphate of copper. B. The amalgamated zinc rod inside the porous cell C C. D. A series of single cells forming a Daniell's battery.]

Professor Grove's battery consists of a flat glazed earthenware vessel containing a flat porous cell. An amalgamated zinc plate is placed outside the porous cell, and a platinum plate inside the latter. The arrangement is put in action by pouring dilute sulphuric acid round the zinc and strong nitric acid inside the porous cell. A set of Grove's nitric acid battery, as manufactured by Messrs. Elliott, Brothers, of 30, Strand, with fifty pairs of sheet platinum, five inches by two inches and a quarter, and double amalgamated zinc plates, flat porous cells, and separate earthenware troughs for each pair, and stout mahogany stand, arranged in ten series of five pairs, will evolve with a proper voltameter one hundred cubic inches of the mixed gases per minute from the decomposition of water, and will exhibit a most brilliant electric light, when arranged as a single series of fifty pairs of plates. Even thirty pairs exhibit the most splendid effects, whilst forty may be regarded as the happy medium, giving all the results that can be desired. (Fig. 182.)

The advantage of employing amalgamated zinc is very prominently ill.u.s.trated whilst using any powerful arrangements of either Daniell's or Grove's batteries, as they will remain for hours quiescent, like a giant asleep, until the terminal wires of the series are brought in contact [Page 197] either through the intervention of some fluid under decomposition or by means of charcoal points. The author had the pleasure of witnessing at King's College some of the effects of an enormous battery, prepared by the late Professor Daniell, and consisting of seventy of his cells.

[Ill.u.s.tration: Fig. 182. A A. Amalgamated zinc plate in flat earthenware trough. Attached to a binding screw is the platinum plate in porous cell, C C. D. A series of single cells forming a Grove's battery.]

A continuous arch of flame was produced between two charcoal points, when distant from each other three quarters of an inch, and the light and heat were so intense that the professor's face became scorched and inflamed, as if it had been exposed to a summer heat. The rays collected by a lens quickly fired paper held in the focus.[C]

[Footnote C: By the light from the same battery photogenic drawings were taken, and the heating power was so great as to fuse with the utmost readiness a bar of platinum one-eighth of an inch square; and all the more infusible metals, such as rhodium, iridium, t.i.tanium, &c., were melted like wax when placed in small cavities in hard graphite and exposed to the current of electricity.]

_Fourth Experiment._

It is by "chemical action" the electricity is produced, and as action and reaction are always equal, but contrary, we are not surprised to find that the electricity from the voltaic battery will in its turn decompose chemically many compound bodies, of which water is one of the most interesting examples. It was in the year 1800, and immediately after Volta's announcement to Sir Joseph Banks of his discovery of the pile, that Messrs. Nicholson and Carlisle constructed the first pile in England, consisting of thirty-six half-crowns, with as many discs of zinc and pasteboard soaked in salt water. These gentlemen, whilst experimenting with the pile, observed that bubbles of gas escaped from the platinum wires immersed in water and connected with the extremities of the Volta's pile, and covering the wires with a gla.s.s tube full of water, on the 2nd of May, 1800, they completed the splendid discovery of the fact that the Volta's current had the power to decompose water and other chemical compounds.

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In 1801, Davy had succeeded to a vacant post in the Royal Inst.i.tution, and on Oct. 6th, 1807, made his transcendent discovery of pota.s.sium with the aid of the voltaic battery, and from that and other experiments inferred that the whole crust of the globe was composed of the oxides of metals. To exhibit the decomposition of water, two platinum plates with proper connecting wires, pa.s.sing to small metallic cups full of mercury, are cemented inside a gla.s.s vessel, which is then filled with dilute sulphuric acid. Just above the platinum plates and over them, stand two gla.s.s tubes also containing the same fluid in contact with the battery.

Two measures of hydrogen are found in one tube, and one of oxygen in the other. (Fig. 183.)

[Ill.u.s.tration: Fig. 183. A A. A finger gla.s.s with two holes drilled to pa.s.s the wires through, which are imbedded in cement up to the platinum plates. B B. Gla.s.s tubes, closed at one end and open at the other, which are placed over the platinum plates to receive the liberated oxygen and hydrogen. The scale at the side shows the respective volumes of two of H to one of O.]

To measure the quant.i.ty power of the voltaic battery, an important instrument invented by Faraday is used. It consists of separate platinum plates cemented in a wooden stand, and over which a capped air-jar with a bent pipe is also cemented. This apparatus contains dilute sulphuric acid of the same strength as that used in the battery under examination, and by taking the time, the quant.i.ty of the mixed oxygen and hydrogen gases producible by a battery per minute is accurately determined, the gases of course being collected in a graduated jar. (Fig. 184.)

[Ill.u.s.tration: Fig. 184. A. Gas jar with cap and bent tube pa.s.sing to the graduated tube C; the jar is cemented in the same stand which carries the connecting cups, wires, and platinum plates, which are bent round each other to improve the action of the voltameter.]

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_Fifth Experiment._

By grouping the simple circles forming a voltaic battery in various numerical relations, the _quant.i.ty_ and _intensity_ effects are modified.

Thus, if a series of thirty pairs of Grove's battery are all connected together in consecutive order, the smallest _quant.i.ty_ and the largest _intensity_ effect is produced.

If changed to two groups of fifteen each, the quant.i.ty is doubled--that is to say, it will produce double the quant.i.ty of the mixed gases from the voltameter with half the intensity.

If arranged in three groups of ten each, it is trebled with a proportional loss of intensity, until the grouping reaches six series of five each, when a maximum supply of the mixed gases is obtained from the voltameter.

In arranging the groups, all the zinc ends of each series are connected, and all the platinum ends are likewise joined by proper wires.

_Sixth Experiment._

A plate-gla.s.s trough, containing a few grains of iodide of pota.s.sium dissolved in water with some starch, is quickly decomposed into its elements by placing in two platinum plates and connecting them with the wires of the voltaic battery. If the gla.s.s trough is divided in the centre with a bit of cardboard, the purple colour of the iodine and starch is shown very beautifully on one side, but not on the other, as iodine is liberated at one pole and the alkali at the other. (Fig. 185.)

[Ill.u.s.tration: Fig. 185. A A. A gla.s.s trough containing the salt dissolved in water, and divided temporarily with a bit of cardboard, B.

C C are the two platinum plates connected with the battery, and the shaded side is supposed to represent the liberation of the iodine.]

_Seventh Experiment._

Some solution of common salt coloured with sulphate of indigo and placed in the trough is decomposed into chlorine, which bleaches one side of the indigo solution, and the alkali liberated on the other does not affect it.

_Eighth Experiment._

Some nitrate of potash dissolved in water and coloured with litmus placed in the gla.s.s trough, changes red on one side of the cardboard by the liberation of acid, and is not affected on the other.

In these experiments the oxygen, iodine, chlorine, and nitric acid are liberated at the electro-positive pole, and are hence termed electro-negative bodies, whilst hydrogen and the alkalies are set free at the electro-negative pole, and are therefore called electro-positive bodies. [Page 200] Faraday has modified these terms, and calls the two cla.s.ses "_anions_" and "_cathions_," and the two poles "anodes" and "cathodes."

Anode, from [Greek: _ana_], up, and [Greek: _hodos_], a way: the way which the sun rises. Anions, from [Greek: _ana_], up, and [Greek: _eimi_], to go: that which goes up; a substance which pa.s.ses to the anode during the pa.s.sage of a current of electricity. Cathode, from [Greek: _kata_], down, and [Greek: _hodos_], a way: the way which the sun sets. Cathion, from [Greek: _kata_], down, and [Greek: _eimi_], to go: that which goes down; a substance which pa.s.ses to the cathode during the pa.s.sage of electricity from the anode to the cathode.

_Ninth Experiment._

In the process of the electrotype is presented a valuable application of the chemical power of the voltaic circle or battery, and it may be conducted either as a single cell operation or by distinct batteries. In the former case the most simple arrangement will suffice; the only articles necessary are--a large mug or tumbler; some brown paper and a ruler; a bit of amalgamated zinc, four inches long and half an inch wide; a short length of copper wire; some black lead, blue vitriol, and oil of vitriol.

The mould from which the electrotype is to be taken can be made of common sealing wax, plaster of Paris, white wax, gutta percha, or fusible alloy. Supposing the first to be selected--viz., a common seal, it is first thoroughly black-leaded,[D] then one end of the copper wire is bent round the top of the amalgamated zinc, and the other is gently warmed and melted into the side of the seal, leaving a small portion uncovered by the wax, which is then well black-leaded. A few ounces of blue vitriol are dissolved in boiling water, and when cold the solution is poured into the tumbler, and the porous cell to contain the mixture of eight parts water to one of sulphuric acid is made by rolling the brown paper three or four times round the ruler and closing the end, and fixing the side with a little sealing wax. The porous cell of brown paper is now filled with the dilute acid, and placed in the tumbler containing the solution of blue vitriol, the amalgamated zinc being arranged in the paper cell, and the attached seal in the copper solution; in about twelve hours a good deposit of copper is produced, and a perfect cast in metal of the seal obtained. (Fig. 186.)

[Footnote D: The application of plumbago, or black lead, for electrotype purposes, was first made by the late lamented Mr. Robert Murray.]

[Ill.u.s.tration: Fig. 186. A A. The tumbler containing the solution of sulphate of copper. B B. The brown paper cell containing the dilute sulphuric acid, inside which is the amalgamated zinc with wire attached to the seal D.]

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Messrs. Elliott provide every kind of convenient vessel for the purpose, and in the picture below it will be noticed that the single cell apparatus, though not so economical as the simple tumbler arrangement already described, is perhaps more convenient for electrotyping. (Fig.

187.)

[Ill.u.s.tration: Fig. 187. A. Single cell apparatus with proper vessel, porous tube, and binding screws. B. A large trough divided by a diaphragm of biscuit-ware or very thin porous wood.]