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The zinc frequently demands but one amalgamation; and the time required either for setting it in action, or for maintaining its operation, is comparatively not worth a thought; and lastly, the expense of working it is reduced to the lowest possible amount, being exactly proportionate to the power obtained.

(57.) Perhaps I may be expected to give an approximation to the relative cost of working the three batteries. In mine it is the cost of the zinc dissolved by the acid: zinc + acid + a local action. In the constant battery it is zinc + acid + sulphate of copper + much local action. Each cell of this, to do any given amount of work, would cost about twice as much as mine. In Grove's battery it is zinc + acid + nitric acid reduced by the hydrogen + nitric acid combined with ammonia, formed during the action of the battery + extensive waste of the zinc = about three times as much as mine.

(58.) The construction of all the various forms of galvanic batteries has now been considered, and the principles also on which the peculiarities of each are founded, have been briefly explained; though if this important branch of our subject were to be alone discussed at a length proportionate to its value, this volume would not be sufficient for the interesting and important matter relating to it.



Signs of a Battery in action, 59. Harris's galvanometer, 60. Spark, 61. Voltaic electricity charges in Leyden jar, 62. Phisiological effects, 63. Magnetism, 64-68. Galvanometers, 68-70. Horseshoe temporary magnet,

71-73. Decomposition cell, voltameters, poles, 74-84. Laws of voltaic decomposition, 85, 86. Table of chemical equivalents, 87. Fluid necessary to decomposition, 88. Conduction of fluids associated with decomposition, 88-90. Intensity necessary for decompositions, 91. Electrolysis; Electrochemical decomposition, 92-98. Daniell's theory, 99. State of the fluid during decomposition, 101. Effect of heat upon fluids, 102. Ohm's formula, 104.

(59.) After describing the various forms of the galvanic battery, we are led to consider the effects which they produce, for these are called the galvanic effects, and the theoretical principle which causes them is termed galvanism.

The sign of a battery in action, is the change going on in each cell of the battery itself. In Daniell's battery it is evinced by a deposit of copper on the negative metal; in Grove's battery, by the evolution of nitrous fumes, and in mine by an evolution of hydrogen. These several actions mark exactly the quantity of current passing; but in the two former batteries, no accurate measure can be readily made, although in the latter, the hydrogen may be collected in one of the cells by means of a glass jar, and the quantity thus exactly ascertained.

This property in my battery is of extreme value to the mechanic and experimentalist, for he can tell at once by the hissing of the hydrogen, whether the connections are all correctly made, and what amount of current is passing, a fact of no small importance when applied to the electrotype.

(60.) The next phenomenon which a battery displays is the power of heating wires of sufficient size to carry the current readily; and by this, the most infusible metals, as platinum, palladium, gold, copper, iron and steel, may be instanta

neously melted. The size of the wires melted, will depend upon the quantity of electricity developed, while the length will depend upon the intensity of the current.

Mr. Snow Harris has ingeniously taken advantage of this property to make an instrument for measuring the voltaic current. It consists of a fine wire passed through a delicate air thermometer, and the expansion of air shows the degree to which the wire is heated. This instrument is a valueless

test, unless both thick and thin wires be used in two experiments, for otherwise but one property of the battery is estimated.

(61.) The next property which a battery displays, is its power of igniting metallic or charcoal points, when joined to the two ends of the battery, and held so that they barely touch, a light is then exhibited equal in brilliancy to that of a little sun. This has been called the spark, and much controversy has taken place among the learned at what distance the spark will pass. Some have asserted that it will pass through some distance; Jacobi, however, considered the distance to be extremely small; but Mr. Gassiot, with that liberality of spirit which alone is a sure test of a man's devotion to science, fitted up 100 series of Professor Daniell's largest batteries, but with them, by the most delicate micrometer, he could not discover that the spark would pass at any appreciable distance, on the contrary this large battery would remain quite inert if the poles were separated by the distance of the thinnest film of paper. In a late number of the Philosophical Magazine, Mr. Crosse has revived the enquiry by stating, that by a very extensive series of water batteries, in his own possession, he has succeeded in obtaining the spark at a short distance. He proposes to enlarge his battery to 1000 cells, in order fully to determine this point.

The spark seems principally to depend upon a combustion of fine particles of metal, and, when charcoal or hard gas coke is used, upon little points of it flying from one pole to another, so that one pole wastes away, and the other increases

till the flame becomes quite encased in a mass of carbonaceous matter. This has always been a serious obstacle to the adaptation of this brilliant arc of flame to practical purposes. The phenomenon of the spark requires intensity for its production: a series of Grove's batteries is best suited for this purpose. Professor Wheatstone, by most ingenious experiments, determined, that the duration of the spark did not last for the one-millionth part of a second.

(62.) The next property evinced by the galvanic battery, is its power of charging a Leyden jar; but this is a property of little importance, and requires great intensity. An extensive series of batteries must be used to effect this object.

(63.) Depending upon the same causes as the last, is the shock; which is a convulsive twitching in the muscles from the intensity of the battery. This singular effect requires generally a series. It is felt only when contact is either made or broken; but if a cut exists in the finger, a small series will illustrate this property.

When we desire to exhibit the effect of the shock upon a dead animal, a pin ought to be run through the skin at the head, and another at its hind leg; every time the poles of a battery are connected or disconnected with these, strong convulsions will take place. If the upper lip be touched with a piece of zinc, and the under part of the tongue with a piece of platinum, or vice versa, a flash of light will be perceived when they are connected, whether the eyelids be open or closed. No explanation can be given of this singular phenomenon. All these phenomena are termed the physiological effects.

(64.) A galvanic battery has the power of producing certain effects which are called magnetic effects, and the supposed principle of magnetism. To describe the term magnetism would be impossible, like galvanism, or electricity, because we are only cognizant of it by its properties. There are but two metals capable of being magnetic, and these are nickel

and iron. The identity of magnetism and electricity has, like all other branches of philosophy, received many important additions from the indefatigable Faraday; but although magnetism is fraught with interest, it will be foreign to the purpose of this work to enter farther into its important details, than to illustrate the effects of galvanism.

(65.) The voltaic current, passing at right angles to a piece of iron, from which it is separated by any non-conducting substance, induces in it the properties which are termed magnetic; for if another piece of iron be now held to it, it will be attracted. The more frequently the same current passes round the iron, the greater will be the power; and for this purpose it is usual to twist wire covered either with silk or cotton round the iron, in order that the same current may pass at right angles a great number of times. When the current ceases, from the connection with the battery being broken, a difference according to the nature of the iron is observed; for if it be the pure malleable soft iron, all magnetism immediately ceases; hence, iron so situated is termed a temporary magnet; but if hard steel is used for the experiment, the magnetism indeed is not so powerful, but it continues for a very long period; hence in this state it is called a permanent magnet.

(66.) A permanent magnet if suspended in such a way that it can vibrate, has one of its poles turned to the north pole of the earth, the other to the south pole; but if a galvanic current be passed round this permanent magnet, in the direction of its axis, the magnet will be instantly deflected at right angles from the current, and upon this principle an instrument has been constructed called the galvanometer.

(67.) The direction in which the needle is deflected, is best remembered by a little device which Professor Daniell describes in his lectures; for by supposing that we ourselves are the conducting wires, and the electric current passes from our head to our heels while we are looking at the magnet, the

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