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north pole will be turned to our right hand. This ingenious device is applicable to every position, provided we are either above, or underneath the plane of the needle.

(68.) Galvanometers are differently constructed, according to the delicacy of the experiments for which they are required. In general it is sufficient to use a needle centered as if it were to be used for a mariner's compass, and a long covered wire is to be passed alternately over and under it, in the direction of the long axis. The two ends of the wire may be connected to mercury cups, to afford a ready means to unite them with the poles of the battery.

(69.) A much more delicate form of galvanometer is constructed by using two needles, so suspended, that the north pole of one is over the south pole of the other.

The polarities are thus neutralized, and no longer under the influence of the earth's magnetism. In this state they are called astatic, and are generally suspended by the finest fibre of silk, so that the slightest voltaic current will act upon them.

It is advisable to allow one needle to have a slight preponderence, in order that the long vibration may not be troublesome. An instrument like this is only necessary for the most minute experiments.

(70.) Another form is termed the tortion galvanometer, because a resistance is afforded by the twisting of an elastic substance. By this we are enabled to appreciate differences in slight currents.

(71.) However useful the instrument may be for all small currents, large quantities of electricity are seldom measured by magnetic effects; but I believe that the right use of the magnet is a very important addition to our instruments for measuring galvanic currents. To estimate the quantity of electricity in any voltaic current, a piece of soft iron is to be bent in the form of a horseshoe, and a good sized covered copper wire is to be wound round it, the two ends being left free for connection with the battery. A piece of soft iron

with a hook attached to it is to be used for the keeper, and the weight which this sustains indicates the amount of electricity.

This instrument is only valuable for comparative experiments; as different results will always be obtained by different magnets, because the quality of iron is found materially to influence the results; but if the same magnet be used, and the wire of sufficient dimensions, and of moderate length, there will be scarcely any appreciable resistance offered to the current; and the relative quantity of electricity evolved, independently of its intensity, can be accurately ascertained.

(72.) Temporary magnets are too frequently made with very thin covered wire, and even great lengths employed. Now in this case, the amount of magnetism induced by any current, ceases to be an exact measure of the quantity of electricity passing, because intensity is required to overcome the resistance afforded to the passage of the current from the wires; and it is from this cause that electro-magnetic engines, possessing, as they do, several magnets, and very often thin wires, require several cells before the current passes at all.

(73.) To give a comparative estimate of the value of this test of the quantity of electricity evolved, and that of whereby the power is estimated by decomposition, perhaps is premature, till the latter mode is fully entered into; but as the magnet requires but little intensity, and that entirely depending upon the size of the wires, it is apparent that one cell of each combination, or form of battery, will suffice for the experiment; if however, the decomposition of water be taken as the test, a sufficient number of cells must be employed, to overcome the resistance; and thus, if many combinations are made the subject of experiment, it will be attended with great cost.

(74.) If the various effects which have been already described are fraught with interest and mysterious obscurity, how much more is the property which the battery possesses, of decomposing various substances, and of overcoming the most

powerful affinities by which bodies are united! To this part of the subject we are again indebted to the labours of which the Royal Institution has been the principal seat, and Sir Humphrey Davey and Faraday the authors, though Daniell and many others have been by no means behind hand in the field of enquiry.

(75.) The decomposing cell is to be reckoned as one of the cells of the battery, with variation in the metals and solution, for the purposes of experiment. Formerly the size of the plates of this cell was reckoned but of little importance, mere wires were employed, but now the plates are made much larger. This fact has been particularly insisted upon by Mr. Grove, and certainly it is a circumstance to be fully attended to.

(76.) There are many forms of the decomposition cell which are useful, according to the purpose for which it is wanted. The simplest is the V shaped tube, which is nothing but a glass tube, bent in the shape of the letter from which it derives its name. A little strip of platinum is to be placed in either part, with the fluid, and these are to be connected, one with the negative, the other with the opposite pole of the battery.

(77.) The next form was devised by Professor Faraday, and is adapted to measure the quantity of gas given off when water is decomposed. To this he gave the name of voltameter, as indicating the amount of electricity passing. Of this form there are many varieties, differing as a small or large quantity of gas is required to be measured. In the former case, two pieces of platinum are fixed about a quarter of an inch apart, and a tube, graduated to cubic inches, is supported over these poles so that the gas may be collected. Sometimes two tubes are employed, one over each pole, the object of which, is to collect separately whatever may be evolved. In other cases, where large quantities have to be estimated, a bent tube passes from the top of the apparatus to a graduated jar, placed in a pneumatic trough. This instrument is made more complicated when the operator is desirous of investigating the changes

which take place in the solution; for then porous tubes of earthenware, bladder, &c. are used to separate the poles, and to prevent the solution on one side passing freely to the other. Substances requiring heat to bring them into a state fit for decomposition, are generally placed in a tube containing two fine platinum wires, which serve as poles, when connected to the battery.

(78.) Having described the usual apparatus to effect decomposition, we have seen that in every case they have, in common, two pieces of platinum, separated from each other, but capable of being connected to either extremity of the battery. These pieces of platinum have the mysterious and incomprehensible names of poles; one of which may be considered as holding the place of the negative metal, and is called the electro-negative pole; and the other, the electropositive pole.

(79.) However, Dr. Faraday disapproving of the name of poles, has called the electro-negative the cathode; and Professor Daniell disapproving of both, has called it the platinode. These three terms, the electro-negative, cathode, and platinode, are synonymous, and are given to that pole which would have been the metal unacted upon, had it been in an ordinary cell of the battery. The opposite pole, holds the same place in the decomposition cell as zinc in the ordinary battery, and technically has the name of the electro positive, anode, or zincode of the battery.

Dr. Faraday has described the poles, as the passages or doors by which the electricity enters into, or passes out of the solution suffering decomposition. On this account he has given the term of electrodes. Good conductors are the best adapted for poles, and for this reason, metals are generally employed; but poles of water, and even of air, have served this office.

(80.) The metal employed for Faraday's voltameters should undergo no change, either from the solution in which it is placed, or from the elements which may be evolved at its sur

face; and in this respect, platinum answers above every other substance, for almost all other metals are liable to be oxydized, or even dissolved.

(81.) The greatest confusion has arisen by not considering the decomposition cell as one of the cells of the battery, but refering the effects to the plate of the battery to which the pole is connected. The reason of this is obvious; for when the terminal plates of a battery are connected with a wire, the circuit is completed, and the platinum of the battery is the platinode, electro-negative, or cathode of the battery. When the circuit is completed, by a liquid to be decomposed, the effect is no longer to be attributed to the terminal plates of the battery, but to the plates in the decomposition cell with which they alternate; so that as, the free zinc of the battery is connected with the platinum of the decomposition cell, the name must not be given from the zinc of the battery, but from the platinum of the decomposition cell; and therefore it is the platinode of the battery. The converse of this applies to the extreme platinum of the battery; for as the platinum in the decomposition cell with which it is connected, holds the place of the zine in the other cells, the platinum being substituted for zinc, to cause the evolution of the oxygen, and to prevent its combination, the name must be given to the platinum in the decomposition cell, which is there holding the place of the zinc, and not to the terminal plate of the battery; it therefore is the zincode, electro-positive, or anode of the battery. Some have given a different explanation of this alteration of the poles, when the circuit is completed, by a solution to be decomposed, instead of metallic wires; they suppose that the two terminal plates in a compound battery, hold no farther place than that of carrying the current; but this will not bear the test of enquiry.

(82.) We have now seen that the decomposition cell, or decomposing apparatus, is nothing but one of the cells of the battery, varied for the purpose of experiment; it necessarily follows the law, that in every cell there is a similar

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