GALVANISM. 



unamalgamated zinc laid on the sand, or hung 

 perpendicular to the face of the copper plate. 



(10.) The Bottle Battery of Faure must not be 

 omitted, on account of its neatness and novelty. 

 It is on Bunsen's principle, but no porous vessel 

 is required, the carbon being at once the porous 

 vessel and negative element. The carbon is made 

 in the shape of a common bottle, with a carbon or 

 platinum stopper to which the wire is attached. 

 Nitric acid is put in it nearly up to the neck, and 

 the stopper prevents the escape of nitrous fumes, 

 which are both disagreeable and dangerous. The 

 outside zinc is put in weak acid, as in Bunsen's 

 cell, and the action is identical. Besides dispens- 

 ing with the porous dish, this form is extremely 

 convenient, and is superior in power to either 

 Bunsen's or Grove's element. 



(n.) The M arid- Davy or Sulphate of Mercury 

 Battery is a Bunsen battery with another liquid in 

 place of nitric acid in the porous dish. This is a 

 solution, or paste, rather, of the white crystalline 

 bisulphate of mercury. Sulphate of zinc is, of 

 course, formed in the outer jar, while metallic 

 mercury is deposited on the carbon, and, from its 

 weight, falls to the bottom of the jar. Any sul- 

 phate of mercury passing through the porous dish 

 will do no harm, as it will only amalgamate the 

 zinc. The arrangement is powerful and ingenious, 

 but it is rather expensive. A battery of eight cells 

 gives as strong a current as twelve of Daniell's, 

 but, with steady action, it falls much more rapidly 

 than Daniell's. For interrupted work, such as 

 telegraphy or ringing of bells, it is better suited, 

 and will serve for three or four months. 



(12.) The Lee lancht Battery is the last we shall 

 mention, as it is one of the most recent, and per- 

 haps one of the most successful. It is a zinc- 

 carbon combination. The carbon or negative 

 plate is put in a porous jar, filled with a mixture 

 of gas-coke or carbon, and the black dioxide of 

 manganese. Both the carbon and manganese 

 have first to be broken and sifted free from dust. 

 For the zinc liquid there is a solution of common 

 sal-ammoniac, or chloride of ammonium. Chloride 

 of zinc is formed in the zinc dish, and this is 

 soluble in the sal-ammoniac solution. In the 

 other jar, the dioxide of manganese is reduced to 

 a lower oxide, and ammonia is formed in small 

 quantities. 



This cell has great electrormotive power, ten of 

 it being as powerful as sixteen Daniells, and it 

 is thus specially suited for long lines of telegraph. 

 It requires very little attention, and the zinc and 

 chemicals need renewing only about once a year. 



GALVANOMETRY, OR THE MEASUREMENT 

 OF CURRENTS. 



The magnetic, chemical, or heating effects of a 

 current may, any or all, be taken as a measure of 

 its strength. Sometimes one test is preferable, 

 sometimes another. But, in general, the most 

 convenient test of current strength is the extent 

 to which it deflects a magnetic needle. 



Nor does the needle only measure the strength ; 

 it also defines the direction of the current by the 

 side to which it swings. Ampere's rule is a simple 

 way of recollecting the connection between the 

 flow of the current and the swing of the needle. 

 It is this : A person supposed to be swimming 

 with the current in the wire, and having his face 

 18 



to the needle, sees its north pole turn to his left. 

 This always supposes the current-wire to lie in 

 the magnetic meridian. 



From this rule, it is obvious that a current 

 passing above a needle turns it to the same hand 

 as an opposite current does beneath the needle. 

 But by doubling a conducting wire, so as to be 

 over and then under the needle, we have clearly 

 the same result. One current will then act twice 

 on the needle, and each time to turn it to the 

 same side. So, by bending the wire round and 

 round the needle, and keeping the different turns 

 separate, we can multiply the effect of the same 

 current to any desired degree. This, then, is the 

 principle of the galvanometer multiplier, or gal- 

 vanometer. There are three forms of it on this 

 principle. 



(i.) The Simple Galvanometer consists of a 

 magnetic needle poised or suspended delicately 

 over a graduated card, and placed within a coil 

 of insulated copper wire, so that its oscillations 

 can be seen. Wire is insulated by being covered 

 with gutta-percha, or by being overspun with 

 cotton, or, far better, silk. This is found to be 

 quite sufficient to prevent the current passing 

 from one turn of the wire to another. 



(2.) The Astatic Galvanometer is the form most 

 commonly used ; it is far more sensitive than the 

 last. In place of a single magnetic needle, an 

 astatic pair is employed. An astatic pair of 

 magnetic needles is simply a pair of similar 

 needles, with their 



were perfectly alike, we should have a perfectly 

 astatic pair, or pair without set; that is, they 

 would remain indifferently in any position, neutral 

 to the direction of the earth's axis. It is impos- 

 sible in practice to have a perfectly astatic pair ; 

 there is always more or less a tendency to set in 

 one direction. The figure (fig. 25) shews how the 

 astatic pair is used to form an astatic galvan- 

 ometer. One needle swings within, and the other 

 without the coil, and we have thus a double 

 improvement on the single needle. First, the 

 magnetic force constraining the needle to keep its 

 north and south position is very small ; only the 

 difference of magnetism in the needles. Second, 

 the current acts again on the outside needle, tend- 

 ing to turn it to the same hand as the inside one, 

 as Ampere's rule at once shews. By such an 

 arrangement, then, a galvanometer of extraordi- 

 nary delicacy may be obtained. Fig. 26 shews the 

 usual form which is given to the instrument 



Round a bobbin, AB, of ivory is wound a coil 

 of fine, silk-covered copper wire. The number of 

 turns and thickness of wire depend on the use 

 we are to put the galvanometer to. About six 

 hundred or eight hundred does for all ordinary 

 purposes, but for some delicate experiments as 

 many as thirty thousand turns have been made. 

 A slit is usually left in the coil, to admit the lower 

 needle ; and the upper needle moves over a 



