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THE POPULAR EDUCATOR. 



higher up in the list to the other. On the other hand, when the 

 junction is cooled, the current will flow in the reverse direction. 

 Now it will bo seen from this table that the greatest effect will 

 be obtained when bismuth and tellurium are the two metals 

 employed. Tho latter of these, however, cannot be procured in 

 any quantity, on account of its rarity ; bismuth and antimony 

 are therefore the pair commonly made use of. A number of 

 short bars of these metals are soldered together end to end, as 

 it is found that the power increases with the number of pairs 

 employed, in the same way as it does with the number of cells 

 in an ordinary battery. Only the alternate junctions must, 

 however, be heated, or else the currents produced will neutralise 

 one another. To accomplish this, the compound bar is bent 

 upon itself at every joint, after the manner shown in Fig. 48 ; 

 and in this way the alternate joints are brought close together, 

 so that by applying heat to one side of the pile the current will 

 be produced. When the instrument is placed in a warm room 

 all parts become equally heated, and therefore no effect is pro- 

 duced on the galvanometer. 



When a very delicate thermo-electric pile is required, some 

 thirty or forty couples are joined together in the way repre- 

 sented in Figs. 46, 47. A band, composed of some non-conduct- 

 ing material, is then placed round the whole pile, so as to keep 

 the bars in their places, and the ends are connected to the 

 binding screws, m n, from which wires pass to the galvanometer. 

 Some insulating material, usually plaster of Paris, is placed 

 between the different bars to prevent their touching one another, 

 and thus forming short circuits for the electric current. The 

 ends of the piles are usually made quite smooth, and are fre- 

 quently covered with lampblack, so that they may absorb the 

 heat better. When carefully constructed, a pile of this kind 

 is extremely sensitive, the mere fact of holding the hand within 

 a foot or two of one end of it being quite sufficient to produce 

 a considerable deflection in the galvanometer. In all delicate 

 experiments on heat, such an instrument is almost indispen- 

 sable, as we shall see when we come to treat of that branch 

 of Physics. As a means, however, of producing electricity for 

 practical purposes, these piles are seldom, if ever, employed. 



The next effect of the electric current which we notice, is one 

 which, at first sight, appears to be of little practical importance, 

 but which has led to very great and important results. It is the 

 influence that a wire, along which a stream of electricity is pass- 

 ing, exerts on a magnetised needle placed near it. The apparatus 

 represented in Fig. 48 'serves well to show this. N s is a stout 

 brass wire, insulated by being supported on glass rods. Along 

 this the electricity passes in the direction shown by the arrows. 

 Just under this there is placed a magnetised needle, a b, balanced 

 on a pivot, and the stand is so arranged that the needle shall 

 be parallel with the wire when the current is shut off. As soon 

 as the electricity is made to pass along in the direction s N, the 

 north pole, a, will be deflected to the left. If the current travels 

 in the opposite direction, this end will point to the right. This 

 experiment is known as Oersted's, the original discovery having 

 been made by him. 



By means of the instrument shown in Fig. 49, these effects 

 may be further investigated. A piece of brass wire is bent into 

 the shape there shown, and small wooden cups, capable of hold- 

 ing a few drops of mercury, are placed on each end, so as to 

 make the connections easily. A piece of wire, with a similar 

 cup, C, is soldered to the other end of the oval thus formed, and 

 a small strip of leather is placed where the wires cross below B, 

 so as to guard against their touching one another there. The 

 magnetic needle is balanced as before in the middle; but, as 

 will be seen, we can now send the current over or under it, or 

 both ways, according to the cups into which we dip our battery 

 wires. If A and c are used, the effect will be the same as before. 

 Now let us use the cups B and c, and thus make the current 

 pass in the same direction, but under the needle instead of over 

 it, and the needle will at once move in the opposite direction. 

 We see, then, that a current passing under the needle has just 

 the contrary effect to one passing in the same direction over it ; 

 that is, a current passing from A to c over the needle will 

 have the same effect as one passing from c to B under it. 

 If, then, we make it pass completely round from A to B, we 

 shall get the effect of both parts of the circuit. Accordingly, 

 when we want a very delicate galvanometer, the wire is wound 

 many times round the needle, each coil, up to a certain limit, 

 increasing the effect. 



Now we shall frequently find it of importance to remember in 

 which way the current deflects the needle, and this may easily 

 be done by Ampere's rule, which may be stated as follows : 

 Imagine the observer placed in the course of the current so as 

 to face the needle, and let the current be supposed to enter at 

 his feet, and pass up to his head. The north pole will always be 

 deflected to the left; or, more briefly, The north pole is always 

 de/lected to the left of the current. This rule should be carefully 

 committed to memory. 



If the current be very weak, it is unable to overcome the 

 directive action of the earth's magnetism on the needle, and 

 hence some more delicate test is needed than is supplied by the 

 ordinary galvanometer, which we have already had occasion to 

 describe. Two needles are therefore taken, having the same 

 amount of magnetic force, and they are mounted on one axle, as 

 in Fig. 50, in such a way that the north pole a of the upper 

 magnet may point in the same direction as the south pole V of 

 the lower one. In this way the directive action is entirely over- 

 come, and the system will remain indifferently in any position. 

 It is then suspended by a fibre of silk, and the only force that 

 has then to be overcome is the torsion (Latin tortum, from 

 torqueo, I twist) or twisting of this. 



A pair of needles thus mounted is called an astatic system. 

 Now the wire of the galvanometer is wound round such a system 

 on the plan shown in Fig. 51, passing between the two, and then 

 under the lower one, and great sensitiveness is thus obtained ; 

 for, as a moment's thought will show, the wire between the 

 two needles acts on eaoh, and as their poles point in different 

 directions, the effect on each tends to turn the system the same 

 way. By increasing the number of coils a very delicate in- 

 strument may be made ; such a one is shown in Fig. 52, the 

 whole being covered by a glass shade to guard from currents 

 of air and other disturbing causes. 



For ordinary purposes, this instrument is all that can be- 

 required. Special researches, however, and recent experiments 

 in connection with long submarine cables, have rendered 

 greater delicacy essential, and instruments have accordingly 

 been made of wondrous susceptibility to the faintest trace of 

 a current. Fresh improvements, too, are being made in almost 

 every instrument. Fig. 53 will, however, convey to our minds 

 a good general idea of the modifications introduced. 



The two needles are exactly equal in power, so that they 

 perfectly neutralise one another, and they are suspended by a. 

 fibre of unspun silk. A milled head, E, is provided under the 

 instrument, by means of which the whole coil can be turned 

 so as exactly to correspond with the needle. Levelling screws, 

 are likewise fixed to the stand, in order that the coil may bo 

 placed perfectly level, and that the thread or needle may not 

 touch the sides of it. The frame, r>, round which the coil 

 is wound, is made of copper, so as to diminish the number of 

 fluctuations made by the needle before coming to rest ; and the 

 face of the instrument is graduated to indicate the exact degree 

 of deviation. Binding screws, H and K, for the battery wires 

 are placed under the instrument. 



In more recent galvanometers, the needle is made as small 

 as possible, so as to weigh but a few grains ; and a very small 

 mirror, made of thin glass silvered, is fixed to it. A dark 

 chimney, pierced with a small hole, is then placed over a bright 

 lamp, and the ray of light emanating from this is made to 

 fall on the mirror, and reflected thence to a divided scale 

 fixed on the wall of a dark room. This ray of light serves 

 as a very long lever, altogether devoid of weight, and thus 

 the instrument indicates most clearly the faintest current; 

 for, as will be readily understood, a motion of the mirror, far 

 too small to be seen by itself, will move the luminous spot 

 on the wall over a considerable space. An instrument of this 

 kind is known as a Eeflecting Galvanometer, and by means of 

 one of them messages have been transmitted through the 

 whole length of the Atlantic cable with a single cell, and that 

 so small that the current from it could scarcely be shown by 

 ordinary tests. 



In graduating a galvanometer, the degree of deflection, if 

 not greater than about 20, is proportional to the strength of the 

 current, so that a current producing a deflection of 16 is 

 twice as powerful as one which only turns the needle 8. Above 

 20, however, the deflection becomes less rapid, and a special 

 table has to bo calculated, showing the relation between the 

 power of the current and the deflection. 



