Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/60

Rh 50 which should be as hard as possible. Watch-spring steel is sometimes used, and ﬁle steel is recommended by son1e authorities. The hardness is important for two reasons,- in the first place, to ensure that the permanent magnetism of the needle sl1all not alter. This is of small importance where permanent deflections are to be observed, provided we can be sure that the direction of the magnetic axis does not alter. In the second place the induced magnetism is less in hard than in soft steel, though not so much less as some writers would lead us to suppose. The best way of avoiding induced magnetism would be to make the needle spherical in form; the advantage thus gained, however, would in most cases be counterbalanced by other ‘defects. The form of the needle has been much varied by different constructors. In the earlier instruments they were made very long, and were suspended like compass needles by means of a. jewelled cup playing on a steel point. We have heard on good authority that for some purposes, such as mounting tangent galvanometer needles, this method of suspension, if carefully carried out, really answers very well. By far the most usual mode of suspension, however, is by means of a raw silk ﬁbre, or by a bundle of such ﬁbres. Weber in- troduced the use of heavy magnets whose moment of inertia and time of oscillation were great. For many purposes such needles have great advantages—where, for instance, the time of oscillation, the logaritlnnic decrement, or the extent of swing of the needle has to be observed. Where, on the co11trary, the galvanometer is to be used merely as an indicator, particularly in detecting transient currents, a light needle of small moment of inertia should be used. Continental constructors, no doubt unduly inﬂuenced by a reverence for Weber’s methods, l1ave failed to realize this; and we have seen few, if indeed any, instruments by them really well suited for measuring resistances with the Wheatstone’s bridge. This principle has been carried farthest in the galvanometers of Sir William Thomson, in some of which the needle with all its appurtenances weighs little over a grain. In some galvanometers (e._r/., certain telegraphic reading instruments) the needle is movable about a horizontal axis, and is weighted so as to be vertical in its undisturbed posi- tion. Owing to the friction at the points where the axis is supported, this method of suspension is useless for sensi- tive instruments. 3. Vhen, as is usual, the. galva11on1eter magnet is 1nov- able in a horizontal plane, the force which balances the electromagnetic force of the current in the multiplier is the horizontal component of the earth’s magnetic force. Each of these forces is proportional to the magnetic moment of the galvanometer needle, and consequently the ratio of the forces, on which depends the magnitude of tlie deﬂexion of the needle, is independent of the magnetic moment of the needle. We cannot therefore increase the sensitivcness of the galvanometer by simply increasing the magnetic moment of the needle. The action of the earth can, how- ever, be counteracted, aud the needle rendered more or less astatie in one or other of two ways. One way is to ﬁx on the same axis of suspension two parallel magnets, whose magnetic moments are as nearly as possible equal, and which are turned opposite ways. The whole system is suspended so that one of the magnets swings inside the multiplier and the other over it, as in fig. 2. In more modern instruments, such as those constructed by Messrs Elliot Brothers, the multiplier consists of two equal coils placed one vertically over the other, each enclosing one of the magnets of the astatie system, as in ﬁg. 3. Another method is to place a magnet, or a system of magnets, in the neighbourhood of the galvanometer, so as to counteract the earth’s force. In general, one magnet will suffice, placed vertically 11ndcr or over the galvanometer, in the magnetic GALVANOMETER meridian, its north pole of course pointing north. For convenience this magnet should be mounted on a vertical graduated rod, with a rough and a ﬁne adjustment. In adjusting the sensi- tiveness of the galvano- meter, it will be useful to recollect that the couple tending to bring the needle back to its posi- tion of equilibrium varies directly as the square of the nu111ber of oscilla- tions which the needle executes in a given time when no current is pass- ing through the multi- plier.‘ As the astatizing ‘_ 111agnet is brought nearer Mg‘ 2' and nearer to the galvanometer, the oscillations of the needle will be seen to become slower and slower, till at last the eupiilibrium becomes unstable, and the needle turns round through 180°; after which, on causing the magnet to approach still farther, the rapidity of oscillation increases. If the damping be very strong, and the mirror very light, an intermediate stage called the aperiodic state is passed through. 4. The normal position of the magnetic axis of the needle, when no current is passing, is parallel to the windings of the multiplier. It is particularly neces- sary that it should be in this position when the galvanometer is being used as a measuring instrument, and it is advisable in any case, since this is the position in which for a given current the 33- 3- electromagnetie action on the needle is greatest. The ﬁnal adjustment might of course be made by moving the multi- plier, but it is far more convenient to move the needle, a magnet being used for the purpose. Sometimes the astatiz- ing magnet is used, but it is better to have a much weaker magnet for the fine adjustment, suspended like the astatizer 011 a vertical axis, having a vertical motion and a motion of rotation. It is better still to use a magnet placed with its axis in the axis of the multiplier, so that it can be slid backwards and forwards at pleasure. We have seen two such magnets placed side by side, with their north and south and their south and north poles together; this gives a differential adjustment which is very convenient. The main advantage of placing magnets in this way is that we can alter the direction of the li11es of force with a minimum effect on the strength of the magnetic ﬁeld. 5. The graduation or reading apparatus in the older instruments consisted of a pointer or index ﬁxed to the magnet (very often it was the magnet itself), playing over a circular graduation centred as nearly as possible in the axis of rotation of the needle. The mirror method of read- ing which prevails in 111ost modern instruments was origin- ally suggested by Poggendorff, and carried out in practice by Gauss and Weber. A mirror is rigidly attached to the magnet, so that the reﬂecting face passes as nearly as pos- sible through the vertical axis of rotation of the needle. The glass of the 111irror should be very thin, otherwise a greater or less correction for its thickness will be necessary. In the szdgjective method 'of reading, a scale is ﬁxed before the mirror, which is usually plane (it must be well made to @ @ 1 This is not exactly true where there is damping: but the rule is suflicient for ordinary purposes.