GALVANOMETER 51 be of any use), and the image of the scale is observed by means of a telescope fixed over or under the centre of the scale. The scale divisions are seen to pass the wires of the telescope, aI1d if a circular scale be used, whose centre is in the axis of suspension of the mirror, the difference between the numbers on the cross wires in any two positions of the magnet is a measure of twice the deflection of the magnet. A correction is necessary when a straight scale is used. The reader who has occasion to use the method will find prac- tical instructions, with tables of corrections, in Wiedemann’s (;'alvcmz'smus, Bd. ii. sec. 181 sqq.; Maxwell's Elect9'icz't_z/ (uul .l[a_r]netism, vol. ii. sec. 450 .979. III the objectz've method, which is more usually practised in this country, the mirror is concave, and reflects the image of a fixed illumin- ated slit (often furnished with a vertical wire where greater accuracy is desired) upon a graduated scale. The readings are proportional to double the dcflexion of the needle, or to the tangent of the double deflexion, according as the scale is circular or straight. 6. By damping is meant the decrease of the extent of the oscillations of the galvanometer needle arising from the dissipation of energy through the resistance of the air, the action of currents induced in neighbouring metallic circuits, the viscosity of the suspension fibre, and so 011. There is always I11ore or less damping owing to the first two causes, and possibly the third ; but in many cases, where it is de- siiable that the oscillations should subside very quickly, the damping is purposely increased. In the older instruments the damping arrangement consisted of Inasses of copper surrounding the magnet. This is carried to the extreme in Wiedemann’s tangent galvanometer, where the needle is ring-shaped, and swings in a ring-shaped cavity not much larger than itself, in the heart of a Inass of copper. In the (lead-beat galvanometers of Sir William Thomson the magnet with its attached Inirror is enclosed in a flat cell, in which it can just move freely to the required extent. The damping, due to the pumping of the air backwards and forwards round the edges of the mirror, is so great that the needle swings off to its position of equilibrium, and remains there without oscillating at all. The saIne result is attained iI1 Varley’s construction by immersing the needle in a cell filled with liquid. 7. The box of shunts is simply a set of resistances; generally there are three,—T‘,tl1,51§th, and ,-,%._—,th of the resistance of the multiplier. When it is required to reduce the sensibility of the galvanometer, the terminals of one of these, say the 313th, are connected with the terminals of the multiplier; we thus have a multiple arc in place of the galvanometer, and the current is divided between its branches in the ratio of their conductivities, so that one- hundredth‘ of the whole current flows through the galvano- Ineter. By means of such a box as we have described, we can therefore send through the galvanometer the whole of any current, or the tenth, hundredth, or thousandth part. It must not be forgotten that the introduction of the shunt diminishes the whole resistance of the galvanometer circuit. In most’cases, however, this is of little moment; where necessary, the alteration may be either compensated? or allowed for. Sensitive Galmnometcrs.—In galvanometers of this class everything is disposed so as to bring the greatest possible number of turns of wire into the neighbourhood of the needle. The needle is therefore made as small and compact as possible, and the windings embrace it as closely as pos- sible, the opening in the centre of the coil being reduced to a minimum. The astatic multiplier (fig. 4) is an instru- ment of this kind which was formerly much used. The 1 See art. ELECTRICITY, p. 43. 9 - I-.'.g,),011I above case by introducing into the galvanometer circuit -mths, —, U-Uths, -f%95°Uths, respectively of the resistance of the multiplier. coil is of flat, rectangular shape, with a narrow central opening just large enough to allow one of the magnets of the astatic system to swing freely. The other magnet swings over a gra- duated circle placed ' on the top of the j: coil, and serves also as an index. SoIne- . times a mirror and - ‘, scale are substituted ‘ ' for the index and _ graduated circle. " __.-_ I The sole on which -;——,—d the coil stands is _ : movable on a fixed _, ' i piece which can be ' ' levelled by means : ,..-. ,..,- of three screws. A . —- "'T"7Il"“'_“1"'é graduation is often : ,,= "" u .'."u-um furnished to mea- """~' .i' I "I .n_.*, 1:: ("If g, sure the angle of .__.,,..... ,,..- rotation of the coil ' z ‘ " about 3, vertical FIG. 4.—Astatic multiplier. axis ; this is useful when the galvanometer has to be graduated or corrected for the torsion of the fibre. In the galvanometers of Sir William Thomson, which are the most sensitive hitherto constructed, the central opening of the coil is circular, being just large enough to allow free play to a small concave mirror a centimetre or so in diameter. Usually the coil is wound in two halves, which can be screwed together with a septum between them, iI1 which is placed the arrangement for suspending the mirror and magnets. In dead—beat instruments the coil is often wound in a single 'piece, and the mirror is arranged in a cell,3 glazed back and front, and fitted into a tube which slides into the core of the coil. Fig. 5 represents a very convenient form of Thomson’s FIG. 5. —Form of Thomson’s Galvanometer. galvanometer, the only specimen of its kind we have seen. The peculiarity of its construction consists in the connexion between the scale and the galvanometer, which saves nmch trouble in adjusting the instrument. It was constructed by Elliot Brothers for the British Association Committee on Electrical Standards. Such a galvanometer as this, pro- vided with a high and low resistance coil, would meet all the wants of most laboratories. In another form called the marine galvanometer, the mirror is strung on a fibre stretched between _two fixed points. In order to keep the needle from being Influenced 3 This arrangement is that adopted vby White of,‘ Glasgow in the
galvanometers made by him after Sir V8 in. Thomson s pattern.