THOMSON'S GALVANOMETER. 227 



A plate of copper S is interposed between the frame and the 

 needle ; it supports the divided scale, and at the same time serves to 

 damp the oscillations. At the top the windings form two sets, and 

 there is a space between them for the introduction of the astatic 

 system; there should be a corresponding slit in the copper plate, 

 which has the drawback that there is a break near where the needle 

 has its greatest velocity, and seriously injures the damping. A glass 

 shade protects the apparatus from air-currents. 



This form of galvanometer has been the object of important 

 researches by Nobili, Melloni, Peclet, Dubois Raymond, De la 

 Provostage, and Desains, on radiant heat, or in physiology. The 

 most delicate instruments are liable to the objection, which has been 

 discussed above, of giving several positions of equilibrium, in conse- 

 quence of the presence of some traces of iron in the copper wire. 

 In order to get rid, in great part, of the difficulties relative to zero, 

 Peclet* coiled all the wire in one bundle, and supported the needle 

 by a bent stirrup, which passed round the coil ; this arrangement got 

 rid of the diametrical slit of the copper plate. 



848. WEBER'S GALVANOMETER. The methods used by Weber f 

 led him to increase greatly the dimensions of the magnetised bar ; 

 he uses a cylinder 10 cm. in length and 15 mm. in diameter, usually 

 hollow, so as to diminish the moment of inertia without materially 

 diminishing the magnetic moment. The magnet is surrounded by 

 a very thick elliptical copper frame, which serves both as damper 

 and as core for the coil. The suspension is by a bundle of silk 

 fibres, and the reading is effected by means of the mirror. 



This arrangement does not allow of the use of systems of astatic 

 needles, but the sensitiveness may be increased by a bifilar suspen- 

 sion which tends to give the bar a direction opposed to that of the 

 terrestrial field (842). 



849. THOMSON'S GALVANOMETERS. In the construction of 

 galvanometers Sir W. Thomson has endeavoured to realise as closely 

 as possible the theoretical conditions. 



The needle consists of a thin plate of steel, of about o'8 cm. 

 diameter, fixed to the back of the mirror which serves for the obser- 

 vations. Instead of a single plate, four or five may be used, placed 

 parallel to each other. The weight of the mirror and of the needle 

 are made as small as possible, and in well-constructed instruments 

 this weight does not exceed 0-05 gramme. The system is suspended 



* PECLET. Ann. de Chim. et de Phys. [3], Vol. II., p. 103. 1841. 

 t WEBER. Ekctrodyn. Maasbestimmimgen^ p. 337. 



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