METHOD OF OSCILLATIONS. 



which is the tube which supports the bifilar suspension, and it allows 

 three electrodes to pass, which are respectively in connection with 

 the quadrants and the needle. These electrodes themselves rest on 

 glass rods, an arrangement which was used by Sir W. Thomson in his 

 portable electrometer. 



Two of the electrodes are connected with the pairs of quadrants. 

 The third is connected to the needle by means of the vessel con- 

 taining sulphuric acid, in which are immersed, on the one hand, a 

 platinum wire attached to the electrode, and, on the other hand, 

 a prolongation in platinum of the axis of the needle ; this pro- 

 longation has one or two small cross needles, which rapidly deaden 

 the oscillations. 



The ordinary arrangement is to make the potentials equal and of 

 opposite sign, by connecting the electrodes respectively with the two 

 poles of a battery formed of three small Volta's or Daniell's elements, 

 the middle of which is to earth. Other things being equal, the 

 sensitiveness is then proportional to the number of cells. 



A thread of unspun silk, made up of single fibres, is attached to 

 the needle by a hook, and is coiled at x the top on a pulley, and passes 

 between the teeth of a screw having threads in contrary directions, 

 or in two V-shaped notches, the distance of which can be varied ; 

 in this way the distance of the wires at the top, and the height of the 

 needle, can be easily regulated. The column itself can rotate by a 

 restrained motion, or by means of a tangent screw, so that the 

 adjustment can be made. 



If the three electrodes are free, it is easy to connect them in any 

 given way, and thus utilise, according to circumstances, the different 

 combinations already described. 



In all experiments with electrometers it is clear that the bodies 

 placed in connection with the electrodes should be as carefully 

 insulated as the electrodes themselves. The losses by even very 

 moist air may be considered as nil, and the loss of electricity must 

 be regarded as taking place entirely by the film of moisture which 

 covers the supports and makes them conductors. This difficulty 

 may be removed by keeping the glass supports, for part at any rate 

 of their length, in an atmosphere dried by sulphuric acid. Figs. 

 155, 156, which explain themselves, show how this may be effected.* 



820. METHOD OF OSCILLATIONS. In order to verify the law 

 of electrical actions, Coulombf investigated also the oscillations 



* MASCART. Journal de Physiqtte, Vol. vii., p. 217. 1878. 

 f COULOMB. Mem. de VAcad. des Sciences, p. 581. 1785. 



