1 70 ELECTROMETRY. 



kind is due to Volta, as has been mentioned above (798), and the 

 absolute electrometer of Thomson is now the most perfect form. 



In order to complete this list, we must include the oscillation 

 apparatus, discharge electrometers, electrical thermometers, polarisa- 

 tion electrometers, capillary electrometers, etc., which do not come 

 within the preceding categories. 



Sir W. Thomson proposes a classification corresponding to 

 another point of view. Idiostatic electroscopes are those in which 

 one of the parts only is electrified, and heterostatic those in which 

 charges due to extraneous sources come into play. Symmetrical 

 electrometers are necessarily heterostatic; the two other kinds may 

 be simply idiostatic. 



In idiostatic electrometers the reciprocal action of the conductors 

 is manifestly a function of the square of the difference of potentials, 

 and the indication does not depend on the sign of the charge. 



In heterostatic instruments the direction of the deflection changes 

 in general with the sign of the electricity, and the deflection is pro- 

 portional to the potential at least, for feeble charges. The sensitive- 

 ness for weak charges may be made far greater than with idiostatic 

 instruments. 



800. COULOMB'S BALANCE. The form of balance employed by 

 Coulomb,* to determine by the torsion of a wire the action exerted 

 between two small electrified spheres, is so well known that it is 

 sufficient to mention the principal parts. The wire supports a 

 horizontal thread of shellac, terminating at one end in a small 

 conducting sphere ; another knob, of the same diameter, placed in 

 the circumference which the first describes, is supported by an 

 insulating stem. In the experiments of Coulomb the diameter of 

 these knobs did not exceed five or six millimetres, and the radius 

 of the circumference which the movable knob described was a little 

 more than ten centimetres. The wire, which was of silver, was 

 75 cm. in length, and was so fine that a force of o'i5 mgr., applied 

 at the end of the needle, was sufficient to twist it through 360. 



The wire is fixed at the top to a graduated drum-head, by which 

 the torsion can be estimated. The deflection of the needle may be 

 measured either by means of a scale fixed in the shade, or more 

 exactly by the mirror method. 



Let us suppose the position of the drum-head to be such that, for 

 the position of equilibrium of the needle, the centre of the 1 movable 

 sphere coincides with that of the fixed knob. When the fixed kn<>b 



'* COULOMB. Mem. de VAcad. des Sciences, 1785, p. 569, etc. 



