AND ITS APPLICATION TO TORSION BALANCES. 
217 
This property may be established by the following methods. 
1st, Let a horizontal needle of glass, or of any substance not magnetic, be 
fixed to the lower extremity of a fine thread of glass, and then made to oscil- 
late : it will be found that these oscillations are isochronous, even when the 
thread has been twisted through several circumferences. Now this isochro- 
nism may be proved to belong only to an elastic thread possessing the property 
enunciated in the preceding proposition. 
2ndly, Suspend a magnetic needle in a horizontal position, by a similar 
glass thread, over the centre of a large circle having its circumference divided 
into degrees. Twist the thread by means of the key as in the common torsion 
balance, and note the degrees of torsion and the corresponding deviations of 
the needle from the magnetic meridian, and it will be found that the sines of 
the deviations are proportional to the corresponding degrees of torsion ; — a 
property which can only belong to elastic threads possessing the property in 
question*. 
5. This perfect elasticity of torsion belonging to threads of glass, may be 
applied with decided advantage to the electric and magnetic balances of 
Coulomb. All that is necessary in those cases is to substitute a thread of glass 
of the proper degree of fineness, for the silver wires employed with so much 
success by the ingenious inventor. The application of this property to the 
construction of a galvanometer and delicate balance, is, I believe, new, and 
therefore I have ventured to lay a description of them before the Society ; but 
before doing this, it may not be improper to describe the best method of 
making the glass threads employed in the construction of these instruments. 
Heat the end of a clean thermometer tube at the flame of a blowpipe, and 
draw it out till it be of the thickness required for fixing in the hole in the end 
of the torsion key, as in the annexed cut. f — „ Direct the 
flame of lamp on the tube at A, till the glass has become sufficiently soft. 
Remove it from the flame, and draw it out rapidly till it be of the length and 
fineness required. By separating the thread from the tube, we shall thus have 
a thread of any degree of fineness, terminated by two thicker portions, which 
may be securely fixed with cement or sealing-wax as circumstances may 
* Biot, Traite de Physique, tom. iii. p. 29. 
MDCCCXXX. 2 P 
