Mr. Ritchie on a Torsion Galvanometer. 31 



the magnetic needles. Take a solid rod of glass, or a piece of 

 a clean thermometer tube, having a very fine bore, and draw 

 out one of its ends, as in the annexed cut. 



Direct the very point of the flame on the thick portion at A, 

 and pull it out, between the two hands, to the proper length. 

 As it is hardly possible to get a thread of glass of the proper 

 length and fineness, at the first trial, it will be found necessary 

 to draw several, and select the one best adapted to the purpose. 

 Two slender darning needles, of the best steel, are then to be 

 selected, the eyes to be broken off, and the ends filed to a point 

 similar to the other ends, and then strongly magnetised in the 

 usual way. The needles are then to be fixed transversely in a 

 piece of straw, or other light substance, about an inch long, and 

 at the distance of about half an inch from each other, with their 

 corresponding poles in opposite directions the one needle in- 

 tended to be above the upper side of the rectangle and the other 

 below it. One end of the glass thread is then to be securely fixed 

 in the end of the straw, or light axis, by means of strong cement 

 or sealing-wax, whilst the other extremity is fixed, in like man- 

 ner, in the centre of the torsion key. A single fibre of silk, 

 having a small weight attached to it, is fixed to the lower end 

 of the axis, and made to pass through a small hole near the 

 lower side of the rectangle, for the purpose of keeping the axis 

 carrying the needles, in the centre of the circular opening in 

 the coil. The upper needle has two pieces of fine straw, several 

 inches long, fixed on its ends, so that the slightest deflection 

 may be readily observed. The extremity of one of the straws 

 is made to oscillate between two upright pieces of glass, to 

 prevent the needle moving over an extensive arc, and thus 

 lengthen the time necessary to complete an observation. The 

 whole will be obvious from the simple inspection of the an- 

 nexed vertical section of the instrument, in which A B is the 

 rectangular coil of wire, NS, S' N', the magnetic needles; C^ 

 the stage with the divided circle and torsion key, and G the 

 glass thread. If, instead of the glass thread, the needle be 



